Atoms, Elements, and Compounds - Spokane Public Schools

Atoms, Elements, 

and Compounds 

Chapter 13 The Atom 

Chapter 14 Elements and the 

Periodic Table 

Chapter 15 Molecules and 

Compounds 

THIS AT HOME 

Matter contains positive and negative 

charges. Put a small handful of puffed cereal 

or puffed rice into a saucer. Inflate and tie a 

balloon. Rub the balloon back and forth on 

your hair, a sweater, or a sock a dozen times. 

Hold the balloon near the cereal. What 

happens? Try this with another kind of puffed 

cereal and compare to the first. Write a 

paragraph about your observations.

Chapter 13 

The Atom 

There is something more to wintergreen-flavored candy (the 

kind with the hole in the middle) than its refreshing taste. When 

you crush one of these candies with your teeth, blue sparks jump 

out of your mouth! You can only see the sparks if you hold a 

mirror up to your mouth in a very dark place, like a closet. 

You will be able to see the light even better if you crush one 

of the candies with a pair of pliers (no mirror required). In order 

to understand why the blue sparks appear, you must know 

what an atom is and what it is made of. After reading this 

chapter on atoms, you can do an Internet search on the term 

triboluminescence to find out why this candy sparks when you 

crush it. 

1. In the past, how did scientists learn about atoms 

if they couldn’t see them? 

2. What makes the atoms of various elements 

different? 

3. What are atoms made of?

13.1Fundamental Particles and Forces 

Scientists once believed atoms were the smallest particles of matter. With the 

advancement of technology, it became clear that atoms themselves are made of simpler 

particles. Today, we believe atoms are made of three basic particles: the proton, electron, 

and neutron. It’s amazing that the incredible variety of matter around us can be built 

from just three subatomic particles! 

Electric charge 

elementary charge - the 

smallest unit of electric charge that 

is possible in ordinary matter; 

represented by the lowercase 

letter e. 

Electric charge is 

a property of 

matter 

Positive and 

negative 

In Chapter 8 you were introduced to electric charge. In that chapter, 

we were interested mostly in electric current, which is moving 

charge. In order to understand atoms, we need to understand just 

where electric charge comes from. One of the two forces that hold 

atoms together comes from the attraction between positive and 

negative charge. 

There are two different kinds of electric 

charge—positive and negative. Because 

there are two kinds of charge, the force 

between electric charges can be either 

attractive or repulsive. A positive and a 

negative charge will attract each other. 

Two positive charges will repel each other. 

Two negative charges will also repel each 

other. 

The elementary 

charge 

We use the letter e to represent the elementary charge. At the size of 

atoms, electric charge always comes in units of +e or –e. It is only 

possible to have charges that are multiples of e, such as +e, +2e, –e, 

–2e, –3e, and so on. Scientists believe it is impossible for ordinary 

matter to have charges that are fractions of e. For example, a charge 

of +0.5e is impossible in ordinary matter. Electric charge only 

appears in whole units of the elementary charge (Figure 13.1). 

Figure 13.1: Just as normal matter 

is divided into atoms, electric charge 

appears only in whole units of the 

elementary charge, e. 

324 UNIT 6 ATOMS, ELEMENTS,AND COMPOUNDS

Inside an atom: Solving the puzzle 

CHAPTER 13: THE ATOM 

The electron 

identified 

An early model of 

an atom 

Testing the model 

with an 

experiment 

An unexpected 

result! 

The nuclear 

model of the 

atom 

The first strong evidence that something smaller than an atom existed 

came in 1897. English physicist J. J. Thomson discovered that 

electricity passing through a gas caused the gas to give off particles 

that were too small to be atoms. The new particles had negative 

electric charge. Atoms have zero charge. Thomson’s particles are now 

known as electrons. Electrons were the first particles discovered that 

are smaller than atoms. 

Thomson proposed that negative electrons were 

sprinkled around inside atoms like raisins in a 

loaf of raisin bread. The “bread” was positively 

charged and the electrons were negatively 

charged. This was the first real model for the 

inside of an atom. As it soon turned out, it was not 

the right model, but it was a good place to start. 

In 1911, Ernest Rutherford, Hans Geiger, and Ernest Marsden did an 

experiment to test Thomson’s model of the atom. They launched 

positively-charged helium ions (a charged atom is an ion) at a very 

thin gold foil (Figure 13.2). They expected most of the helium ions to 

be deflected a little as they plowed through the gold atoms. 

They found something quite unexpected. Most of the helium ions 

passed right through with no deflection at all. Even more surprising, 

a few bounced back in the direction they came! This unexpected result 

prompted Rutherford to remark, “It was as if you fired a five- inch 

(artillery) shell at a piece of tissue paper and it came back and hit you!” 

The best way to explain the pass-through result was if a gold atom 

was mostly empty space. If most of the helium ions hit nothing, they 

wouldn’t be deflected. The best way to explain the bounce-back result 

was if nearly all the mass of a gold atom were concentrated in a tiny, 

hard core at the center. Further experiments confirmed Rutherford’s 

idea about this hard core. We now know that every atom has a tiny 

nucleus, which contains more than 99% of the atom’s mass. 

electron - a particle with an 

electric charge (–e) found inside of 

atoms but outside the nucleus. 

nucleus - the tiny core at the 

center of an atom containing most 

of the atom’s mass and all of its 

positive charge. 

Figure 13.2: Rutherford’s famous 

experiment led to the discovery of the 

nucleus. 

13.1 FUNDAMENTAL PARTICLESAND FORCES 

325

Three particles make up all atoms 

Protons and 

neutrons 

The nucleus 

contains most of 

the mass 

Protons 

Today we know that the nucleus contains protons and neutrons. 

have positive charge, opposite of electrons. The charge on a 

Neutrons 

proton (+e) and an electron (–e) are exactly equal and opposite. 

have zero electric charge. 

Protons and neutrons are much 

more massive than electrons. A 

proton has 1,836 times as much 

mass as an electron. A neutron 

has about the same mass as a 

proton. The chart below 

compares electrons, protons, and 

neutrons in terms of charge and 

mass. Because protons and 

neutrons have so much more 

mass, more than 99% of an 

atom’s mass is in the nucleus. 

proton - a particle found in the 

nucleus with a positive charge 

exactly equal and opposite to the 

electron. 

neutron - a particle found in the 

nucleus with mass similar to the 

proton but with zero electric 

charge. 

Electrons define 

the volume of an 

atom 

Electrons take up the region outside the nucleus in a region called the 

electron cloud. The diameter of an atom is really the diameter of the 

electron cloud (Figure 13.3). Compared to the tiny nucleus, the 

electron cloud is enormous, more than 10,000 times larger than the 

nucleus. As a comparison, if an atom were the size of a football 

stadium, the nucleus would be the size of a pea, and the electrons 

would be equivalent to a small swarm of gnats buzzing around the 

stadium at extremely high speed. Can you imagine how much empty 

space there would be in the stadium? An atom is mostly empty space! 

Figure 13.3: The overall size of an 

atom is the size of its electron cloud. The 

nucleus is much, much smaller. 


Forces inside atoms 


Electromagnetic 

forces 

Strong nuclear 

force 

Weak force 

Gravity 

Electrons are bound to the nucleus by the attractive force between 

electrons (–) and protons (+). The electrons don’t fall into the nucleus 

because they have kinetic energy, or momentum. The energy of an 

electron causes it to move around the nucleus instead of falling in 

(Figure 13.4). A good analogy is Earth orbiting the Sun. Gravity 

creates a force that pulls the Earth toward the Sun. Earth’s kinetic 

energy causes it to orbit the Sun rather than fall straight in. While 

electrons don’t really move in orbits, the energy analogy is 

approximately right. 

Because of electric force, all the positively charged protons in the 

nucleus repel each other. So, what holds the nucleus together? There 

is another force that is even stronger than the electric force. We call 

it the strong nuclear force. The strong nuclear force is the strongest 

force known to science (Figure 13.5). This force attracts neutrons and 

protons to each other and works only at the extremely small distances 

inside the nucleus. If there are enough neutrons, the attraction 

from the strong nuclear force wins out over repulsion from the 

electromagnetic force and the nucleus stays together. In every atom 

heavier than helium, there is at least one neutron for every proton in 

the nucleus. 

There is another nuclear force called the weak force. The weak force is 

weaker than both the electric force and the strong nuclear force. If you 

leave a single neutron outside the nucleus, the weak force eventually 

causes it to break down into a proton and an electron. The weak force 

does not play an important role in a stable atom, but comes into action 

in certain special cases when atoms break apart. 

The force of gravity inside the atom is much weaker even than the 

weak force. It takes a relatively large mass to create enough gravity to 

make a significant force. We know that particles inside an atom do not 

have enough mass for gravity to be an important force on the scale of 

atoms. But there are many unanswered questions. Understanding 

how gravity works inside atoms is an unsolved mystery in science. 

Figure 13.4: The negative electrons 

are attracted to the positive protons in 

the nucleus, but their momentum keeps 

them from falling in. 

Figure 13.5: When enough neutrons 

are present, the strong nuclear force 

wins out over the repulsion between 

positively charged protons and pulls the 

nucleus together tightly. The strong 

nuclear force is the strongest force in 

327 

the 

universe that we know of. 

13.1 FUNDAMENTAL PARTICLESAND FORCES

How atoms of various elements are different 

The atomic 

number is the 

number of 

protons 

Elements have 

unique atomic 

numbers 

Complete atoms 

are electrically 

neutral 

How is an atom of one element different from an atom of another 

element? The atoms of different elements contain varying numbers of 

protons in the nucleus. For example, all atoms of carbon have six 

protons in the nucleus and all atoms of hydrogen have one proton in 

the nucleus (Figure 13.6). Because the number of protons is so 

important, it is called the atomic number. The atomic number of an 

element is the number of protons in the nucleus of every atom of that 

element. 

Atoms of the same element always have the 

same number of protons in the nucleus. 

Each element has a unique 

atomic number. On a periodic 

table of elements, the atomic 

number is usually written above 

or below the atomic symbol. An 

atom with only one proton in its 

nucleus is the element hydrogen, 

atomic number 1. An atom with 

six protons is the element carbon, 

atomic number 6. Atoms with 

seven protons are nitrogen, atoms 

with eight protons are oxygen, 

and so on. 

Because protons and electrons attract each other with very large 

forces, the number of protons and electrons in a complete atom is 

always equal. For example, hydrogen has one proton in its nucleus 

and one electron outside the nucleus. The total electric charge of a 

hydrogen atom is zero because the negative charge of the electron 

cancels the positive charge of the proton. Each carbon atom has six 

electrons, one for each of carbon’s six protons. Like hydrogen, a 

complete carbon atom is electrically neutral. 

atomic number - the number of 

protons in the nucleus of an atom. 

The atomic number determines 

what element the atom represents. 

Figure 13.6: Atoms of the same 

element always have the same number 

of protons in the nucleus. 


Isotopes 

Isotopes 

The isotopes of 

carbon 

Calculating number of the 

neutrons nucleusin a 

All atoms of the same element have the same number of protons in the 

nucleus. However, atoms of the same element may have different 

numbers of neutrons in the nucleus. Isotopes are atoms of the same 

element that have different numbers of neutrons. 

Figure 13.7 shows three isotopes of carbon that exist in nature. 

Most carbon atoms have six protons and six neutrons in the nucleus. 

However, some carbon atoms have seven or eight neutrons. They 

are all carbon atoms because they all contain six protons, but they 

are different isotopes of carbon. The isotopes of carbon are called 

carbon-12, carbon-13, and carbon-14. The number after the name is 

called the mass number. The mass number of an isotope tells you 

the number of protons plus the number of neutrons. 

How of 13 many and a neutrons mass number are present of 27? in an aluminum atom that has an atomic number 

1. Looking for: You are asked to find the number of neutrons. 

2. Given: You are given the atomic number and the mass number. 

3. Relationships: Use the relationship: protons + neutrons = mass number. 

4. Solution: Plug and solve: neutrons = 27 - 13 = 14 

The aluminum atom has 14 neutrons. 

Your 

a. How 

turn... 

24? Answer: many neutrons 12 are present in a magnesium atom with a mass number of 

b. Find Answer: the number 20 of neutrons in a calcium atom that has a mass number of 40. 


isotopes - atoms of the same 

element that have different 

numbers of neutrons in the 

nucleus. 

mass number - the number of 

protons plus the number of 

neutrons in the nucleus. 

Figure 13.7: The isotopes of carbon. 


329

Radioactivity 

What if there are 

too many 

neutrons? 

Radioactivity 

Alpha decay 

Beta decay 

Gamma decay 

Almost all elements have one or more isotopes that are stable. 

“Stable” means the nucleus stays together. For complex reasons, the 

nucleus of an atom becomes unstable if it contains too many or too 

few neutrons relative to the number of protons. If the nucleus is 

unstable, it breaks apart. Carbon has two stable isotopes, carbon-12 

and carbon-13. Carbon-14 is radioactive because it has an unstable 

nucleus. An atom of carbon-14 eventually changes into an atom of 

nitrogen-14. 

If an atomic nucleus is unstable for any reason, the atom eventually 

changes into a more stable form. Radioactivity is a process in which 

the nucleus spontaneously emits particles or energy as it changes 

into a more stable isotope. Radioactivity can change one element into 

a completely different element. For example, carbon-14 is radioactive 

and eventually becomes nitrogen-14. 

In alpha decay, the nucleus ejects two protons and two neutrons 

(Figure 13.8). Check the periodic table and you can quickly find that 

two protons and two neutrons are the nucleus of a helium-4 (He-4) 

atom. Alpha radiation is actually fast-moving He-4 nuclei. When 

alpha decay occurs, the atomic number is reduced by two because two 

protons are removed. The atomic mass is reduced by four because two 

neutrons go along with the two protons. For example, uranium-238 

undergoes alpha decay to become thorium-234. 

Beta decay occurs when a neutron in the nucleus splits into a proton 

and an electron. The proton stays in the nucleus, but the high energy 

electron is ejected and is called beta radiation. During beta decay, the 

atomic number increases by one because one new proton is created. 

