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Chapter 2 Chemistry, Matter, and Life 23
Structure of Atoms
An element is composed of atoms that are all of the same kind.
The element gold is made up entirely of gold atoms, and the
element silver is made up entirely of silver atoms. An atom is
the smallest particle of an element that still retains the properties
of that element, and it is almost unbelievably small. It takes
over 100 million average-sized atoms lined up side by side to
make 1 inch, or 2.54 centimeters.
Even though it is extremely small, an atom is made up of
still smaller subunits or subatomic particles called protons,
neutrons, and electrons. A dense region, called the nucleus,
contains the protons and neutrons. Electrons are outside the
nucleus. The number and nature of the subatomic particles
in the atoms of an element determine the physical and
chemical characteristics of the element. Protons, located in the
nucleus, have a positive electrical charge, and each has a mass
of 1 atomic mass unit (amu). The number of protons in the
nucleus is called the atomic number. All the atoms in an element
have the same number of protons in the nucleus so they
have the same atomic number. Elements are arranged by their
atomic number in the periodic table. Figure 2-1 is a simplified
and abbreviated periodic table that shows the first 54 elements.
The atomic number is above the chemical symbol in the periodic
table.
Neutrons, also found in the nucleus, have the same mass
as protons but have no charge. Protons and neutrons together
account for the mass of the atom, and their number, collectively,
is called the mass number of an atom. The element
sodium is made up of sodium atoms, which have 11 protons
and 12 neutrons in the nucleus. The atomic number of sodium
is 11 (the number of protons), and the atomic mass number is
23 (the number of protons plus neutrons).
Electrons are minute, negatively charged particles with
almost no mass. Their number and arrangement determine how
an atom reacts. Electrons are located in the space surrounding
the nucleus. The number of negatively charged electrons in
an atom is always equal to the number of positively charged
protons so that the atom is electrically neutral. The sodium
atom, described in the previous paragraph, has 11 protons and
12 neutrons in the nucleus. Because the number of electrons
equals the number of protons, there will be 11 electrons in
the space surrounding the nucleus. It is impossible to know
where a given electron will be at any given time, but it is possible
to predict the region in which it will be located. Electrons
are located in energy levels, or shells, around the nucleus. In
general, electrons with higher energy levels are located in shells
farther away from the nucleus than electrons with lower energy
levels. The shell closet to the nucleus has the lowest energy level
and can hold two electrons. The next higher energy level can
hold eight electrons. Higher energy levels can hold more than
eight electrons, but an atom is most stable when there are eight
electrons in the outermost shell, which has the highest energy
level. Simplified diagrams of the atomic structure of some biologically
important elements are shown in Figure 2-2.
Isotopes
The number of neutrons in the nucleus may vary for different
atoms of a given element, which changes the atomic weight.
For example, most hydrogen atoms have one proton and one
electron, which gives an atomic weight of 1 amu. A small number
of hydrogen atoms have a neutron in the nucleus with the
proton, and this gives them an atomic weight of 2 amu. This
is called deuterium. It still has the characteristics of hydrogen
because it has one proton. Figure 2-3 illustrates the structure of
an atom of deuterium. Atoms of a given element that have different
numbers of neutrons, and consequently different atomic
weights, are called isotopes. Isotopes are included in the calculations
of an element’s atomic weight. For example, the periodic
table in Figure 2-1 gives the atomic weight of hydrogen as
1.01. This value includes the amount of the isotope deuterium
that occurs with normal hydrogen.
H
1
He
2
1.01
3
4
5
6
7
8
9
4.00
10
Li
6.94
11
Na
Be
9.01
12
Mg
H
1.01
1
Atomic number
Symbol
Atomic weight
B
10.81
13
Al
C
12.01
14
Si
N
14.01
15
P
O
16.00
16
S
F
19.00
17
Cl
Ne
20.18
18
Ar
22.99
19
24.31
20
21
22
23
24
25
26
27
28
29
30
26.98
31
28.09
32
30.97
33
32.07
34
35.45
35
39.95
36
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
39.10
37
40.08
38
44.96
39
47.88
40
50.94
41
52.00
42
54.94
43
55.85
44
58.93
45
58.69
46
63.55
47
65.39
48
69.72
49
72.61
50
74.92
51
78.96
52
79.90
53
83.80
54
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
85.47
87.62
88.91
91.22
92.91
95.94
97.91
101.07 102.91 106.42 107.87 112.41 114.82 118.71 121.75 127.60 126.90 131.29
Figure 2-1 An abbreviated and simplified periodic table of the elements from hydrogen to xenon,
atomic numbers 1 through 54.