55
CHAPTER TWO
Chemical Basis of Life
of a hydrogen atom is in the F
rst shell; the two electrons of a
helium atom F ll its F rst shell; and of the three electrons of a
lithium atom, two are in the F rst shell and one is in the sec-
ond shell. Lower energy shells, closer to the nucleus, must
be F lled F rst.
The number of electrons in the outermost shell of an
atom determines whether it will react with another atom.
Atoms react in a way that leaves the outermost shell com-
pletely F lled with electrons, achieving a more stable struc-
ture. This is sometimes called the
octet rule,
because, except
for the F rst shell, it takes eight electrons to F ll the shells in
most of the atoms important in living organisms.
Atoms such as helium, whose outermost electron shells
are F
lled, already have stable structures and are chemically
inactive or
inert
(they cannot form chemical bonds). Atoms
with incompletely F
lled outer shells, such as those of hydro-
gen or lithium, tend to gain, lose, or share electrons in ways
that empty or F
ll their outer shells. In this way, they achieve
stable structures.
Atoms that gain or lose electrons become electrically
charged and are called
ions
(i
onz). An atom of sodium, for
example, has eleven electrons: two in the F
rst shell, eight
in the second shell, and one in the third shell. This atom
tends to lose the electron from its outer (third) shell, which
leaves the second (now the outermost) shell F
lled and the
new form stable
(f
g. 2.4
a
)
. In the process, sodium is left
with eleven protons (11
+
) in its nucleus and only ten elec-
trons (10
). As a result, the atom develops a net electrical
charge of 1
+
and is called a sodium ion, symbolized Na
+
.
A chlorine atom has seventeen electrons, with two in the
F
rst shell, eight in the second shell, and seven in the third
shell. An atom of this type tends to accept a single electron,
F
lling its outer (third) shell and becoming stable. In the pro-
cess, the chlorine atom is left with seventeen protons (17
+
)
in its nucleus and eighteen electrons (18
). As a result, the
atom develops a net electrical charge of 1
and is called a
chloride ion, symbolized Cl
.
Positively charged ions are called
cations
(kat
i-onz),
and negatively charged ions are called
anions
(an
i-onz).
Ions with opposite charges attract, forming
ionic bonds
(i-on
ik bondz). Sodium ions (Na
+
) and chloride ions (Cl
)
uniting in this manner form the compound sodium chloride
(NaCl), or table salt (F
g. 2.4
b
). Similarly, hydrogen atoms
(H
2
O), as
f gure 2.2
shows. Table sugar, baking soda, natural
gas, beverage alcohol, and most drugs are compounds.
A molecule of a compound always consists of deF
nite
types and numbers of atoms. A molecule of water (H
2
O),
for instance, always has two hydrogen atoms and one oxy-
gen atom. If two hydrogen atoms combine with two oxygen
atoms, the compound formed is not water, but hydrogen
peroxide (H
2
O
2
).
Bonding of Atoms
Atoms combine with other atoms by forming links called
bonds.
Chemical bonds result from interactions of electrons.
The electrons of an atom occupy one or more regions
of space called
electron shells
that encircle the nucleus.
Because electrons have a level of energy characteristic of the
particular shell they are in, the shells are sometimes called
energy shells.
Each electron shell can hold a limited number
of electrons. The maximum number of electrons that each of
the F rst three shells can hold for elements of atomic number
18 and under is
First shell (closest to the nucleus)
2 electrons
Second shell
8 electrons
Third shell
8 electrons
More complex atoms may have as many as eighteen elec-
trons in the third shell.
SimpliF
ed diagrams such as those in
f gure 2.3
are used
to show electron conF
guration in atoms. The single electron
H
H
OO
H
H
O
H
H
O
H
H
O
H
H
O
H
H
O
H
H
O
OO
O
O
H
H
H
H
H
H
H
H
H
H
FIGURE 2.2
Under certain conditions, hydrogen
molecules can combine with oxygen molecules,
forming water molecules.
FIGURE 2.3
Electrons orbit the atomic nucleus. The single electron
of a hydrogen atom moves within its F
rst shell. The two electrons of
a helium atom F
ll its F
rst shell. Two of the three electrons of a lithium
atom are in the F
rst shell, and one is in the second shell.
Lithium (Li)
Helium (He)
Hydrogen (H)
+
0
0
+
+
0
0
+
+
0
0
+
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