The mass number stays the same because the atom lost a neutron but 

gained a proton. 

Gamma decay is how the nucleus gets rid of excess energy. In gamma 

decay, the nucleus emits pure energy in the form of gamma rays. The 

number of protons and neutrons stays the same. 

stable - a nucleus is stable if it 

stays together. 

radioactive - a nucleus is 

radioactive if it spontaneously 

breaks up, emitting particles or 

energy in the process. 

Figure 13.8: Two common 

radioactive decay reactions. 


13.1 Section Review 

1. Which of the following statements regarding electric charge is 

TRUE? 

a. A positive charge repels a negative charge and attracts other 

positive charges. 

b. A positive charge attracts a negative charge and repels other 

positive charges. 

2. Is electric charge a property of just electricity or is charge a 

property of all atoms? 

3. Which of the drawings in Figure 13.9 is the most accurate model of 

the interior of an atom? 

4. There are four forces in nature. Name the four forces and rank 

them from strongest to weakest. 

5. There are three particles inside an atom. One of them has zero 

electric charge. Which one is it? 

6. All atoms of the same element have (choose one): 

a. the same number of neutrons 

b. the same number of protons 

c. the same mass 

7. The atomic number is: 

a. the number of protons in the nucleus 

b. the number of neutrons in the nucleus 

c. the number of neutrons plus protons 

8. The diagram in Figure 13.10 shows three isotopes of the element 

carbon. Which one is radioactive? 

9. Radioactive means: 

a. an atom gives off radio waves 

b. the nucleus of an atom is unstable and will eventually change 

c. the electrons in an atom have too much energy 


Figure 13.9: Question 3. 



331

13.2Electrons in the Atom 

Atoms interact with each other through their electrons. This is why almost all the 

properties of the elements (except mass) are due to electrons. Chemical bonds involve 

only electrons, so electrons determine how atoms combine into compounds. We find a 

rich variety of matter because electrons inside atoms are organized in unusual and 

complex patterns. Exactly how electrons create the properties of matter was a puzzle 

that took bright scientists a long time to figure out! 

The spectrum 

The spectrum is a 

pattern of colors 

Spectroscopes 

and spectral lines 

Almost all the light you see comes from atoms. For example, light is 

given off when electricity passes through the gas in a fluorescent bulb 

or a neon sign. When scientists look carefully at the light given off by 

a pure element, they find that the light does not include all colors. 

Instead, they see a few very specific colors, and the colors are 

different for different elements (Figure 13.11). Hydrogen has a red 

line, a green line, a blue and a violet line in a characteristic pattern. 

Helium and lithium have different colors and patterns. Each 

different element has its own characteristic pattern of colors called a 

spectrum. The colors of clothes, paint, and everything else around 

you come from this property of elements that allows them to emit or 

absorb light of only certain colors. 

Each individual color in a spectrum is called a spectral line because 

each color appears as a line in a spectroscope. A spectroscope is a 

device that spreads light into its different colors. The illustration 

below shows a spectroscope made with a prism. The spectral lines 

appear on the screen at the far right. 

spectrum - the characteristic 

colors of light given off or 

absorbed by an element. 

spectral line - a bright colored 

line 

spectroscope 

in a spectroscope. 

- an instrument 

that separates light into a 

spectrum. 

Figure 13.11: When light from 

energized atoms is directed through a 

prism, spectral lines are observed. Each 

element has its own distinct pattern of 

spectral lines. 


The Bohr model of the atom 


Energy and color 

Neils Bohr 

Explaining the 

spectrum 

In Chapter 10 you learned that light is a form of pure energy that 

comes in tiny bundles called photons. The amount of energy in a 

photon determines the color of the light. Red light has lower energy 

and blue light has higher energy. Green and yellow light have energy 

between red and blue. The fact that atoms only emit certain colors of 

light tells us that something inside an atom can only have certain 

values of energy. 

Danish physicist Neils Bohr proposed the concept of energy levels to 

explain the spectrum of hydrogen. In Bohr’s model, the electron in a 

hydrogen atom must be in a specific energy level. You can think of 

energy levels like steps on a staircase. You can be on one step or 

another, but you cannot be between steps except in passing. Electrons 

must be in one energy level or another and cannot remain in between 

energy levels. Electrons change energy levels by absorbing or emitting 

light (Figure 13.12). 

When an electron moves from a higher energy level to a lower one, the 

atom gives up the energy difference between the two levels. The 

energy comes out as different colors of light. The specific colors of the 

spectral lines correspond to the differences in energy between the 

energy levels. The diagram below shows how the spectral lines of 

hydrogen come from electrons falling from the 3 rd , 4 th , 5 th , and 6 th 

energy levels down to the 2 nd energy level. 

energy level - one of the discrete 

allowed energies for electrons in 

an atom. 

Figure 13.12: When the right 

amount of energy is absorbed, an 

electron in a hydrogen atom jumps to a 

higher energy level. When the electron 

falls back to the lower energy, the atom 

releases the same amount of energy it 

absorbed. The energy comes out as light 

of a specific color. 

13.2 ELECTRONSINTHE ATOM 

333

The quantum theory 

Quantum versus 

classical 

Everything is 

fuzzy in the 

quantum world 

The uncertainty 

principle 

Understanding 

the uncertainty 

principle 

Quantum theory says that when things get very small, like the size 

of an atom, matter and energy do not obey Newton’s laws or other 

laws of classical physics. That is, the classical laws are not obeyed in 

the same way as with a larger object, like a baseball. According to the 

quantum theory, when a particle (such as an electron) is confined to a 

small space (inside an atom) then the energy, momentum, and other 

variables of the particle become restricted to certain specific values. 

You are used to thinking of a “particle” like a 

grain of sand. A sand grain is small but you 

can easily imagine it has a definite shape, 

size, position, and speed. According to 

quantum theory, particles the size of electrons 

are fundamentally different. Instead, when 

you look closely, an electron is “smeared out” 

into a wave-like “cloud.” There is no exact 

position where the entire electron is. 

If an electron is spread out into a cloud, how can you locate its exact 

position in an atom? You can’t! The work of German physicist Werner 

Heisenberg (1901–1976) led to Heisenberg’s uncertainty principle. 

According to the uncertainty principle, a particle’s position, 

momentum, energy, and time can never be precisely known. 

The uncertainty principle arises because the quantum world is so 

small. To “see” an electron you have to bounce a photon of light off 

it, or interact with the electron in some way (Figure 13.13). Because 

the electron is so small, even a single photon moves it and changes 

its motion. That means the moment you use a photon to locate an 

electron, you push it, so you no longer know precisely how fast it was 

going. In fact, any process of observing in the quantum world changes 

the very system you are trying to observe. The uncertainty principle 

exists because measuring any variable disturbs the others in an 

unpredictable way. 

quantum theory - the theory 

that describes matter and energy 

at very small (atomic) sizes. 

uncertainty principle - it is 

impossible to know variables 

exactly in the quantum world. 

Figure 13.13: The act of observing 

anything in the quantum world means 

disturbing in unpredictable ways the 

very thing you are trying to observe. 


Electrons and energy levels 


The energy levels 

are at different 

distances from 

the nucleus 

The positive nucleus attracts negative electrons like gravity attracts a 

ball down a hill. The farther down the “hill” an electron slides, the less 

energy it has. Conversely, electrons have more energy farther up the 

hill, and away from the nucleus. The higher energy levels are farther 

from the nucleus and the lower energy levels are closer. 

Orbitals 

The energy levels in an atom 

are grouped into different 

shapes called orbitals. 

The electron 

cloud 

Bohr’s model of electron energy levels was incomplete. Electrons are 

so fast and light that their exact position within an atom cannot be 

defined. Remember, in the current model of the atom, we think of the 

electrons in an atom as moving around the nucleus in an area called 

an electron cloud. The energy levels occur because electrons in the 

cloud are at different average distances from the nucleus. 

The s-orbital is spherical and 

holds two electrons. The first 

two electrons in each energy 

level are in the s-orbital. 

Rules for energy 

levels 

Quantum 

mechanics 

Inside an atom, electrons always obey these rules: 

• The energy of an electron must match one of the energy levels in 

the atom. 

• Each energy level can hold only a certain number of electrons, and 

no more. 

• As electrons are added to an atom, they settle into the lowest 

unfilled energy level. 

Energy levels are predicted by quantum mechanics, the branch of 

physics that deals with the microscopic world of atoms. While 

quantum mechanics is outside the scope of this book, you should know 

that it is a very accurate theory and it explains energy levels. 

The p-orbitals hold 6 electrons 

and are aligned along the three 

directions on a 3-D graph. 


335

The energy levels in an atom 

How electrons fill 

in the energy 

levels 

How the energy 

levels fill 

The first energy level can accept up to two electrons. The second and 

third energy levels hold up to eight electrons each. The fourth and 

fifth energy levels hold 18 electrons (Figure 13.14). A good analogy is 

to think of the electron cloud like a parking garage in a crowded city. 

The first level of the garage only has spaces for two cars, just as the 

first energy level only has spaces for two electrons. The second level 

of the garage can hold eight cars just as the second energy level can 

hold eight electrons. Each new car that enters the garage parks in the 

lowest unfilled space, just as each additional electron occupies the 

lowest unfilled energy level. 

The number of electrons in an atom depends on the atomic number 

because the number of electrons equals the number of protons. 

That means each element has a different number of electrons and 

therefore fills the energy levels to a different point. For example, a 

helium atom (He) has two electrons (Figure 13.15). The two electrons 

completely fill up the first energy level (diagram below). The next 

element is lithium (Li) with three electrons. Since the first energy 

level only holds two electrons, the third electron must go into the 

second energy level. The diagram shows the first 10 elements which 

fill the first and second energy levels. 

Figure 13.14: Electrons occupy 

energy levels around the nucleus. The 

farther away an electron is from the 

nucleus, the higher the energy it 

possesses. 

Figure 13.15: A helium atom has two 

protons in its nucleus and two electrons. 




1. The pattern of colors given off by a particular atom is called: 

a. an orbital 

b. an energy level 

c. a spectrum 

2. Which of the diagrams in Figure 13.16 corresponds to the element 

lithium? 

3. When an electron moves from a lower energy level to a higher 

energy level the atom: 

a. absorbs light 

b. gives off light 

c. becomes a new isotope 

4. Two of the energy levels can hold eight electrons each. Which 

energy levels are these? 

5. How many electrons can fit in the fourth energy level? 

6. The element beryllium has four electrons. Which diagram in 

Figure 13.17 shows how beryllium’s electrons are arranged in the 

first four energy levels? 

7. Which two elements have electrons only in the first energy level? 

a. hydrogen and lithium 

b. helium and neon 

c. hydrogen and helium 

d. carbon and oxygen 

8. On average, electrons in the fourth energy level are: 

a. farther away from the nucleus than electrons in the second 

energy level 

b. closer to the nucleus than electrons in the second energy level 

c. about the same distance from the nucleus as electrons in the 

second energy level 




337

Aim For the Stars: Dr. Shirley Ann Jackson 

Chapter 13 Connection 

What is it like to study something you cannot see, like atoms? What sort of scientist studies atoms? Dr. Shirley Ann Jackson is a well-known 

scientist. She has studied atoms and the particles inside atoms for several decades. How did Dr. Jackson become an atomic physicist? She’s 

been curious about the way things work for as long as she can remember. 

Speeding sisters 

Brushing sweaty curls back from 

their faces, twelve year old Shirley 

Jackson and her younger sister 

Gloria steered their homemade gocart 

between two others, lining up 

the front wheels with a chalk line 

drawn across the alley. A small 

crowd of kids gathered to watch the 

three carts race down a steep hill in 

their Washington, D.C. 

neighborhood. 

The Jackson girls had built their gocart 

from old wheels, axles, bicycle 

handlebars, and other junk parts. 

Their father provided wood for the 

base and lessons on using his 

workshop tools. They had spent hours figuring out ways to 

make their “hot rod” go faster. 

Now Gloria climbed into the cart. Shirley crouched behind, 

hands on the smaller girl’s back. A neighbor kid yelled, “On 

your mark, get set...GO!” and Shirley pushed with all her 

might, propelling Gloria and the go-cart down the steep hill. 

Faster and faster Gloria sped down the hill, cruising across 

the finish line well ahead of her two competitors. Shirley 

raced down hill on foot to congratulate her. 

But just before she reached her sister, Shirley was stopped 

in her tracks. A boy in one of the other carts, furious with 

jealousy, jumped on the Jacksons’ hot rod, breaking it in two. 

The girls stared at him in disbelief. Gloria blinked back 

tears. Shirley wanted to smack him. But she took a deep 

breath, and head held high, she grabbed Gloria’s hand. 

“We’ll be back,” she glared. The two 

of them dragged their broken cart 

home. With their father’s 

encouragement, they rebuilt it and 

returned to race again. 

It was the summer of 1958. Shirley 

was about to enter seventh grade. 

Her high test scores placed her in an 

honors program with a challenging 

curriculum. Shirley was one of the 

school’s first African-American 

students to participate. 

Directing her path 

As she looks back at this time, Dr. 

Shirley Ann Jackson says four 

significant factors shaped the 

direction her life would take. The first two were inherited 

traits: her father’s interest in science and technology and her 

mother’s love of reading. 

The other two were national events. In 1954, the supreme 

court decision, Brown v. the Board of Education of Topeka, 

Kansas, ended the practice of racial segregation in schools, 

giving Shirley the opportunity for a high-quality education. 

In October 1957, the Soviet Union launched a satellite into 

space. People in the United States were stunned. As Shirley 

entered high school, an intense interest in attracting young 

people into math and science careers emerged. Across the 

country, school boards put money into better science labs. 

Government and corporations provided college scholarships 

to talented students. There was a great sense of urgency 

about doing science. 

338

Shirley enjoyed all her classes, but math was her favorite. 

She loved the way math could describe patterns and bring to 

light hidden connections. So when it was time to apply to 

college, a teacher suggested Massachusetts Institute of 

Technology—a school known for its excellent math, science, 

and engineering programs. 

