Atoms are the smallest
form of elements.
BEFORE, you learned
•All matter is made of atoms
•Elements are the simplest
NOW, you will learn
• Where atoms are found and
how they are named
• About the structure of atoms
• How ions are formed
proton p. 11
neutron p. 11
nucleus p. 11
electron p. 11
atomic number p. 12
atomic mass number p. 12
isotope p. 12
ion p. 14
EXPLORE The Size of Atoms
How small can you cut paper?
Cut the strip of paper in half. Cut one of
these halves in half.
Continue cutting one piece of paper in half
as many times as you can.
• strip of paper about
30 centimeters long
WHAT DO YOU THINK?
•How many cuts were you able to make?
•Do you think you could keep cutting the paper
forever? Why or why not?
The word element is
related to elementary,
which means “basic.”
All matter is made of atoms.
Think of all the substances you see and touch every day. Are all of
these substances the same? Obviously, the substances that make up
this book you’re reading are quite different from the substances in
the air around you. So how many different substances can there be?
This is a question people have been asking for thousands of years.
About 2400 years ago, Greek philosophers proposed that everything
on Earth was made of only four basic substances—air, water, fire, and
earth. Everything else contained a mixture of these four substances.
As time went on, chemists came to realize that there had to be more
than four basic substances. Today chemists know that about 100 basic
substances, or elements, account for everything we see and touch.
Sometimes these elements appear by themselves. Most often, however,
these elements appear in combination with other elements to make new
substances. In this section, you’ll learn about the atoms of the elements
that make up the world and how these atoms differ from one another.
Chapter 1: Atomic Structure and the Periodic Table 9
Atom Concentrations by Mass
Types of Atoms in Earth’s Crust and
Atoms of the element hydrogen account for about
90 percent of the total mass of the universe.
Hydrogen atoms make up only about 1 percent of
Earth’s crust, however, and most of those hydrogen
atoms are combined with oxygen atoms in the form
of water.The graph on the left shows the types of
atoms in approximately the top 100 kilometers of
The distribution of the atoms of the elements
in living things is very different from what it is in
Earth’s crust. Living things contain at least 25 types
of atoms. Although the amounts of these atoms
vary somewhat, all living things—animals, plants,
and bacteria—are composed primarily of atoms of
oxygen, carbon, hydrogen, and nitrogen. As you can
see in the lower graph on the left, oxygen atoms
account for more than half your body’s mass.
SOURCE: CRC Handbook of Chemistry and Physics
Check Your Reading
What is the most common element
in the universe?
Names and Symbols of Elements
Elements get their names in many different ways. Magnesium, for
example, was named for the region in Greece known as Magnesia.
Lithium comes from the Greek word lithos, which means “stone.”
Neptunium was named after the planet Neptune. The elements
einsteinium and fermium were named after scientists Albert Einstein
and Enrico Fermi.
Each element has its own unique symbol. For some elements,
the symbol is simply the first letter of its name.
hydrogen (H) sulfur (S) carbon (C)
The symbols for other elements use the first letter plus one other
letter of the element’s name. Notice that the first letter is capitalized
but the second letter is not.
aluminum (Al) platinum (Pt) cadmium (Cd) zinc (Zn)
The origins of some symbols, however, are less obvious. The symbol
for gold (Au), for example, doesn’t seem to have anything to do with
the element’s name. The symbol refers instead to gold’s name in Latin,
aurum. Lead (Pb), iron (Fe), and copper (Cu) are a few other elements
whose symbols come from Latin names.
BD 10 Unit: Chemical Interactions
Each element is made of a different atom.
In the early 1800s British scientist John Dalton proposed that each
element is made of tiny particles called atoms. Dalton stated that all
of the atoms of a particular element are identical but are different
from atoms of all other elements. Every atom of silver, for example, is
similar to every other atom of silver but different from an atom of iron.
Dalton’s theory also assumed that atoms could not be divided into
anything simpler. Scientists later discovered that this was not exactly
true. They found that atoms are made of even smaller particles.
Learn more about
The Structure of an Atom
A key discovery leading to the current model of the atom was that
atoms contain charged particles. The charge on a particle can be either
positive or negative. Particles with the same type of charge repel each
other—they are pushed apart. Particles with different charges attract
each other—they are drawn toward each other.
Atoms are composed of three types of particles—electrons, protons,
and neutrons. A proton is a positively charged particle, and
a neutron is an uncharged particle. The neutron has approximately
the same mass as a proton. The protons and neutrons of an atom are
grouped together in the atom’s center. This combination of protons
and neutrons is called the nucleus of the atom. Because it contains
protons, the nucleus has a positive charge. Electrons are negatively
charged particles that move around outside the nucleus.
Remember to make a
frame for neutron, proton,
and electron and for
other vocabulary terms.
The Atomic Model
Atoms are made of protons, neutrons, and electrons.
The electron cloud has
a negative charge.
The nucleus has an
overall positive charge.
Particle Charges and Mass
Particle Mass Charge
Electron 1 –1
Proton 2000 +1
Neutron 2000 0
Which part of the atom has a negative charge?
Chapter 1: Atomic Structure and the Periodic Table 11
Build a model of
Atoms are extremely small, about 10 –10
meters in diameter. This means that you could
fit millions of atoms in the period at the end
of this sentence. The diagram on page 11,
picturing the basic structure of the atom, is
not drawn to scale. In an atom the electron
cloud is about 10,000 times the diameter of
Millions of atoms
could fit in a space
the size of this dot.
It would take you
500 years to count
the number of atoms
in a grain of salt.
Electrons are much smaller than protons
or neutrons—about 2000 times smaller. Electrons also move about
the nucleus very quickly. Scientists have found that it is not possible
to determine their exact positions with any certainty. This is why we
picture the electrons as being in a cloud around the nucleus.
