essentials of anatomy and physiology; 5e

26 Some Basic Chemistry
Na + Cl = NaCl
+ –
Figure 2–2. Formation of an ionic bond. An atom of sodium loses an electron to an
atom of chlorine. The two ions formed have unlike charges, are attracted to one another,
and form a molecule of sodium chloride.
QUESTION: Why is the charge of a sodium ion 1?
Another example is the bonding of chlorine to calcium.
An atom of calcium has two electrons in its outermost
shell and tends to lose those electrons in order
to become stable. If two atoms of chlorine each gain
one of those electrons, they become chloride ions.
The positive and negative ions are then attracted to
one another, forming a molecule of calcium chloride,
CaCl 2
, which is also a salt. A salt is a molecule made
of ions other than hydrogen (H ) ions or hydroxyl
(OH ) ions.
Ions with positive charges are called cations. These
include Na , Ca 2 , K , Fe 2 , and Mg 2 . Ions with
negative charges are called anions, which include Cl ,
SO 4
2
(sulfate), and HCO 3
(bicarbonate). The types
of compounds formed by ionic bonding are salts,
acids, and bases. (Acids and bases are discussed later in
this chapter.)
In the solid state, ionic bonds are relatively strong.
Our bones, for example, contain the salt calcium carbonate
(CaCO 3
), which helps give bone its strength.
However, in an aqueous (water) solution, many ionic
bonds are weakened. The bonds may become so weak
that the bound ions of a molecule separate, creating a
solution of free positive and negative ions. For example,
if sodium chloride is put in water, it dissolves, then
ionizes. The water now contains Na ions and Cl
ions. Ionization, also called dissociation, is important
to living organisms because once dissociated, the ions
are free to take part in other chemical reactions within
the body. Cells in the stomach lining produce
hydrochloric acid (HCl) and must have Cl ions to do
so. The chloride in NaCl would not be free to take
part in another reaction since it is tightly bound to the
sodium atom. However, the Cl ions available from
ionized NaCl in the cellular water can be used for the
synthesis, or chemical manufacture, of HCl in the
stomach.
COVALENT BONDS
Covalent bonds involve the sharing of electrons
between atoms. As shown in Fig. 2–3, an atom of oxygen
needs two electrons to become stable. It may
share two of its electrons with another atom of oxygen,
also sharing two electrons. Together they form a
molecule of oxygen gas (O 2
), which is the form in
which oxygen exists in the atmosphere.
An atom of oxygen may also share two of its electrons
with two atoms of hydrogen, each sharing its
single electron (see Fig. 2–3). Together they form a
molecule of water (H 2
O). When writing structural
formulas for chemical molecules, a pair of shared electrons
is indicated by a single line, as shown in the formula
for water; this is a single covalent bond. A double
covalent bond is indicated by two lines, as in the formula
for oxygen; this represents two pairs of shared
electrons.
The element carbon always forms covalent bonds;
an atom of carbon has four electrons to share with
other atoms. If these four electrons are shared with
four atoms of hydrogen, each sharing its one electron,
a molecule of methane gas (CH 4
) is formed. Carbon
may form covalent bonds with other carbons, hydrogen,
oxygen, nitrogen, or other elements. Organic