It was difficult to get into MIT, but Shirley’s father advised 

her to “Aim for the stars so that you can reach the treetops, 

and at any rate you will get off the ground.” 

She did far more than get off the ground. She was accepted 

to MIT, receiving corporate and community scholarships! 

University Life 

Shirley was one of 43 women in her freshman class of 900 

students, and one of ten African-Americans out of 4,000 

undergraduates. Sometimes she felt isolated and lonely. She 

volunteered in a hospital pediatric ward and tutored 

children at a local YMCA. This, she says, helped her keep 

perspective. 

At first, Shirley thought she might choose math as her major 

at MIT. But her favorite freshman class turned out to be 

Physics: A New Introductory Course (which the students 

called “PANIC” for short!). In PANIC, she discovered the 

fascinating world of subatomic particles—the tiniest things 

in the universe. 

Shirley enjoyed conducting experiments in the huge physics 

labs at MIT, which reminded her of the time she had spent in 

her father’s workshop. But what really captured her interest 

was using mathematics to predict the behavior of subatomic 

particles. She decided to stay at MIT and pursue a Ph.D. in 

theoretical high energy particle physics. 

In 1973, Shirley Ann Jackson became the first African- 

American woman to receive a Ph.D. from MIT, and one of the 

first two African-American women in the United States to 

receive a doctorate in physics. 

Working in Research, Government, and Education 

Dr. Jackson’s career in physics has taken many interesting 

turns. First, she researched subatomic particles at Fermilab 

near Chicago and CERN in Geneva, Switzerland. Both labs 

have huge machines called particle accelerators which send 

particles zooming around at nearly the speed of light and 

then smash them into each other. By studying the energy 

released by the collisions, scientists can deduce properties of 

the subatomic particles, such as their mass and charge. 

Next, Dr. Jackson turned her attention to condensed matter 

physics, working on semiconductors and other material 

systems at AT&T Bell laboratories. 

In 1995, Dr. Jackson was 

appointed the Chairman of the 

U.S. Nuclear Regulatory 

Commission by President Bill 

Clinton. She worked to increase 

nuclear plant safety in the 

United States and with 

international organizations to 

enhance worldwide nuclear 

safety. 

Dr. Jackson became president of 

Rensselaer Polytechnic Institute 

in 1999. In 2005 she was 

described by Time Magazine as 

“Perhaps the ultimate role model 

for 

Questions: 

women in science.” 

1. Think about the physics involved in Shirley’s go-cart races. 

What force caused the motion? What force resisted motion? 

What changes do you think the girls made to increase the gocart’s 

speed? 

2. Resilience is defined as the ability to bounce back after a 

painful experience. How did Shirley demonstrate this 

character trait? 

3. Find out more about the particle accelerators at Fermilab or 

CERN. How large are they? How are they shaped? 

To find out more about Dr. Jackson, check out this book: 

O’Connell, Diane. (2005). Strong Force: The Story of Physicist Shirley Ann 

Jackson. New York: Scholastic, Inc. 

UNIT 6 ATOMS, ELEMENTS,AND COMPOUNDS 


339


Half Life 

Radioactivity is how we describe any process where the 

nucleus of an atom emits particles or energy. All radioactive 

elements have a half-life. This means that there is a certain 

length of time after which half of the radioactive element has 

decayed. Radioactive elements have an unstable nucleus, 

which decays into an different type of atom with a more 

stable nucleus. As it decays, it releases radiation. 

Materials: 

can of pennies, graph paper 

What you will do 

Your teacher has given you a 

can of pennies to represent the 

atoms of a sample of a newly 

discovered, radioactive 

element. You will use the 

pennies to simulate the process 

of radioactive decay. Upon 

completion of the simulation, 

you will construct a graph of 

your data. 

Shake your can of pennies and spill them out onto a tray or 

table. 

1. Remove all pennies that are “heads” up and count them. 

2. Record these as decayed atoms in a table like the one 

below. 

Trial Sample Number # of atoms decayed 

1 

2 

3 

Trial Sample Number # of atoms decayed 

4 

5 

6 

7 

8 

9 

10 

3. Put the rest of the pennies back into the can and shake 

them again. 

4. Spill them out onto the tray or table, and again, remove 

and count the “heads”. 

5. Repeat this process until there are no pennies left. 

Applying your knowledge 

a. Graph your data. The sample number will be on the x- 

axis and the number of decayed atoms per sample will be 

on the y-axis. Label the axes clearly and provide a title for 

the graph. 

a. Describe what your graph looks like. 

b. How many trials did it take for half of your original 

number of pennies to decay to “heads up”? 

c. How many trials did it take for all your pennies to decay? 

d. Would it make a difference if you graphed the number of 

“tails” up instead? 

e. If you were to put a sticker on one of the pennies and 

repeat the activity, could you predict in which trial the 

marked penny would decay? 

f. Another student did this activity, and on the third shake 

12 pennies decayed. Can you tell how many pennies the 

other student started with? 

340

Chapter 13 Assessment 

Vocabulary 

Select the correct term to complete the sentences. 

atomic number 

energy level 

neutron 

quantum theory 

Section 13.1 

1. The sum of protons plus neutrons in the nucleus of an atom 

is known as the ____. 

2. The negatively charged particle found outside the nucleus of 

an atom is called a(n) ____. 

3. The core of the atom containing most of the atom’s mass and 

all of its positive charge is called a(n) ____. 

4. A light particle with a negative charge, found in atoms, is 

called a(n) ____. 

5. A neutral particle with nearly the same mass as the proton 

is the ____. 

6. The number of protons in an atom, unique to each element is 

known as the ____. 

7. A nucleus that spontaneously breaks apart, emitting 

particles or energy is referred to as ____. 

8. Atoms of the same element containing different numbers of 

neutrons are called ____. 

Section 13.2 

electron 

isotopes 

nucleus 

radioactive 

elementary charge 

mass number 

spectral line 

spectroscope 

9. One of the allowed energies for electrons in an atom is 

known as an ____. 

10. The theory that describes matter and energy at atomic sizes 

is the ____. 

11. A bright colored line produced by a spectroscope is a ____. 

12. An instrument that is used to separate light into spectral 

lines is a ____. 

Concepts 

Section 13.1 

1. Explain why Rutherford assumed most of the atom to be 

empty space. 

2. Explain how Rutherford concluded that positive charge was 

concentrated in a small area. 

3. How did Rutherford’s model of the atom differ from 

Thomson’s model? 

4. Summarize the characteristics of the electron, proton and 

neutron, comparing their relative mass, charge and location 

within the atom by completing the table below. 

Particle 

Place in 

Atom 

Charge 

Relative 

Mass 

electron ? 1 

proton +1 neutron ? ? ? 

5. Name the four forces of nature and compare their relative 

strengths. 

6. Explain the effect of the electromagnetic and strong forces 

on the structure of the atom. 

7. What do the atomic number and mass number tell you about 

an atom? 

8. Compare the number of protons and electrons in a neutral 

atom. 

9. Compare the mass number and atomic number for isotopes 

of an element. Explain your answer. 

CHAPTER 13 THE ATOM 341

10. Describe the radioactive disintegrations known as alpha, 

beta, and gamma decay. 

Section 13.2 

11. Which particle in an atom is most responsible for its 

chemical properties? 

12. What is the source of the light you see? 

13. How can a spectroscope be used to identify an element 

heated to incandescence? 

14. Cite evidence that atoms are restricted to have only certain 

amounts of energy. 

15. How did Neils Bohr explain spectral lines? 

16. What is the difference between an electron in ground state 

and one in an excited state? 

17. What would occur if electron were to move from a certain 

energy level to a lower energy level? 

18. Summarize the uncertainty principle. 

19. Why can’t the position of an electron be determined with 

certainty? 

20. How is the location of an electron described? 

Problems 

Section 13.1 

1. Which of the following charges do NOT appear in normal 

matter? 

a. +2e d. -5.4e 

b. b. +1/4e e. +3/4e 

c. c. -4e f. -1e 

2. What charge would an atom have if it lost 1 electron? 

342 CHAPTER 13 THE ATOM 

3. A neutral atom has 7 protons and 8 neutrons. Determine its: 

a. mass number 

b. atomic number 

c. number of electrons 

4. A carbon atom contains 6 protons in the nucleus. If an atom 

of carbon-14 were to undergo alpha decay, determine each of 

the following for the new element: 



c. number of protons 

d. number of neutrons 

5. A uranium atom contains 92 protons in the nucleus. If an 

atom of uranium-238 were to undergo alpha decay, 

determine each of the following for the new element: 



c. number of protons 

d. number of neutrons 

Section 13.2 

6. If electrons in the hydrogen atom become excited and then 

fall back to the 2nd energy level from levels 3, 4, 5, and 6, 

four colors of light are emitted: violet, red, blue-violet and 

blue-green. 

a. Which transition is responsible for the blue-violet light: 

6 to 2, 5 to 2, 4 to 2 or 3 to 2? 

b. If an electron on the 2nd level were struck by a photon, 

then it could be excited to the 6th energy level. What 

color photon would be absorbed by the electron? 

7. An atom has an atomic number of 6. Sketch a diagram that 

correctly represents the electron arrangement in energy 

levels around the nucleus.

Chapter 14 

Elements and the Periodic 

Table 

What are metals like? Think of things that are made with metals 

like aluminum, copper, iron, and gold. What do they have in 

common? They are usually shiny, and they can often be bent into 

different shapes without breaking. Did you know there is a 

metal that is shiny, but is so soft it can be cut with a knife? This 

metal is very reactive. If you place a piece of this metal in water, 

it will race around the surface, and the heat given off is often 

enough to melt the metal and ignite the hydrogen gas that is 

produced! This strange metal is called sodium. You can look at 

the periodic table of elements to find other metals that behave 

like sodium. In this chapter, you will become familiar with how 

you can predict the properties of different elements by their 

location on the periodic table. 

1. How are the elements arranged in the periodic 

table? 

2. What sort of information can the periodic table 

of elements give you? 

3. Why does the periodic table have the shape that 

it does?

14.1The Periodic Table of the Elements 

Long before scientists understood atoms, they grouped elements by their chemical 

properties. In this section, you will learn how the periodic table gives us a way to 

organize all the known elements. The periodic table also shows how chemical properties 

are related to the arrangement of electrons inside the atom. 

Physical and chemical properties 

Physical 

properties 

Properties that you can measure or see through direct observation 

are called physical properties. For example, water is a colorless, 

odorless substance that exists as a liquid at room temperature. Gold 

is shiny, exists as a solid at room temperature, and can be hammered 

into very thin sheets. Physical properties include color, texture, 

density, brittleness, and state (solid, liquid, or gas). Melting point, 

boiling point, and specific heat are also physical properties. 

physical properties - 

characteristics of matter that can 

be seen through direct observation 

such as density, melting point, and 

boiling point. 

chemical properties - 

characteristics of matter that can 

only be observed when one 

substance changes into a different 

substance, such as iron into rust. 

Physical changes 

are reversible 

Physical changes, such as melting, boiling, or bending are usually 

reversible. When water freezes, it undergoes a physical change from a 

liquid to a solid. This does not change the water into a new substance. 

It is still water, only in solid form. The change can easily be reversed 

by melting the water. Bending a steel bar is another physical change. 

Bending changes the shape of the bar, but it is still steel. 

Chemical change 

Chemical 

properties 

Properties that can only be observed when one substance changes 

into a different substance are called chemical properties. For 

example, if you leave an iron nail outside, it will eventually rust 

(Figure 14.1). A chemical property of iron is that it reacts with oxygen 

in the air to form iron oxide (rust). 

Oxygen 

Rust 

Chemical 

changes are hard 

to reverse 

Any change that transforms one substance into a different substance 

is called a chemical change. The transformation of iron into rust is a 

chemical change. Chemical changes are not easily reversible. Rusted 

iron will not turn shiny again even if you remove it from the oxygen 

in the air. 

BEFORE 

AFTER 

Figure 14.1: Rusting is an example 

of a chemical change. 


The periodic table 

elements How many 

there? are 

The modern periodic table 

Organization of the periodic table 

Humans have long wondered how many elements make up our 

universe. The only way to tell if a substance is an element is to try 

and break it down into other substances by any possible means. A 

substance that can be chemically broken apart cannot be an element. 

As of this writing, scientists have identified 113 different elements, 

and five more are expected to be confirmed in the near future. Only 

about 90 of these elements occur naturally. The others are made in 

laboratories. 

As chemists worked on finding the true elements, they noticed that 

some elements acted like other elements. For example, the soft metals 

lithium, sodium, and potassium always combine with oxygen in a ratio 

of two atoms of metal per atom of oxygen (Figure 14.2). By keeping 

track of how each element combined with other elements, scientists 

began to recognize repeating patterns. From this data, they developed 

the first periodic table of the elements. The periodic table organizes 

the elements according to how they combine with other elements 

(chemical properties). 

The periodic table is organized in order 

of increasing atomic number. The 

lightest element (hydrogen) is at the 

upper left. The heaviest (#118) is on the 

lower right. Each element corresponds 

to one box in the periodic table, 

identified with the element symbol. 

The periodic table is further divided into periods and groups. Each 

horizontal row is called a period. Across any period, the properties of 

the elements gradually change. Each vertical column is called a 

group. Groups of elements have similar properties. The main group 

elements are Groups 1–2 and 13–18 (the tall columns of the periodic 

table). Elements in Groups 3 through 12 are called the transition 

elements. The inner transition elements, called lanthanides and 

actinides, are usually put below to fit on a page. 

CHAPTER 14: ELEMENTSANDTHE PERIODIC TABLE 

periodic table - a chart that 

organizes the elements by their 

chemical properties and increasing 

atomic number. 

period - a row of the periodic 

group 

table. 

- a column of the periodic 

table. 

Figure 14.2: The metals lithium, 

sodium, and potassium all form 

compounds with a ratio of 2 atoms of the 

metal to 1 atom of oxygen. All the 

elements in Group 1 of the periodic table 

form similar compounds. 

