Essentials of Anatomy and Physiology

atom, the number of protons () equals the number of
electrons (); therefore, an atom is electrically neu-
tral. The electrons, however, are important in that
they may enable an atom to connect, or bond, to other
atoms to form molecules. A molecule is a combina-
tion of atoms (usually of more than one element) that
are so tightly bound together that the molecule
behaves as a single unit.
Each atom is capable of bonding in only very spe-
cific ways. This capability depends on the number and
the arrangement of the electrons of the atom.
Electrons orbit the nucleus of an atom in shells or
energy levels. The first, or innermost, energy level
can contain a maximum of two electrons and is then
considered stable. The second energy level is stable
when it contains its maximum of eight electrons. The
remaining energy levels, more distant from the
nucleus, are also most stable when they contain eight
electrons, or a multiple of eight.
A few atoms (elements) are naturally stable, or
uninterested in reacting, because their outermost
energy level already contains the maximum number of
electrons. The gases helium and neon are examples of
these stable atoms, which do not usually react with
other atoms. Most atoms are not stable, however, and
tend to gain, lose, or share electrons in order to fill
their outermost shell. By doing so, an atom is capable
of forming one or more chemical bonds with other

atoms. In this way, the atom becomes stable, because

its outermost shell of electrons has been filled. It is

these reactive atoms that are of interest in our study of

anatomy and physiology.

CHEMICAL BONDS

A chemical bond is not a structure, but rather a force

or attraction between positive and negative electrical

charges that keeps two or more atoms closely associ-

ated with each other to form a molecule. By way of

comparison, think of gravity. We know that gravity is

not a “thing,” but rather the force that keeps our feet

on the floor and allows us to pour coffee with consis-

tent success. Molecules formed by chemical bonding

often have physical characteristics different from those

of the atoms of the original elements. For example,

the elements hydrogen and oxygen are gases, but

atoms of each may chemically bond to form molecules

of water, which is a liquid.

The type of chemical bonding depends upon the

tendencies of the electrons of atoms involved, as you

will see. Four kinds of bonds are very important to the

chemistry of the body: ionic bonds, covalent bonds,

disulfide bonds, and hydrogen bonds.

IONIC BONDS

An ionic bondinvolves the loss of one or more elec-

trons by one atom and the gain of the electron(s) by

another atom or atoms. Refer to Fig. 2–2 as you read

the following.

An atom of sodium (Na) has one electron in its out-

ermost shell, and in order to become stable, it tends to

lose that electron. When it does so, the sodium atom

has one more proton than it has electrons. Therefore,

it now has an electrical charge (or valence) of 1 and

is called a sodium ion(Na). An atom of chlorine has

seven electrons in its outermost shell, and in order to

become stable tends to gain one electron. When it

does so, the chlorine atom has one more electron than

it has protons, and now has a charge (valence) of 1.

It is called a chloride ion (Cl).

When an atom of sodium loses an electron to an

atom of chlorine, their ions have unlike charges (pos-

itive and negative) and are thus attracted to one

another. The result is the formation of a molecule of

sodium chloride: NaCl, or common table salt. The

bond that holds these ions together is called an ionic

bond.

Some Basic Chemistry 25

Second energy level

First energy level

Proton [+]

Neutron

Nucleus

Electrons [--]

Figure 2–1. An atom of carbon. The nucleus contains
six protons and six neutrons (not all are visible here). Six
electrons orbit the nucleus, two in the first energy level
and four in the second energy level.
QUESTION:What is the electrical charge of this atom as
a whole?