The negative electrons remain associated with the nucleus because
they are attracted to the positively charged protons. Also, because
electrical charges that are alike (such as two negative charges) repel
each other, electrons remain spread out in the electron cloud. Neutral
atoms have no overall electrical charge because they have an equal
number of protons and electrons.
Gold has 79 protons
and 79 electrons.
If all atoms are composed of the same particles, how can there be more
than 100 different elements? The identity of an atom is determined
by the number of protons in its nucleus, called the atomic number.
Every hydrogen atom—atomic number 1—has exactly one proton in
its nucleus. Every gold atom has 79 protons, which means the atomic
number of gold is 79.
The iso- in isotope is from
the Greek language, and it
Atomic Mass Numbers
The total number of protons and neutrons in an atom’s nucleus is
called its atomic mass number. While the atoms of a certain element
always have the same number of protons, they may not always have
the same number of neutrons, so not all atoms of an element have the
same atomic mass number.
All chlorine atoms, for instance, have 17 protons. However, some
chlorine atoms have 18 neutrons, while other chlorine atoms have
20 neutrons. Atoms of chlorine with 18 and 20 neutrons are called
chlorine isotopes. Isotopes are atoms of the same element that have
a different number of neutrons. Some elements have many isotopes,
while other elements have just a few.
check your reading
How is atomic mass number different from atomic number?
12 Unit: Chemical Interactions
Isotopes have different numbers of neutrons.
atomic mass number = 35
atomic mass number = 37
A particular isotope is designated by the name of the element and
the total number of its protons and neutrons. You can find the number
of neutrons in a particular isotope by subtracting the atomic number
from the atomic mass number. For example, chlorine-35 indicates the
isotope of chlorine that has 18 neutrons. Chlorine-37 has 20 neutrons.
Every atom of a given element always has the same atomic number
because it has the same number of protons. However, the atomic mass
number varies depending on the number of neutrons.
Masses of Atomic Particles
How can you model the relative masses
of atomic particles?
Use a paper clip to represent the mass of an electron. Determine its mass.
Find a substance in the classroom (sand, clay, water) from which you could
make a model representing the mass of a proton or neutron. The mass of a
proton or neutron is about 2000 times the mass of an electron.
Measure out the substance until you have enough of it to make your model.
•large paper clip
• other items
WHAT DO YOU THINK?
•What substance did you use to make your model?
•What was the model’s mass?
•What do you conclude about the masses of
CHALLENGE The diameter of an electron is approximately
1/2000 that of a proton. What two objects could represent
each of these to scale?
Chapter 1: Atomic Structure and the Periodic Table 13
MAIN IDEA WEB
Make a main idea web to
organize what you know
Atoms form ions.
An atom has an equal number of electrons and protons. Since each
electron has one negative charge and each proton has one positive
charge, atoms have no overall electrical charge. An ion is formed when
an atom loses or gains one or more electrons. Because the number of
electrons in an ion is different from the number of protons, an ion
does have an overall electric charge.
Formation of Positive Ions
Consider how a positive ion can be formed from an atom. The left
side of the illustration below represents a sodium (Na) atom. Its nucleus
contains 11 protons and some neutrons. Because the electron cloud
surrounding the nucleus consists of 11 electrons, there is no overall
charge on the atom. If the atom loses one electron, however, the
charges are no longer balanced. There is now one more proton than
there are electrons. The ion formed, therefore, has a positive charge.
A positive ion is
smaller than the
atom that formed
it because it has
Sodium Atom (Na)
Sodium Ion (Na + )
Notice the size of the positive ion. Because there are fewer electrons,
there is less of a repulsion among the remaining electrons. Therefore,
the positive ion is smaller than the neutral atom.
Positive ions are represented by the symbol for the element with a
raised plus sign to indicate the positive charge. In the above example,
the sodium ion is represented as Na + .
Some atoms form positive ions by losing more than one electron.
In those cases, the symbol for the ion also indicates the number of
positive charges on the ion. For example, calcium loses two electrons
to form an ion Ca 2+ ,and aluminum loses three electrons to form Al 3+ .
Check Your Reading
What must happen to form a positive ion?
BD 14 Unit: Chemical Interactions
Formation of Negative Ions
The illustration below shows how a negative ion is formed. In this
case the atom is chlorine (Cl). The nucleus of a chlorine atom contains
17 protons and some neutrons. The electron cloud has 17 electrons, so
the atom has no overall charge. When an electron is added to the
chlorine atom, a negatively charged ion is formed. Notice that a
negative ion is larger than the neutral atom that formed it. The extra
electron increases the repulsion within the cloud, causing it to expand.
A negative ion is
larger than the
atom that formed
it because it has
Chlorine Atom (Cl) Chloride Ion (Cl – )
Negative ions are represented by placing a minus sign to the right
and slightly above the element’s symbol. The negative chloride ion in
the example, therefore, would be written as Cl – .Ifan ion has gained
more than one electron, the number of added electrons is indicated by
a number in front of the minus sign. Oxygen (O), for example, gains
two electrons when it forms an ion. Its symbol is O 2– .
1. Which two atoms are most
common in Earth’s crust? in
the human body?
2. What are the particles that
make up an atom?
3. What happens when an atom
forms an ion?
4. Infer Magnesium and sodium
atoms are about the same size.
How does the size of a magnesium
ion with a 2+ charge
compare with that of a sodium
ion with a single + charge?
5. Compare The atomic number
of potassium is 19. How does
potassium-39 differ from
6. Analyze When determining
the mass of an atom, the
electrons are not considered.
Why can scientists disregard
Chapter 1: Atomic Structure and the Periodic Table 15