345 

14.1 THE PERIODIC TABLEOFTHE ELEMENTS

Reading the periodic table 

Metals, 

nonmetals, and 

metalloids 

Most of the elements are metals. A metal is typically shiny, opaque, 

and a good conductor of heat and electricity as a pure element. Metals 

are also ductile, which means they can be bent into different shapes 

without breaking. With the exception of hydrogen, the nonmetals are 

on the right side of the periodic table. Nonmetals are poor conductors 

of heat and electricity. Solid nonmetals are brittle and appear dull. 

The elements on the border between metals and nonmetals are called 

Metalloids. Silicon is an example of a metalloid element with 

properties in between those of metals and nonmetals. 

1 

H 

1 

hydrogen 

Li 

3 

lithium 

Na 

11 

sodium 

K 

19 

potassium 

Rb 

37 

rubidium 

Cs 

55 

cesium 

Fr 

87 

francium 

2 

Be 

4 

beryllium 

Mg 

12 

magnesium 

Ca 

20 

calcium 

Sr 

38 

strontium 

Ba 

56 

barium 

Ra 

88 

radium 

3 4 5 6 7 8 9 10 11 12 

Sc Ti V Cr Mn Fe Co Ni Cu Zn 

21 

Y 

39 

22 

Zr 

40 

Hf 

72 

Main Group Elements 

Transition Elements 

ROWS = PERIODS 

23 

Nb 

41 

Ta 

73 

24 

Mo 

42 

W 

74 

25 

Tc 

43 

Re 

75 

26 

scandium titanium vanadium chromium manganese iron 

Ru 

44 

yttrium zirconium niobium molybdenum technetium ruthenium 

Periodic Table of the Elements 

Os 

76 

hafnium tantalum tungsten rhenium osmium 

Non metals 

Metals 

COLUMNS = GROUPS 

27 

Rh 

45 

Ir 

77 

28 

Pd 

46 

Pt 

78 

Metalloids 

29 

Ag 

47 

Au 

79 

30 

Cd 

48 

Hg 

80 

metal - elements that are typically 

shiny and good conductors of heat 

and electricity. 

nonmetal - elements that are 

poor conductors of heat and 

electricity. 

13 14 15 16 17 

B C N O F 

5 

Al 

13 

Ga 

31 

In 

49 

Tl 

81 

6 

Si 

14 

Ge 

32 

Sn 

50 

Pb 

82 

7 

P 

15 

As 

33 

cobalt nickel copper zinc gallium germanium arsenic 

Sb 

51 

rhodium palladium silver cadmium indium tin antimony 

Bi 

83 

iridium platinum gold mercury thallium lead bismuth 

8 

S 

16 

Se 

34 

Te 

52 

Po 

84 

9 

Cl 

17 

Br 

35 

I 

53 

At 

85 

18 

He 

2 

Ne 

10 

boron carbon nitrogen oxygen fluorine neon 

Ar 

18 

aluminum silicon phosphorus sulfur chlorine argon 

Kr 

36 

selenium bromine krypton 

Xe 

54 

tellurium iodine xenon 

Rn 

86 

polonium astatine radon 

Rf Db Sg Bh Hs Mt Uun Uuu Uub Uuq 

104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 

rutherfordium dubnium seaborgium bohrium hassium meitnerium ununnilium unununium ununbium ununquadium 

helium 

Ac 

346 UNIT 6 ATOMS, ELEMENTS,AND COMPOUNDS 

La 

57 

89 

Ce 

58 

Th 

90 

Pr 

59 

Pa 

91 

Nd 

60 

U 

92 

Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 

61 62 63 64 65 66 67 68 69 70 71 

lanthanum cerium praseodymium neodymium promethium samarium europium gadolinium terbium dysprosium holmium erbium thulium ytterbium lutetium 

actinium thorium protactinium uranium 

Np Pu Am Cm Bk Cf Es Fm Md No Lr 

93 94 95 96 97 98 99 100 101 102 103 

neptunium plutonium americium curium berkelium californium einsteinium fermium mendelvium nobelium lawrencium

Atomic mass 


Atomic mass 

units 

Atomic mass and 

isotopes 

The mass of individual atoms is so small that the numbers are difficult 

to work with. To make calculations easier, scientists came up with the 

atomic mass unit (amu). One atomic mass unit is about the mass of a 

single proton (or a neutron). In laboratory units, 1 amu is 1.66 10 –24 

grams. That’s 0.00000000000000000000000166 gram! 

The atomic mass is the average mass (in amu) of an atom of each 

element. Atomic masses differ from mass numbers because most 

elements in nature contain more than one isotope (see chart below). 

For example, the atomic mass of lithium is 6.94 amu. That does NOT 

mean there are 3 protons and 3.94 neutrons in a lithium atom! On 

average, out of every 100 g of lithium, 94 grams are Li-7 and 6 grams 

are Li-6 (Figure 14.3). The average atomic mass of lithium is 6.94, 

because of the mixture of isotopes. 

atomic mass unit - a unit of 

mass equal to 1.66 10 –24 grams, 

which is one twelfth the mass of 

the isotope carbon-12. 

atomic mass - the average mass 

of all the known isotopes of an 

element, expressed in amu. 

Figure 14.3: Naturally occurring 

elements have a mixture of isotopes. 

14.1 THE PERIODIC TABLEOFTHE ELEMENTS 

347

Groups of the periodic table 

Alkali metals 

Group 2 metals 

Halogens 

Noble gases 

Transition metals 

The different groups of the periodic table have similar 

chemical properties. For example the first group is known 

as the alkali metals. This group includes the elements 

lithium (Li), sodium (Na), and potassium (K). The alkali 

metals are soft and silvery in their pure form and are highly 

reactive. Each of them combines in a ratio of two to one with 

oxygen. For example, lithium oxide has two atoms of lithium 

per atom of oxygen. 

The group two metals include beryllium (Be), magnesium 

(Mg), and calcium (Ca). These metals also form oxides 

however they combine one-to-one with oxygen. For example, 

beryllium oxide has one beryllium atom per oxygen atom. 

The halogens are on the opposite side of the periodic table. 

These elements tend to be toxic gases or liquids in their 

pure form. Some examples are fluorine (F), chlorine (Cl), 

and bromine (Br). The halogens are also very reactive and 

are rarely found in pure form. When combined with alkali 

metals, they form salts such as sodium chloride (NaCl) and 

potassium chloride (KCl). 

On the far right of the periodic table are the noble gases, 

including the elements helium (He), neon (Ne), and argon 

(Ar). These elements do not naturally form chemical bonds 

with other atoms and are almost always found in their pure 

state. They are sometimes called inert gases for this reason. 

In the middle of the periodic table are the transition metals, 

including titanium (Ti), iron (Fe), and copper (Cu). These 

elements are usually good conductors of heat and electricity. 

For example, the wires that carry electricity in your school 

are made of copper. 

alkali metals - elements in the 

first group of the periodic table. 

halogens - elements in the group 

containing fluorine, chlorine, and 

bromine, among others. 

noble gases - elements in the 

group containing helium, neon, 

and argon, among others. 


Energy levels and the periodic table 


Row 1 is the first 

energy level 

Row 2 is the 

second energy 

level 

Row 3 is the third 

energy level 

Outer electrons 

The periods (rows) of the periodic table correspond to the energy levels 

in the atom (Figure 14.4). The first energy level can accept up to two 

electrons. Hydrogen has one electron and helium has two. These two 

elements complete the first period. 

The next element, lithium (Li), has three electrons. Lithium begins 

the second period because the third electron goes into the second 

energy level. The second energy level can hold eight electrons so there 

are eight elements in the second row of the periodic table, ending with 

neon. Neon (Ne) has 10 electrons, which completely fills the second 

energy level. 

Potassium (K) has 11 electrons, and starts the third period because 

the eleventh electron goes into the third energy level. We know of 

elements with up to 118 electrons. These elements have their 

outermost electrons in the seventh energy level. 

As we will see in the next chapter, the outermost electrons in an atom 

are the ones that interact with other atoms. The outer electrons are 

the ones in the highest energy level. Electrons in the completely filled 

inner energy levels do not participate in forming chemical bonds. 

Figure 14.4: The rows (periods) of 

the periodic table correspond to the 

energy levels for the electrons in an 

349 

atom. 

14.1 THE PERIODIC TABLEOFTHE ELEMENTS


1. Which of the following (pick 2) are physical properties of matter 

and NOT chemical properties? 

a. melts at 650 °C 

b. density of 1.0 g/mL 

c. forms molecules with two oxygen atoms 

2. Groups of the periodic table correspond to elements with: 

a. the same color 

b. the same atomic number 

c. similar chemical properties 

d. similar numbers of neutrons 

3. Which element is the atom in Figure 14.5? 

4. Name three elements that have similar chemical properties to 

oxygen. 

5. The atomic mass unit (amu) is 

a. the mass of a single atom of carbon 

b. one millionth of a gram 

c. approximately the mass of a proton 

d. approximately the mass of electron 

6. Which element belongs in the empty space in Figure 14.6? 

7. The outermost electrons of the element vanadium (atomic #23) 

are in which energy level of the atom? How do you know? 

8. The elements fluorine, chlorine, and, bromine are in which group 

of the periodic table? 

a. the alkali metals 

b. the oxygen-like elements 

c. the halogens 

d. the noble gases 

9. Which three metals are in the third period (row) of the periodic 

table? 




14.2Properties of the Elements 


The elements have a wide variety of chemical and physical properties. Some are solid at 

room temperature, like copper. Others are liquid (like bromine) or gas (like oxygen). Some 

solid elements (like zinc) melt at very low temperatures and some melt at very high 

temperatures (like titanium). Chemically, there is an equally wide variety of properties. 

Some elements, like sodium, form salts that dissolve easily in water. Other elements, like 

neon, do not form compounds with any other elements. 

Room temperature appearance 

Most elements 

are solid at room 

temperature 

What this tells us 

about 

intermolecular 

forces 

Most of the pure elements are solid at room temperature. Only 11 of 

the 92 naturally occurring elements are a gas, and 10 of the 11 are 

found on the far right of the periodic table. Only 2 elements (Br and 

Hg) are liquid at room temperature. 

An element is solid when intermolecular forces are strong enough to 

overcome the thermal motion of atoms. At room temperature, this is 

true for most of the elements. The noble gases and elements to the far 

right of the periodic table are the exception. These elements have 

completely filled or nearly filled energy levels (Figure 14.7). When an 

energy level is completely filled, the electrons do not interact strongly 

with electrons in other atoms, reducing intermolecular forces. 

Figure 14.7: The noble gases have 

completely filled energy levels. All of 

the elements that are gas at room 

temperature have filled or nearly filled 

energy levels. 

351 

14.2 PROPERTIESOFTHE ELEMENTS

Periodic properties of the elements 

The pattern in 

melting and 

boiling points 

We said earlier that the periodic table arranges elements with 

common properties in groups (columns). The diagram below shows 

the melting and boiling points for the first 36 elements. The first 

element in each row always has a low melting point (Li, Na, K). The 

melting (and boiling) points rise toward the center of each row and 

then decrease again. 

periodicity - the repeating 

pattern of chemical and physical 

properties of the elements. 

Periodicity The pattern of melting and boiling points is an example of 

periodicity. Periodicity means properties repeat each period (row) 

of the periodic table (Figure 14.8). Periodicity tells us a property is 

strongly related to the filling of electron energy levels. Melting points 

reflect the strength of intermolecular forces. The diagram shows that 

intermolecular forces are strongest when energy levels are about half 

full (or half empty). Elements with half filled energy levels have the 

greatest number of electrons that can participate in bonding. 

Figure 14.8: One of these graphs 

shows periodicity and the other does not. 

Can you tell which one is periodic? 

The top graph shows the energy it takes 

to remove an electron. The bottom graph 

shows the atomic weight. 


Thermal and electrical conductivity 


Metals are good 

electrical 

conductors 

Metals are good 

conductors of 

heat 

Nonmetals are 

typically 

insulators 

Electricity is something we often take for 

granted because we use it every day. 

Fundamentally, electricity is the 

movement of electric charge, usually 

electrons. Some materials allow electrons 

to flow easily through them. If you 

connected a battery and a bulb through 

one of these materials, the bulb would 

light. We call these materials electrical 

conductors. Copper and aluminum are excellent electrical 

conductors. Both belong to the family of metals, which are elements in 

the center and left-hand side of the periodic table (Figure 14.9). 

Copper and aluminum are used for almost all electrical wiring. 

If you hold one end of a copper pipe with 

your hand and heat the other end with a 

torch, your hand will quickly get hot. That 

is because copper is a good conductor of 

heat as well as electricity. Like copper, 

most metals are good thermal conductors. That is one reason pots and 

pans are made of metal. Heat from a stove can pass easily through the 

metal walls of a pot to transfer energy to the food inside. 

Elements to the far right of the periodic 

table are not good conductors of electricity 

or heat, especially since many are gases. 

Because they are so different from metals, 

these elements are called nonmetals. 

Nonmetals make good insulators. An 

insulator is a material that slows down or 

stops the flow of either heat or electricity. 

Air is a good insulator. Air is made of 

oxygen, nitrogen, and argon. 

electrical conductor - a 

material that allows electricity to 

flow through easily. 

thermal conductor - a material 

that allows heat to flow easily. 

Figure 14.9: Dividing the periodic 

table into metals, metalloids, and 

nonmetals. 

14.2 PROPERTIESOFTHE ELEMENTS 

353

Metals and metal alloys 

Steel is an alloy 

of iron and 

carbon 

Aluminum is light 

Titanium is both 

strong and light 

Brass 

When asked for an example of a metal, many people immediately 

think of steel. Steel is made from iron, which is the fourth most 

abundant element in the Earth’s crust. However, steel is not pure 

iron. Steel is an alloy. An alloy is a solid mixture of one or more 

elements. Most metals are used as alloys and not in their pure 

elemental form. Common steel contains mostly iron with a small 

percentage of carbon. Stainless steel and high-strength steel alloys 

also contain small percentages of other elements such as chromium, 

manganese, and vanadium. More that 500 different types of steel are 

in everyday use (Figure 14.10). 

