57
CHAPTER TWO
Chemical Basis of Life
atoms form two bonds, nitrogen atoms form three bonds,
and carbon atoms form four bonds. Symbols and lines can
be used to represent the bonding capacities of these atoms,
as follows:
H
N
C
O
Symbols and lines show how atoms bond and are
arranged in various molecules. One pair of shared elec-
trons, a single covalent bond, is depicted with a single line.
Sometimes atoms may share two pairs of electrons (a double
covalent bond), or even three pairs (a triple covalent bond),
represented by two and three lines, respectively. Illustrations
of this type, called
structural formulas
(f g. 2.6)
, are useful,
but they cannot adequately capture the three-dimensional
forms of molecules. In contrast,
f
gure 2.7
shows a three-
dimensional (space-F lling) representation of a water mol-
ecule.
Different types of chemical bonds share electrons to dif-
ferent degrees. At one extreme is an ionic bond in which
atoms gain or lose electrons. At the other extreme is a cova-
lent bond that shares electrons equally. In between lies a
covalent bond in which electrons are not shared equally,
resulting in a molecule whose shape gives an uneven dis-
tribution of charges. Such a molecule is called
polar.
Unlike
an ion, a polar molecule has an equal number of protons
and electrons, but one end of the molecule has more than
its share of electrons, becoming slightly negative, while the
other end of the molecule has less than its share, becoming
slightly positive. Typically, polar covalent bonds form where
hydrogen atoms bond to oxygen or nitrogen atoms. Water is
a polar molecule
(f
g. 2.8
a
)
.
The attraction of the positive hydrogen end of a polar
molecule to the negative nitrogen or oxygen end of another
polar molecule is called a
hydrogen bond.
These bonds are
weak, particularly at body temperature. ±or example, below
0°C, the hydrogen bonds between water molecules shown in
F gure 2.8
b
are strong enough to form ice. As the temperature
rises, increased molecular movement breaks the hydrogen
bonds, and water becomes liquid. Even at body temperature,
+−
11p
+
12n
0
11p
+
12n
0
17p
+
18n
0
17p
+
18n
0
Sodium atom (Na)
Chlorine atom (Cl)
Chloride ion (Cl
)
Sodium ion (Na
+
)
(b)
Bonded ions
These oppositely charged particles attract electrically and join by
an ionic bond.
(c)
Salt crystal
Ionically bonded substances form arrays such as a crystal of NaCl.
(a)
Separate atoms
If a sodium atom loses an electron to a chlorine atom, the sodium
atom becomes a sodium ion (Na
+
), and the chlorine atom becomes
a chloride ion (Cl
).
Sodium chloride
Na
+
Cl
FIGURE 2.4
An ionic bond forms when (
a
) one atom gains and
another atom loses electron(s) and then (
b
) oppositely charged ions
attract. (
c
) Ionically bonded substances may form crystals.
FIGURE 2.5
A hydrogen molecule forms when two hydrogen atoms share a pair of electrons. A covalent bond forms between the atoms.
+
+
+
+
Hydrogen atom
+
H
Hydrogen molecule
H
2
Hydrogen atom
H
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