Aluminum is a metal widely used for structural applications. 

Aluminum alloys are not quite as strong as steel, but aluminum has 

one third the density of steel. Aluminum alloys are used when weight 

is a factor, such as for airplane construction. The frames and skins of 

airplanes are built of aluminum alloys (Figure 14.11). 

Titanium combines the strength and 

hardness of steel with the light weight of 

aluminum. Titanium alloys are used for 

military aircraft, racing bicycles, and 

other high performance machines. 

Titanium is expensive because it is 

somewhat rare and difficult to work with. 

Brass is a hard, gold-colored metal 

alloy. Ordinary (yellow) brass is 

an alloy of 72% copper, 24% zinc, 

3% lead, and 1% tin. Hinges, door 

knobs, keys, and decorative objects 

are made of brass because brass is 

easy to work with. Because it 

contains lead, however, you should 

never eat or drink from anything 

made of ordinary (yellow) brass. 

steel - an alloy of iron and carbon. 

Figure 14.10: Nails are made of 

steel that contains 95% iron and 

5% carbon. Kitchen knives are made 

of stainless steel that is an alloy 

containing vanadium and other metals. 

Figure 14.11: This aircraft is made 

mostly from aluminum alloys. 

Aluminum combines high strength and 

light weight. 


Carbon and carbon-like elements 


Carbon is an 

important 

element for life 

Diamond and 

graphite 

Silicon 

Silicon and 

semiconductors 

Carbon represents less than 1/100th of a percent of the Earth’s crust 

by mass, yet it is the element most essential for life on our planet. 

Virtually all the molecules that make up plants and animals are 

constructed around carbon. The chemistry of carbon is so important it 

has its own name, organic chemistry (Figure 14.12), which is the 

subject of Chapter 18. 

Pure carbon is found in nature in two very 

different forms. Graphite is a black solid made 

of carbon that becomes a slippery powder when 

ground up. Graphite is used for lubricating 

locks and keys. Diamond is also pure carbon. 

Diamond is the hardest natural substance 

known and also has the highest thermal 

conductivity of any material. Diamond is so 

strong because every carbon atom in diamond 

is bonded to four neighboring atoms in a 

tetrahedral crystal. 

Directly under carbon on the periodic table is the element silicon. 

Silicon is the second most abundant element in the Earth’s crust, 

second only to oxygen. Like carbon, silicon has four electrons in its 

outermost energy level. This means silicon can also make bonds with 

four other atoms. Sand, rocks, and minerals are predominantly made 

from silicon and oxygen (Figure 14.13). Most gemstones, such as 

rubies and emeralds, are compounds of silicon and oxygen with traces 

of other elements. In fact, when you see a glass window, you are 

looking at (or through) pure silica (SiO 2 ). 

Perhaps silicon’s most famous application today is for making 

semiconductors. Virtually every computer chip and electronic device 

uses crystals of very pure silicon (Figure 14.14). The area around San 

Jose, California is known as Silicon Valley because of the electronics 

companies located there. Germanium, the element just below silicon 

on the periodic table is also used to make semiconductors. 

Figure 14.12: Organic chemistry is 

the chemistry of living organisms and is 

based on the element carbon. 

Figure 14.13: Sand and glass are 

two common materials based on silicon. 

Figure 14.14: 

Microelectronics 

are constructed on 

crystals of pure 

silicon. 


355

Nitrogen, oxygen, and phosphorus 

Nitrogen and 

oxygen make up 

most of the 

atmosphere 

Oxygen in rocks 

and minerals 

Liquid nitrogen 

Oxygen and 

nitrogen in living 

organisms 

Phosphorus 

Nitrogen is a colorless, tasteless, and odorless gas that makes 

up 77 percent of Earth’s atmosphere. Oxygen makes up another 

21 percent of the atmosphere (Figure 14.15). Both oxygen and 

nitrogen gas consist of molecules with two atoms (N 2 , O 2 ). 

Oxygen is only 21 percent of the atmosphere however oxygen is by far 

the most abundant element in Earth’s crust. Almost 46 percent of the 

Earth’s crust is oxygen (Figure 14.16). Because it is so reactive, all of 

this oxygen is bonded to other elements in rocks and minerals in the 

form of oxides. Silicon dioxide (SiO 2 ), calcium oxide (CaO), aluminum 

oxide (Al 2 O 3 ), and magnesium oxide (MgO) are common mineral 

compounds. Hematite, an oxide of iron (Fe 2 O 3 ) is a common ore from 

which iron is extracted. 

With a boiling point of –196 °C, liquid nitrogen is used for rapid 

freezing in medical and industrial applications. A common treatment 

for skin warts is to freeze them with liquid nitrogen. 

Oxygen and nitrogen are crucial to living animals and plants. For 

example, proteins and DNA both contain nitrogen. Nitrogen is part of 

a key ecological cycle. Bacteria in soil convert nitrogen dioxide (NO 2 ) 

in the soil into complex proteins and amino acids. These nutrients are 

taken up by the roots of plants, and later eaten by animals. Waste 

and dead tissue from animals is recycled by the soil bacteria that 

return the nitrogen to begin a new cycle. 

Directly below nitrogen in the periodic table is 

phosphorus. Phosphorus is a key ingredient of 

DNA, the molecule responsible for carrying the 

genetic code in all living creatures. One of 

phosphorus’ unusual applications is in “glow-inthe-dark” 

plastic. When phosphorus atoms 

absorb light, they store energy and give off a 

greenish glow as they slowly re-emit the energy. 

Figure 14.15: The Earth’s 

atmosphere is predominantly nitrogen 

and oxygen. 

Figure 14.16: Oxygen makes up 46% 

of the mass of Earth’s crust. This 

enormous quantity of oxygen is bound 

up in rocks and minerals. 




1. Name two elements that are liquid at room temperature. 

2. Which of the following is NOT true about the noble gases? 

a. They have completely filled energy levels. 

b. They have weak intermolecular forces. 

c. They do not bond with other elements in nature. 

d. They have boiling points above room temperature. 

3. Describe what it means if a chemical or physical property is 

periodic. 

4. Name three elements that are good conductors of electricity. 

5. Name three elements that are good conductors of heat. 

6. A metalloid is an element that: 

a. has properties between those of a metal and a nonmetal 

b. is a good thermal conductor but a poor electrical conductor 

c. is a good electrical conductor but a poor thermal conductor 

d. belongs to the same group as carbon in the periodic table 

7. Steel is a metallic-like material but is not a pure element. What is 

steel? 

8. Almost all of the oxygen on the planet Earth is found in the 

atmosphere. Is this statement true or false? 

9. This element is abundant in Earth’s crust and combines with 

oxygen to form rocks and minerals. Which element is it? 

10. An element that has strong intermolecular forces is most likely to 

have: 

a. a boiling point below room temperature 

b. a melting point below room temperature 

c. a boiling point very close to its melting point 

d. a very high melting point 

11. Which element in Figure 14.17 is likely to be a good conductor of 

electricity? 

12. Which element in Figure 14.17 is likely to be a good insulator? 

One of the elements with atomic 

number less than 54 does not exist 

in nature. It was created in the 

laboratory. Which element is this 

and how was it discovered? 

Figure 14.17: Questions 11 and 12. 


357


Digging for Gold 

Look around the room you’re in. Can you find any objects 

containing gold? 

Most people, when asked this question, look for jewelry. 

Necklaces and rings are easy places to find gold. But if you 

pointed to a computer, a telephone jack, or a wheelchair, you 

would also be correct. The physical properties of gold make it 

useful in surprising ways. You can find gold in astronaut 

gear, airplane windshields, and even in some people’s 

eyelids! 

Soft gold 

If you had a lump of pure gold in your hand, you would be 

able to squish it easily with pliers. Scientists call this 

property malleability. Because it is so malleable, gold is 

useless for jewelry unless it is combined with other 

metals—usually silver and copper. Have you heard of “14 

karat gold?” This means that the “gold” is 14/24ths gold and 

10/24ths something else. Pure gold is 24 karat. 

The malleability of pure gold is useful in other ways. Pure 

gold can be hammered into thin sheets only 1,000 atoms 

thick! One cubic centimeter of gold can be pounded into a 

sheet stretching out over three square meters. Typically, a 

thicker one-square meter sheet of gold foil is formed. This 

thickness is easier to handle and more durable. Pure gold 

foil, also known as gold leaf, covers the domes of important 

public buildings like the New York Metropolitan Opera 

House. Prized for its beauty, gold leaf also lasts a lot longer 

than paint. 

You can also take a lump of gold and draw it out into a thin 

wire. Scientists call this property ductility. An ounce of gold 

can be shaped into a wire five miles long! 

A great conductor 

Gold is a great conductor of both heat and electricity. This 

means that very small currents can travel along thin gold 

wires at a wide range of temperatures. Why should you care 

about this? For one thing, there’s a gold plated membrane in 

the mouthpiece of your cell phone that helps turn the sound 

of your voice into an electrical current. Without it, your cell 

phone might fail you in very hot or cold weather. 

There are two metals that have similar thermal and 

electrical conductivity to gold: silver and copper. They are 

less expensive than gold, so why aren’t they used in your cell 

phone instead? 

Here’s the reason: silver turns black and copper turns green 

when exposed to air for awhile. This process is called 

tarnishing. It happens because these metals undergo a 

chemical reaction with substances in the air. The tarnish 

disrupts the flow of electrons and also acts as an insulator, 

holding in heat. 

Gold is non-reactive in air 

Gold, on the other hand, doesn’t react with substances in air. 

As a result, it’s often used for electrical contacts in items 

that are frequently unplugged and plugged back in, like 

telephone jacks and computer connectors. 

Gold wire is used to connect the microprocessor to the 

controls of an electric wheelchair. Wheelchairs have to 

survive exposure to rain, salt water, high and low 

temperatures, and constant use. Gold holds up under these 

conditions better than any other metal. 

358

Gold reflectors 

There is no metal that reflects infrared energy better than 

gold. Gold is used to coat the face shield on an astronaut’s 

helmet, providing protection from the sun’s strong rays. It’s 

also used to coat the windows of airplane cockpits. The 

coating keeps the crew cooler when the plane is sitting on 

hot tarmac. An electric current runs through the gold coating 

during cold weather to melt frost and prevent fogging. 

Eye doctors have found another use for gold. Accidents or 

diseases sometimes prevent an eyelid from fully closing. If 

the cornea dries out, infection or blindness can result. Eye 

doctors can solve this problem by implanting a thin strip of 

gold into the upper eyelid. Tiny muscles contract to open the 

eyelid. When those muscles relax, the weight of the gold 

causes the eyelid to drop fully closed. 

Gold is used for two reasons: its high density and 

malleability mean that just a small thin strip is needed, and 

gold does not react with body substances like blood or tears, 

so it won’t break down over time. 

However, doctors are now searching for a new material to 

use. The reason? People with metal implanted in their bodies 

can’t have MRI scans. If doctors could find a non-metal 

alternative to gold, people with eyelid implants would still 

have access to this valuable diagnostic tool. 

Questions: 

1. Now that you have read this Connection, look around the 

room again. Can you name two more objects that contain 

gold? 

2. List four important physical properties of gold. Compare/ 

contrast these with another common natural product, rubber. 

3. You have learned about how gold is used in everyday 

objects. It also has some interesting high-tech uses. Find out 

how gold is used in the world’s largest telescope at the Keck 

observatory in Hawaii, or why gold-coated mylar sheets are 

wrapped around satellites. 



359


Name That Element 

Each element on the periodic table 

has a chemical symbol that is an 

abbreviation of the element’s name. 

Unlike the abbreviations for a U.S. 

state, these symbol-abbreviations are 

not always obvious. Many are 

derived from the element’s name in a 

language such as Latin or German. 

The chemical symbol for silver is 

“Ag”. Note that the first letter in the 

symbol is upper case and the second 

is lower case. Writing symbols this 

way allows us to represent all of the 

elements without getting confused. There is a big difference 

between the element cobalt, with its symbol Co, and the 

compound carbon monoxide, written as CO. In this activity, 

you’ll make a set of flashcards for 30 elements and then play 

a game to see who in your class knows their elements. 

Materials: 

30 blank 8 10 cards and markers 


1. Each person in the class writes the symbol of one of the 

elements from the list on one of the large cards. Make 

sure you write the chemical symbol large enough so you 

can see it all the way across the classroom. The elements 

suggested below are some of the most common elements. 

C Cu O N He 

H Cl Mg Na K 

S Ca Mn Fe Br 

B Cs Ag Au Pb 

I Si Al F Ne 

Ba Be Cr Ni Hg 

2. The teacher collects all the cards and stands in front of 

the class. 

3. The first two players stand next to each other. The 

teacher holds up a chemical symbol card and the first 

player to correctly give the name of the element moves 

on to the next player. The player who didn’t answer sits 

down. 

4. The game goes all the way around the classroom, with 

the player who names the element moving on and the 

other player sitting down. 

5. The player who is left standing at the end of the game is 

the winner. 


a. Find the element whose chemical symbol comes from the 

Latin word aurum which means “shining dawn”. 

b. What word does the chemical symbol for lead, Pb, come 

from? 

c. Find the element whose chemical symbol comes from the 

Latin word natrium? 

d. Which element comes from the Latin word for coal? 

e. Another game to play is to see who can come up with the 

longest word spelled completely with chemical symbols. 

Some examples are life, from lithium (Li) and iron (Fe), 

and brook, from bromine (Br), oxygen (O), and potassium 

(K). 

360


Vocabulary 


group 

period 

physical properties 

atomic mass 

Section 14.1 

1. A chart that organizes elements by their chemical properties 

and increasing atomic number is the ____. 

2. Characteristics of matter that can be seen through direct 

observation are called ____. 

3. A row of the periodic table is referred to as a(n) ____. 

4. Characteristics of matter that can be observed only as one 

substance changes to another are called ____. 

5. A column of the periodic table is known as a(n) ____. 

6. The average mass of all known isotopes of an element, 

expressed in amu, is the ____. 

7. Elements that are generally poor conductors, and in solid 

form, generally dull and brittle, are called ____. 

Section 14.2 

chemical properties 

periodicity 

steel 

electrical conductor 

8. A repeating pattern of chemical and physical properties of 

the elements is called ____. 

9. A material that allows heat to flow easily is called ____. 

10. A material that slows or stops the flow of heat or electricity 

is called a(n) ____. 

11. An alloy of iron and carbon is called ____. 

insulator 

nonmetals 

periodic table 

thermal conductor 

12. A material that allows electricity to flow easily is a(n) ____. 

Concepts 

Section 14.1 

1. Label each of the following changes or properties as being a 

physical (P) or chemical (C) property: 

a. One cm 3 of water has a mass of one gram. 

b. Burning hydrogen in the presence of oxygen produces 

water. 

c. Candle wax will melt when heated gently. 

d. An iron nail left outside for a year will rust. 

e. To raise the temperature of 1 kg of lead 1°C requires 

130 joules of energy. 

f. If ice is heated enough, the ice will change to steam. 

2. Melting, boiling, and bending are considered physical 

changes, but burning is a chemical change. Explain why this 

is so. 

3. How may a substance be tested to determine whether or not 

it is an element? 

4. Supply the missing number for each of the following: 

a. The number of naturally occurring elements. 

b. The atomic number of the heaviest element. 

c. The atomic number of the lightest element. 

d. The total number of elements identified (as of the 

publication of this book). 

5. Describe the difference between a period and a group on the 

periodic table. 

6. What property of elements was used to organize the periodic 

table? 

7. Describe the difference between the mass number and the 

atomic mass of an element. 

CHAPTER 14 ELEMENTSANDTHE PERIODIC TABLE 361

8. Identify each of the following as a metal (M), nonmetal (N), 

or metalloid (T): 

a. Includes most of the elements. 

b. As solid they are dull, poor conductors, and brittle. 

c. Generally located on the right side of the periodic table. 

d. Ductile 

e. Share properties in between metals and nonmetals. 

9. Briefly describe each group below and give an example of 

one element in each group: 

a. alkali metals 

b. halogens 

c. noble gases 

10. How does the energy level of an element on the periodic 

table compare to its period number? 

Section 14.2 

11. Most elements occur as solids at room temperature. 

a. Name the 2 elements found as liquids at room 

temperature. 

b. Name 5 elements (out of 11) found as gases at room 

temperature. 

12. Explain why the elements in group 18 are all gases. 

13. Name two properties that display periodicity across the 

periodic table. 

14. Name 3 elements that are good conductors of both heat and 

electricity. 

15. Name 3 elements that are poor conductors that are good 

insulators of both heat and electricity. 

16. Carbon is not an exceptionally abundant element but is the 

most essential element for life on our planet. Why? 

17. Name 2 reasons why silicon is an important element 

economically. 

18. Name the following: 

a. The two most abundant gases and their approximate 

percentage of occurrence in our atmosphere. 

b. The most abundant element in Earth’s crust and its 

percentage of occurrence. 

Problems 

Section 14.1 

1. How many electrons can be held in energy level one? In 

energy level two? 

2. Aluminum has 13 electrons. Which is its outermost energy 

level and how many electrons are found in it? 

3. Name the elements found at the following positions: 

a. Group 1, period 2 

b. Period 4, group 9 

4. Give the symbol, name, and atomic number of the two 

elements in period 4 most similar to cobalt (Co). 

Section 14.2 

5. Using the diagram of the periodic table below, state one 

property for each element indicated on the table. 

A 

B 

C 

D 

362 CHAPTER 14 ELEMENTSANDTHE PERIODIC TABLE

Chapter 15 

Molecules and Compounds 

What do aspirin, plastic wrap, and vinegar have in common? 

Give up? They are all compounds made from different 

combinations of the same three atoms: carbon, hydrogen, and 

oxygen. By themselves, these atoms cannot reduce pain, keep 

food fresh, or season food. But when they are chemically 

combined in certain ways to form compounds, they can be useful 

in many ways. Study this chapter to learn how millions upon 

millions of compounds and molecules are made from 

combinations of less than 100 basic elements. 

. 

1. What does the chemical formula H2O mean? 

2. What are chemical bonds, and how do they 

form? 

3. How do scientists show the shape of a molecule?

15.1Compounds and Chemical Bonds 

Most matter exists as compounds, not pure elements. That’s because most pure elements 

are chemically unstable. They quickly combine with other elements to make compounds. 

For example, water, H 2 O, is a compound of hydrogen and oxygen. The iron in a nail 

combines with the oxygen in water or air to make a compound called iron oxide, better 

known 

Most 

as 

matter 

rust. This 

is 

chapter 

in the 

is 

form 

about how 

of compounds 

and why elements 

and 

combine 

mixtures 

into compounds. 

Compounds and 

mixtures 

Acompound contains two or more different elements that are 

chemically bonded together. For example, water (H 2 O) is a compound 

of hydrogen and oxygen atoms bonded together. A mixture contains 

two or more elements and/or compounds that are not chemically 

bonded together. The atmosphere is a mixture of oxygen and nitrogen 

but it is not a compound. Nitrous oxide (N 2 O) is a compound because 

oxygen and nitrogen atoms are bonded together in a molecule. 

compound - a substance whose 

smallest particles include more 

than one element chemically 

bonded together. 

mixture - a substance that 

includes more than one type of 

element and/or compound that are 

not bonded together. 

Salt and sugar 

are compounds 

Most matter is 

mixtures of 

compounds 

Virtually everything you eat and everything in your kitchen is a 

compound (or mixture of compounds). Salt is a compound of sodium 

and chlorine. Sugar is a compound of carbon, hydrogen, and oxygen. 

“Pure sugar” is not an element in the chemical sense. The “pure 

sugar” you buy in the store is a compound called sucrose that 

combines two other compounds, glucose and fructose (Figure 15.1). 

Meat and vegetables are mixtures of many compounds, including 

proteins and fats. These compounds are made of hundreds or even 

thousands of atoms. You school building is also made of mixtures— 

wood and stone are both mixtures of many compounds. 

Figure 15.1: “Pure” sugar is a 

compound made up of two chemically 

bonded simple sugars: glucose and 

fructose. 


Covalent bonds and molecules 

CHAPTER 15: MOLECULESAND COMPOUNDS 

Electrons form 

chemical bonds 

Achemical bond forms when atoms transfer or share electrons. Two 

atoms that are sharing one or more electrons are chemically bonded 

and move together. In a water molecule, each hydrogen atom shares 

its single electron with the oxygen atom at the center. Almost all the 

elements form chemical bonds easily. This is why most of the matter 

you experience is in the form of compounds. 

A chemical bond forms when atoms transfer 

or share electrons. 

Covalent bonds Acovalent bond is formed when atoms share electrons. The bonds 

between oxygen and hydrogen in a water molecule are covalent bonds 

(Figure 15.2). There are two covalent bonds in a water molecule, 

between the oxygen and each of the hydrogen atoms. Each bond 

represents one electron. In a covalent bond, electrons are shared 

between atoms, not transferred. 

chemical bond - a bond formed 

between atoms through the 

sharing or transferring of 

electrons. 

covalent bond - a type of 

chemical bond formed by shared 

electrons. 

molecule - a group of atoms held 

together by covalent bonds. 

Molecules 

A group of atoms held together by covalent bonds is called a molecule. 

Water is a molecule, and so are the two different sugar molecules 

shown on the previous page. Other examples of molecules are 

methane (CH 4 ), ammonia (NH 3 ), oxygen (O 2 ), and nitrogen (N 2 ). In 

the case of oxygen and nitrogen, the bond between atoms is a double or 

triple covalent bond. A double bond involves two electrons and a triple 

bond involves three electrons per atom. 

Figure 15.2: In a covalent bond, the 

shared electrons act like ties that hold a 

molecule together. 

15.1 COMPOUNDSAND CHEMICAL BONDS 

365

Chemical formulas and diagrams 

The chemical 

formula 

Molecules are represented by a chemical formula. The chemical 

formula tells you how many of each kind of atom are in the molecule. 

For example, the chemical formula for water is H 2 O. The subscript 2 

indicates there are two hydrogen atoms in a water molecule. The 

chemical formula also tells you that water always contains twice as 

many hydrogen atoms as oxygen atoms. This is important to know if 

you wish to make water from the elements oxygen and hydrogen. 

chemical formula - identifies the 

number and element of each type 

of atom in a compound. For 

example, the chemical formula 

Fe 2 O 3 is for a compound with iron 

(Fe) and oxygen (O) in a ratio of 2 

iron atoms for every 3 oxygen 

atoms. 

The shape of a molecule is also important to its function and 

properties. For this reason, molecules are represented by structural 

diagrams that show the shape and arrangement of atoms. Single 

bonds between atoms are shown with solid lines connecting the 

element symbols. Double and triple bonds are shown with double and 

triple lines. Both the chemical formulas and structural diagrams are 

shown in Figure 15.3. 

Of course, real molecules are threedimensional, 

not flat as shown in 

the structural diagram. For 

example, a methane molecule has 

the shape of a 4-sided pyramid 

called a tetrahedron. Each 

hydrogen atom is at a corner of the 

tetrahedron and the carbon atom is 

at the center. 

Figure 15.3: Chemical formulas and 

structural diagrams. 


Structure and function 


Properties come 

from the 

molecule 

Properties 

depend on the 

exact chemical 

formula 

The properties of a compound depend much more on the exact 

structure of its molecule than on the individual elements of which it is 

made. As a good example, aspirin is a molecule made from carbon, 

hydrogen, and oxygen according to the chemical formula C 9 H 8 O 4 

(Figure 15.4). Aspirin relieves swelling and reduces pain in humans. 

By themselves, the elements (H, C, O) do not have the property of 

reducing pain. In fact, other molecules formed from the same elements 

have very different properties than aspirin. For example, polyethylene 

plastic wrap and formaldehyde (a toxic preservative) are also made 

from carbon, oxygen, and hydrogen. The beneficial properties of 

aspirin come from the specific combination of exactly 8 hydrogen, 

9 carbon, and 4 oxygen atoms in a particular structure. If the ratio of 

elements was changed, for example removing even one hydrogen, the 

resulting molecule would not have the properties of aspirin. 

Figure 15.4: An aspirin molecule 

(acetylsalicylic acid). 

Properties also 

depend on 

molecular 

structure 

The structure of a molecule is very important to the properties of a 

compound. The same 21 atoms in aspirin can be combined in other 

structures with the same chemical formula! The resulting molecules 

are something completely different (see diagram above) and do not 

have the beneficial properties of aspirin. Both chemical formula and 

the structure of the molecules determine the properties of a compound. 

Like many modern medicines, the 

active ingredient in aspirin was 

first discovered in nature. In fact, 

aspirin’s pain-relieving properties 

were known and used in its natural 

form long before scientists learned 

of it. Research the discovery of 

aspirin to find out the intriguing 

story of this widely used medicine. 


367

Ionic compounds 

An ion is a 

charged atom 

Ionic bonds 

Ionic compounds 

do not form 

molecules 

The chemical 

formula for ionic 

compounds 

Ions may be 

multiply charged 

Not all compounds are made of molecules. For example, sodium 

chloride (NaCl) is a compound of sodium (Na) and chlorine (Cl) in a 

ratio of one sodium atom per chlorine atom. The difference is that in 

sodium chloride, the electron is essentially transferred from the 

sodium atom to the chlorine atom. When atoms gain or lose an 

electron they become ions. An ion is a charged atom. By losing an 

electron, the sodium atom becomes a sodium ion with a charge of +1. 

By gaining an electron, the chlorine atom becomes a chloride ion with 

a charge of –1 (note that when chlorine becomes an ion, the name 

changes to chloride). 

Sodium and chlorine form an ionic bond because the positive sodium 

ion is attracted to the negative chloride ion. Ionic bonds are bonds in 

which electrons are transferred from one atom to another. 

Ionic bonds are not limited to a single pair of atoms, like covalent 

bonds. In sodium chloride each positive sodium ion is attracted to all 

of the neighboring chloride ions (Figure 15.5). Likewise, each chloride 

ion is attracted to all the neighboring sodium atoms. Because the 

bonds are not just between pairs of atoms, ionic compounds do not 

form molecules! In an ionic compound, each atom bonds with all of its 

neighbors through attraction between positive and negative charge. 

Like molecular compounds, ionic compounds also have fixed ratios of 

elements. For example, there is one sodium ion per chloride ion in 

sodium chloride. This means we can use the same type of chemical 

formula for ionic compounds and molecular compounds. 

Sodium chloride involves the transfer of one electron. However, ionic 

compounds may also be formed by the transfer of two or more 

electrons. A good example is magnesium chloride (MgCl 2 ). The 

magnesium atom gives up two electrons to become a magnesium ion 

with a charge of +2 (Mg 2+ ). Each chlorine atom gains one electron to 

become a chloride ion with a charge of –1 (Cl – ). The ion charge is 

written as a superscript after the element (Mg 2+ , Fe 3+ ,Cl – , etc.). 

ion - an atom that has an electric 

charge different from zero. Ions 

are created when atoms gain or 

lose electrons. 

ionic bond - a bond that 

transfers an electron from one 

atom to another resulting in 

attraction between oppositely 

charged ions. 

Figure 15.5: Sodium chloride is an 

ionic compound in which each positive 

sodium ion is attracted to all of its 

negative chloride neighbors and vice 

versa. 


Why chemical bonds form 


Atoms form 

bonds to reach a 

lower energy 

state 

Chemical 

reactivity 

Imagine pulling tape off a surface. It takes energy to pull the tape off. 

It also takes energy to separate atoms that are bonded together. If it 

takes energy to separate bonded atoms, then the same energy must be 

released when the bond is formed. Energy is released when chemical 

bonds form. Energy is released because atoms bonded together have 

less total energy than the same atoms separately. Like a ball rolling 

downhill, atoms form compounds because the atoms have lower 

energy when they are together in compounds. For example, one carbon 

atom and four hydrogen atoms have more total energy apart than they 

do when combined in a methane molecule (Figure 15.6). 

All elements, except the noble gases, form chemical bonds. However, 

some elements are much more reactive than others. In chemistry, 

reactive means an element readily forms chemical bonds, often 

releasing energy. For example, sodium is a highly reactive metal. 

Chlorine is a highly reactive gas. If pure sodium and pure chlorine are 

placed together, a violent explosion occurs as the sodium and chlorine 

combine and form ionic bonds. The energy of the explosion is the 

energy given off by the formation of the chemical bonds. 

Figure 15.6: The methane (CH 4 ) 

molecule has lower total energy than 

four separate hydrogen atoms and one 

separate carbon atom. 

Some elements 

are more reactive 

than others 

The closer an element is to having the same number of electrons as a 

noble gas, the more reactive the element is. The alkali metals are very 

reactive because they are just one electron away from the noble gases. 

The halogens are also very reactive because they are also one electron 

away from the noble gases. The beryllium group and the oxygen group 

are less reactive because each element in these groups is two electrons 

away from a noble gas. 

The noble gases (He, Ne, Ar, etc.) 

are called inert because they do 

not ordinarily react with anything. 

You can put sodium in an 

atmosphere of pure helium and 

nothing will happen. However, 

scientists have found that a few 

noble gases DO form compounds, 

in very special circumstances. 

Research this topic and see if you 

can find a compound involving a 

noble gas. 


369


1. What is the difference between a compound and a mixture? 

2. Give one example of a compound and one example of a mixture. 

3. How many atoms of chlorine (Cl) are in the carbon tetrachloride 

molecule (CCl 4 )? 

4. Which of the diagrams in Figure 15.7 is the correct structural 

diagram for carbon tetrachloride (CCl 4 )? 

5. Write a chemical formula for a compound that has two atoms of 

oxygen (O) and three atoms of iron (Fe). 

6. What is the chemical formula for the molecule in Figure 15.8? 

7. How many atoms of hydrogen are in a molecule of acetic acid 

(HC 2 H 3 O 2 )? 


8. Which of the molecules above has the chemical formula C 3 H 8 ? 

9. Which of the following statements is FALSE? 

a. The properties of a compound depend more on which elements 

are present and less on the structure of the molecule. 

b. The properties of a compound depend more on the structure of 

the molecule and less on which elements are present. 

10. Chemical bonds form because: 

a. the atoms have more energy bonded together than separated 

b. the atoms have less energy bonded together than separated 



15.2Electrons and Chemical Bonds 


The discovery of energy levels in the atom solved a 2,000-year-old mystery. The mystery 

was why elements combined with other elements only in particular ratios (or not at all). 

For example, why do two hydrogen atoms bond with one oxygen atom to make water? 

Why isn’t there a molecule with three (H 3 O) or even four (H 4 O) hydrogen atoms? Why 

does sodium chloride have a precise ratio of one sodium ion to one chloride ion? Why don’t 

helium, neon, and argon form compounds with any other element? The answer has to do 

with energy levels and electrons. 

Valence electrons 

valence electrons - electrons in 

the highest unfilled energy level of 

an atom. These electrons 

participate in chemical bonds. 

What are valence 

electrons? 

Most elements 

bond to reach 8 

valence electrons 

Light elements 

bond to reach 2 

valance electrons 

Hydrogen is 

special 

Chemical bonds are formed only between the electrons in the highest 

unfilled energy level. These electrons are called valence electrons. 

You can think of valence electrons as the outer “skin” of an atom. 

Electrons in the inner (filled) energy levels do not “see” other atoms 

because they are shielded by the valence electrons. For example, 

chlorine has 7 valence electrons. The first 10 of chlorine’s 17 electrons 

are in the inner (filled) energy levels (Figure 15.9). 

It turns out that 8 is a magic number for chemical bonding. All the 

elements heavier than boron form chemical bonds to try and get to a 

configuration with eight valence electrons (Figure 15.10). Eight is a 

preferred number because 8 electrons are a complete (filled) energy 

level. The noble gases already have a magic number of 8 valence 

electrons. They don’t form chemical bonds because they don’t need to! 

For elements with atomic number 5 (boron) or less, the magic number 

is 2 instead of 8. For these light elements, 2 valence electrons 

completely fills the first energy level. The elements H, He, Li, Be, and 

B form bonds to reach the magic number of 2 valence electrons. 

Because of its single electron, hydrogen can also have 0 valence 

electrons! Zero is a magic number for hydrogen, as well as 2. This 

flexibility makes hydrogen a very “friendly” element; hydrogen can 

bond with almost any other element. 

Figure 15.9: Chlorine has 7 valence 

electrons. The other 10 electrons are in 

filled (inner) energy levels. 

Figure 15.10: Chlorine and sodium 

bond so each can reach a configuration 

371 

with 8 valence electrons. 

15.2 ELECTRONSAND CHEMICAL BONDS

Valence electrons and the periodic table 

Period 2 

elements 

Each successive 

element has one 

more valence 

electron 

The illustration below shows how the electrons in the elements in the 

second period (lithium to neon) fill the energy levels. Two of lithium’s 

three electrons go in the first energy level. Lithium has one valence 

electron because its third electron is the only one in the second 

energy level. 

Going from left to right across a period, each successive element has 

one more valence electron. Beryllium has two valence electrons, 

boron has three, and carbon has four. Each element in the second 

period adds one more electron until all 8 spots in the second energy 

level are full at atomic number 10, which is neon, a noble gas. Neon 

has 8 valence electrons 

Figure 15.11: Water (H 2 O) and 

beryllium oxide (BeO). 

Bonding Oxygen has 6 valence electrons. To get to the magic number of 8, 

oxygen needs to add two electrons. Oxygen forms chemical bonds that 

provide these two extra electrons. For example, a single oxygen atom 

combines with two hydrogen atoms because each hydrogen can 

supply only one electron. Oxygen combines with one beryllium atom 

because beryllium can supply two valence electrons to give oxygen its 

preferred number of 8 (Figure 15.11). 

Double bonds Carbon has four valence electrons. That means two oxygen atoms can 

share 2 electrons bond with a single carbon atom, each oxygen sharing two of carbon’s 

four valence electrons. The bonds in carbon dioxide (CO 

372 

2 ) are double 

bonds because each bond involves 2 electrons (Figure 15.12). 


Figure 15.12: Carbon forms two 

double bonds with oxygen to make 

carbon dioxide.

Lewis dot diagrams 


Dot diagrams of 

the elements 

Dot diagrams of 

molecules 

A clever way to keep track of valence electrons is to draw Lewis dot 

diagrams. A dot diagram shows the element symbol surrounded by 

one to eight dots representing the valence electrons. Each dot 

represents one electron. Lithium has one dot, beryllium has two, boron 

has three, etc. Figure 15.13 shows the dot diagrams for the first 10 

elements. 

Each element forms bonds to reach one of the magic numbers of 

valence electrons: 2 or 8. In dot diagrams of a complete molecule, each 

element symbol has either 2 or 8 dots around it. Both configurations 

correspond to completely filled (or empty) energy levels. 

Example dot 

diagrams 

Dot diagrams 

Carbon has four dots and hydrogen has one. One carbon atom bonds 

with four hydrogen atoms because this allows the carbon atom to have 

eight valence electrons (8 dots)—four of its own and four shared from 

the hydrogen atoms. The picture above shows dot diagrams for carbon 

dioxide (CO 2 ), ammonia (NH 3 ), methane (CH 4 ), and carbon 

tetrachloride (CCl 4 ), a flammable solvent. 

Draw the dot diagram for nitric acid, H2O. 

1. Looking for: Dot diagram 

2. Given: Chemical formula H2O 

3. Relationships: 

4. Solution: add diagram answer 

Figure 15.13: Lewis dot diagrams 

show valence electrons as dots around 

the element symbol. Atoms form bonds 

to get eight valence electrons by sharing 

with other atoms. 

373 

15.2 ELECTRONSAND CHEMICAL BONDS

Oxidation numbers 

Oxidation 

numbers 

Oxidation 

numbers and the 

periodic table 

A sodium atom always ionizes to become Na+ (a charge of +1) when it 

combines with other atoms to make a compound. Therefore, we say 

that sodium has an oxidation number of 1+. An oxidation number 

indicates the charge on the remaining atom (ion) when electrons are 

lost, gained, or shared in chemical bonds. Table 15.1 shows the 

oxidation numbers for some elements. Notice that the convention for 

writing oxidation numbers is the opposite of the convention for 

writing the charge. When writing the oxidation number, the positive 

(or negative) symbol is written after the number, not before it. 

Oxidation numbers correspond closely to an element’s group on the 

periodic table. All of the alkali metals have oxidation numbers of 1+ 

since these elements all prefer to lose one electron in chemical bonds. 

All of the halogens have an oxidation number of 1– because these 

elements prefer to gain an electron in chemical bonds. The diagram 

below shows the trend in oxidation numbers across the periodic table. 

Most transition metals have complicated oxidation numbers because 

they have many more electrons. 

oxidation number - indicates 

the charge of an atom when an 

electron is lost, gained, or shared 

in a chemical bond. An oxidation 

number of +1 means an electron 

is lost, –1 means an electron is 

gained. 

Table 15.1: Some oxidation numbers 

atom gained electrons 

lostor 

oxidation number 

K loses 1 1+ 

Mg loses 2 2+ 

Al loses 3 3+ 

P gains 3 3– 

Se gains 2 2– 

Br gains 1 1– 

Ar loses 0 0 

What is fluorine’s (F) oxidation 

number? If you think it is 1–, you 

are right. Like the other halogens, 

fluorine gains one electron, one 

negative charge, when it bonds 

with other atoms. 


Predicting a chemical formula 

Oxidation 

numbers in a 

compound add 

up to zero 

Example, carbon 

tetrachloride 

Predict a chemical formula 

When elements combine in molecules and ionic compounds, the total 

electric charge is always zero. This is because any electron donated by 

one atom is accepted by another. The rule of zero charge is easiest to 

apply using oxidation numbers. The total of all the oxidation numbers 

for all the atoms in a compound must be zero. This important rule 

allows you to predict many chemical formulas. 

The oxidation numbers for all the atoms in a 

compound must add up to zero. 

To see how this works, consider the compound carbon tetrachloride 

(CCl 4 ). Carbon has an oxidation number of 4+. Chlorine has an 

oxidation number of 1-. It takes four chlorine atoms to cancel with 

carbon’s 4+ oxidation number. 


Multiple oxidation numbers 

Many periodic tables list multiple 

oxidation numbers for most 

elements. This is because more 

complex bonding is possible. This 

course gives you the fundamental 

ideas but there is much more! 

When multiple 

oxidation numbers 

are shown, the 

most common one 

is usually in bold 

type. For example, 

nitrogen has 

possible oxidation 

numbers of 5+, 4+, 

3+, 2+ and 3- even though 3– is 

the most common. 

Iron of 2–. and Predict oxygen the combine chemical to formula a compound. of this compound. Iron (Fe) has an oxidation number of 3+. Oxygen (O) has an oxidation number 

1. Looking for: Chemical formula 

2. Given: Oxidation numbers Fe 3+ and O 2– 

3. Relationships: The oxidation numbers for all the atoms in a compound must add up to zero. 

4. Solution: Three total oxidation oxygen atoms number contribute of 6+. Therefore, the total oxidation the chemical number formula of 6–. is Fe2O3. It takes only two iron atoms to get a 

Your 

a. Predict 

turn... 

the chemical formula of the compound containing beryllium (2+) and fluorine (1–). Answer: BeF2 

15.2 ELECTRONSAND CHEMICAL BONDS 

375

Ionic and covalent bonds 

Why bonds are 

ionic or covalent 

Sodium chloride 

is ionic 

Widely separated 

elements form 

ionic compounds 

Nearby elements 

form covalent 

compounds 

Whether or not a compound is ionic or covalently bonded depends on 

how much each element “needs” an electron to get to a magic number 

(2 or 8). Elements that are very close to the noble gases tend to give or 

take electrons rather than share them. These elements often form 

ionic bonds rather than covalent bonds. 

As an example, sodium has one electron more than the noble gas 

neon. Sodium has a very strong tendency to give up that electron and 

become a positive ion. Chlorine has one electron less than argon. 

Therefore, chlorine has a very strong tendency to accept an electron 

and become a negative ion. Sodium chloride is an ionic compound 

because sodium has a strong tendency to give up an electron and 

chlorine has a strong tendency to accept an electron. 

On the periodic table, strong electron donors are the left side (alkali 

metals). Strong electron acceptors are on the right side (halogens). 

The farther separated two elements are on the periodic table, the 

more likely they are to form an ionic compound. 

Covalent compounds form when elements have roughly equal 

tendency to accept electrons. Elements that are nonmetals and 

therefore close together on the periodic table tend to form covalent 

compounds with each other because they have approximately equal 

tendency to accept electrons. Compounds involving carbon, silicon, 

nitrogen, and oxygen are often covalent. 

Predicting bonds ionic or covalent 

Potassium (Br) to make (K) the combines salt, potassium with bromine 

bromide ionic or covalently (KBr). Is this bonded likely compound? to be an 

1. Looking for: Ionic covalent or bond 

2. Given: and Br 

3. Relationships: K electron is a strong 

Br is a strong donor. 

electron acceptor. 

4. Solution: compound KBr is an ionic 

because Br are from K and 

opposite of the periodic sides 

table. 

Your 

a. Is silica 

turn... 

or covalently (SiO2) bonded likely to be an ionic compound? 

b. 

Answer: 

Is calcium 

covalent 

be an ionic fluoride covalently (CaF2) bonded likely to 

compound? Answer: ionic 




1. Atoms form chemical bonds using: 

a. electrons in the innermost energy level 

b. electrons in the outermost energy level 

c. protons and electrons 

2. Which of the diagrams in Figure 15.14 shows an element with 

three valence electrons? What is the name of this element? 

3. Which of the following elements will form a double bond with 

oxygen making a molecule with one atom of the element and one 

atom of oxygen. 

a. lithium 

b. boron 

c. beryllium 

d. nitrogen 

4. Name two elements that have the Lewis dot diagram shown in 

Figure 15.15. 

5. The oxidation number is: 

a. the number of oxygen atoms an element bonds with 

b. the positive or negative charge acquired by an atom in a 

chemical bond 

c. the number of electrons involved in a chemical bond 

6. Name three elements that have an oxidation number of 3+. 

7. What is the oxidation number for the elements shown in 

Figure 15.16? 

8. When elements form a molecule, what is TRUE about the 

oxidation numbers of the atoms in the molecule? 

a. The sum of the oxidation numbers must equal zero. 

b. All oxidation numbers from the same molecule must be 

positive. 

c. All oxidation numbers from the same molecule must be 

negative 



Figure 15.16: Question 7 

15.2 ELECTRONSAND CHEMICAL BONDS 

377

Spiderman’s Favorite Compound 


378 

What is your favorite compound? Maybe it’s 

C 6 H 5 (CH 2 ) 2 NH 2 , phenylethylamine - that’s one of the lively 

compounds in chocolate. Maybe your favorite compound is 

C 16 H 10 N 2 O 2 , indigo - that’s the blue dye used for jeans. 

What do you think Spiderman’s favorite compound is? You 

guessed it - the complex protein known as SPIDER SILK! 

Amazing spider silk 

Spiderman releases a super 

strong thread of silk and 

swings from one towering 

skyscraper to another, as if 

suspended from a steel 

cable. Spiderman stops a 

runaway train with spider 

silk. These super spidey 

tricks are Hollywood 

gimmicks, but did you know 

that real spider silk is 

stronger by weight than steel? If you had a thread of steel 

and a thread of spider silk that were both the same 

diameter, the spider silk would be about five times stronger! 

Spider silk is also incredibly elastic. The sticky silk a spider 

uses to capture its prey can stretch two to four times its 

original length before breaking. There are not many 

materials, man-made or natural, that have both strength and 

toughness. Spider silk is strong and tough. Spider silk is an 

amazing compound. 

Spiders and silk 

Most spiders are silk thread factories. They produce the 

fibers and spin them into all sorts of neat structures that 

may look delicate and filmy, but looks can be deceiving! You 

probably know that spider webs are used to capture prey. 

The unsuspecting fly becomes trapped in the sticky silk 

threads. Sometimes the prey SLAMS into the web, and the 

silk threads absorb this energy without breaking. One of the 

only materials that can absorb energy like this is man-made 

Kevlar, which is used in bullet proof vests.

How Spiders Use Spider Silk 

Webs 

Shelters 

Egg-sacs 

Draglines 

Parachuting 

Swathing 

sticky silk is elastic, and prey can’t 

rebound 

some spiders make burrows or nests 

from the silk threads 

spider silk can form a protective case 

around eggs 

threads can connect the spider to its 

web, and non-sticky threads are used 

in the spokes of the web so the spider 

itself doesn’t get trapped! 

silk can be released and caught by 

the wind to transport young 

spiders can wrap their prey in silk 

threads to immobilize them 

Chemical structure 

Spider silk is a natural 

protein. Proteins are a 

huge group of 

compounds that are 

found in all living 

things. Spider silk 

protein belongs to a 

sub-group called 

scleroproteins. These 

include collagen 

(found in ligaments) 

and keratin (hair and 

nails). Each sub-unit 

of a protein is called 

an amino acid. One of 

the most common amino acids used by the spider to build its 

protein silk threads is alanine. This is a simple amino acid. 

Where does the spider get the amino acid units to make the 

long silk protein chains? Some of the amino acids come from 

the spider’s high protein diet of insects. But there is another 

way. Many spiders eat their own webs to reuse the amino 

acids to make a new web. This is a natural recycling 

program! 

Humans and spider silk 

People have long benefited from the amazing properties of 

spider silk. Greeks of long ago used spider webs to cover 

bleeding wounds. Some cultures used spider silk to catch 

small fish, and some still use spider silk for small fish nets. 

Silk was used before World War II as the crosshairs in 

spotting scopes on guns and in some telescopes. What do we 

use spider silk for now? Well, there are not many commercial 

uses of spider silk, despite it’s incredible strength and 

toughness. You can only imagine how difficult it would be to 

maintain “spider farms” to harvest spider silk. Spiders tend 

to eat one another when in close quarters. Spider silk is 

thinner than a human hair. It is difficult to work with. 

However - scientists are extremely interested in studying 

spider silk to learn how the proteins are structured, and how 

the spider spins the thread. The amazing strength and 

toughness of spider silk can be better understood (and 

perhaps copied) if more is known about the chemical makeup 

of silk proteins. 

Man-made spider silk 

Researchers are trying to figure out how to make spider silk 

in the laboratory. This has not been an easy thing to figure 

out. An artificial method of spinning spider silk is not even 

close to being perfected. In fact, the research on how to make 

artificial spider silk is in its early stages. Perhaps one day 

you will help unravel the mystery of how to spin spider silk. 

Then maybe Spiderman could be more than a comic book 

superhero! 

Questions: 

1. What is an amino acid, and what does it have to do with 

spider silk? 

2. How do spiders recycle the compounds they use to make 

spider silk? 

3. Why would it be difficult to harvest spider silk directly from 

spiders? 

UNIT 6 MOTIONAND FORCE 


379

Plastics and Evaporation 


380 

Polymers are materials made of long chains of repeating 

units. Starch (found in potatoes and pasta) is a natural 

polymer made up of a long chain of repeating simple sugar 

units. Plastic is a man-made polymer. Plastic wrap has 

many household uses. In this activity, you will investigate 

different polymers used in plastic wrap. 

Plastic wrap 

There are three different types of plastic wrap sold in stores. 

Polyethylene wrap is sold as Handiwrap®. Polyvinyl chloride 

is sold as Reynolds Wrap®. Polyvinylidene chloride is sold as 

Saran Wrap. The physical properties of these plastic 

wraps make them easy to use as a cover for containers or as 

a protective wrap for food and other items. These polymers 

can keep moisture and air either in or out, depending on 

your wrapping purpose. Are these different polymers equally 

good at preventing evaporation? Let’s test them to see. 

Materials 

• 9 identical drinking glasses or jars (baby food or jelly jars 

work well) 

• 3 squares each of each brand of plastic wrap. Cut the squares 

so you don’t stretch the wrap. Make sure each square is the 

same size. 

Water 

• 9 rubber bands 


1. Fill each jar or glass to the halfway point with water. 

Make sure each container is identical, and put the same 

amount of water in each. 

2. Mark the level of the water in each container. A Sharpie 

marker works well. If you need to erase the mark when 

you are done, you can use a little rubbing alcohol. 

3. Place a piece of plastic wrap over each container. Do not 

stretch the wrap. Fasten tightly with a rubber band. Be 

sure you use a system to know what type of wrap is on 

each container. 

4. Keep the containers together, and be sure they are 

exposed to the same conditions. 

5. Check and record the water level in each container on a 

regular basis. It may take a couple of weeks to notice 

differences. 

6. Keep checking the containers until you can conclude for 

sure that there IS or IS NOT a difference between the 

wraps. 

Permeability 

You have tested the permeability of three different plastic 

polymers. Permeability is the ability of a substance, like 

plastic, to allow another substance to pass through it. In this 

case, you tested the ability of each polymer to allow water 

vapor to pass through it. You could test the permeability of 

plastic wrap to air by wrapping apple slices. What kind of 

results would you see and record? 


a. Was there a difference in each wrap’s ability to prevent 

evaporation? Use your results to defend your answer. 

b. Do some research on each polymer. Should there be a 

difference in water vapor permeability between the three 

wraps? Explain. 

c. Will you use the results of this experiment to buy a 

particular brand of plastic wrap? If so, which one? 

Explain why you would or wouldn’t choose a particular 

wrap.


Vocabulary 


compound 

mixture 

chemical formula 

valence electrons 

Section 15.1 

1. When two atoms share or trade electrons a(n) ____ is 

formed. 

2. An atom that has acquired a positive or negative charge is 

called a(n) ____. 

3. To represent the number and type of each element in a 

molecule, chemists write a ____. 

4. A substance whose smallest particles include more than one 

element chemically bonded together is a ____. 

5. The type of chemical bond formed when atoms share 

electrons is the ____ bond. 

6. A substance made of two or more elements or compounds 

not chemically bonded to each other is a(n) ____. 

7. A bond formed when an electron is transferred from one 

atom to another is a(n) ____. 

8. A group of atoms held together by covalent bonds in a 

specific ratio form a(n) ____. 

Section 15.2 

covalent 

chemical bond 

ionic bond 

9. Electrons in the highest unfilled energy level of an atom, 

that may participate in chemical bonds are called ____. 

10. The number which indicates the charge on an atom when an 

electron is lost, gained or, shared is called the ____. 

ion 

molecule 

oxidation number 

Concepts 

Section 15.1 

1. What is the chemical formula for water? What atoms make 

up this compound? 

2. What is the difference between a compound and a mixture? 

3. List 3 examples of a mixture and 3 examples of a compound. 

4. Why do atoms form compounds instead of existing as single 

atoms? 

5. What type of bond holds a water molecule together? 

6. What do we call the particle that is a group of atoms held 

together by covalent bonds? 

7. List 4 examples of a molecule. 

8. What does the subscript “2” in H 2 0 mean? 

9. What do the subscripts in the formula for ethane represent? 

10. Name the two most important factors in determining the 

properties of a compound. 

11. Summarize the differences between a covalent compound 

and an ionic compound. 

12. What happens when chemical bonds form? Why? 

13. Which group of elements usually don’t form chemical bonds? 

14. Name a very reactive group of metals and a very reactive 

group of nonmetals. Why do they behave this way? 

CHAPTER 15 MOLECULESAND COMPOUNDS 381

Section 15.2 

15. When atoms form chemical bonds, which of their electrons 

are involved in the bonds? 

16. How many electrons represent a complete (filled) outermost 

energy level for elements heavier than Boron (atomic 

number greater than 5)? 

17. Noble gases usually don’t form chemical bonds. Why? 

18. What is so special about hydrogen when it comes to forming 

bonds? 

19. Each successive element on a period table going from left to 

right across a period has what? 

20. In a Lewis dot diagram, what is represented by the dots 

surrounding the element symbol? 

21. How many valence electrons does oxygen have? How many 

more electrons are needed to fill the outermost energy level? 

22. How does the oxidation number indicate if an electron will 

be lost or gained by the bonding atom? 

23. Using the periodic table, what is the oxidation number of: 

a. calcium 

b. aluminum 

c. fluoride 

24. What is the total electric charge on molecules and 

compounds? 

25. Elements close to the noble gases tend to form what type of 

bond? 

26. Elements that are widely separated on the periodic table 

tend to form____ compounds. 

27. Elements that are close together on the periodic table tend 

to form ____ compounds. 

28. Strong electron donors are on the ____ side of the periodic 

table, while strong electron acceptors are on the ____ side. 

Problems 

1. Label each of the diagrams below as a mixture, compound, 

or separate elements. 

2. For each of the formulas for molecules listed below, name 

each element and how many of atoms of each element are in 

that molecule. 

a. C 6 H 12 O 6 

b. CaCO 3 

c. Al 2 O 3 

3. Predict the formula for a molecule containing carbon (C) 

with an oxidation number of 4+ and oxygen (O) with an 

oxidation number of 2-. 

4. Which of the following would be a correct chemical formula 

for a molecule of N 3- and H + ? 

a. HNO 3 

b. H 3 N 6 

c. NH 3 

382 CHAPTER 15 MOLECULESAND COMPOUNDS


5. Referring to the diagram of the periodic table in chapter 7, 

determine which element in each pair is more active: 

a. Li or Be 

b. Ca or Sc 

c. P or S 

d. O or Ne 

Section 15.3 

6. In order for nitrogen to form a compound 

with other elements, how many 

additional electrons are required to give 

nitrogen the required number of electrons 

in its outermost energy level? 

7. Draw Lewis dot diagrams for the 

following: 

a. An atom of hydrogen (one valence electron) 

b. An atom of oxygen (6 valence electrons). 

c. A molecule of water, H 2 O. 

d. A molecule of carbon dioxide, CO 2 . 

8. Using the periodic table: 

a. Determine the oxidation number of Ca and Cl. 

b. Write the chemical formula for calcium chloride. 

9. Give the most common oxidation number and how many 

electrons are gained or lost for the following elements: 

a. oxygen (O) 

b. boron (B) 

c. lithium (Li) 

d. potassium (Na) 

e. magnesium (Mg) 

f. aluminum (Al) 

g. carbon (C) 

h. iodine (I) 

10. Use the periodic table to determine the type of bond most 

likely formed between the elements: 

a. carbon and oxygen 

b. lithium and fluorine 

c. carbon and carbon 

d. carbon and nitrogen 

11. Carbon and oxygen combine to form a gas called carbon 

dioxide. Carbon (C) has an oxidation number of 4+ and 

oxygen (O) has an oxidation number of 2-. 

a. What is the total of the oxidation numbers for all the 

atoms in carbon dioxide? 

b. Predict the formula for carbon dioxide. 

c. Is carbon dioxide an ionic or covalently bonded 

compound? 

12. Carbon and hydrogen combine to form a gas called methane. 

Carbon (C) has an oxidation number of 4+ and hydrogen has 

an oxidation number of 1-. 

a. What is the total of all the oxidation numbers for all the 

atoms in methane? 

b. Predict the formula for methane. 

c. Is methane an ionic or covalently bonded compound? 

13. The chemical formula for a molecule of glucose is C 6 H 12 O 6 . 

a. How many atoms of carbon are in a molecule of glucose? 

b. How many atoms of hydrogen are in a molecule of 

glucose? 

c. How many atoms of oxygen are in a molecule of glucose? 

CHAPTER 15 MOLECULESAND COMPOUNDS 

383

Atoms, Elements, and Compounds - Spokane Public Schools

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