Fundamentals of Anatomy and Physiology

20

BONDS AND ENERGY

Atoms combine chemically with one another in one of two
ways, that is, they form bonds. Chemical bonds are
formed when the outermost electrons are transferred
(gained or lost) or shared between atoms. When the atoms-
of two or more different elements combine in this way, a
compound (such as water, H 2 O) is created. This symbol
H 2 O also represents a molecule. A molecule or
compound is the smallest combination or particle retaining-
all the properties of the compound itself.
One type of bond is called an ionic bond. This kind
of bond is formed when one atom gains electrons while the
other atom loses electrons from its outer-most level or
orbit. Atoms that gain electrons become negatively
charged, whereas those that lose electrons become
positively charged, each having originally been electrically
neutral. The new charged atoms are called ions.
Negatively charged ions (Cl 2 , for example)-
are attracted to positively charged ions (Na^1 ). The
-resulting force that binds these ions together is an ionic
bond. Referring to Figure 2-3, notice that the sodium atom
has a completely filled innermost level with two electrons,
a completely filled second level with eight electrons, but
only one electron in its third level. The chlorine atom has a
completely filled -innermost level with two electrons, a
completely filled second level with eight electrons, but only
seven in its third level. Because eight electrons fill the
outermost

Chapter 2

level in forming the ionic bond, sodium loses its one

electron to the chlorine atom’s outermost level, thus filling

chlorine’s outermost level with eight electrons. The

resulting compound, sodium chloride (Na^1 Cl^2 ), is common

table salt formed by an ionic bond, held together by the

attraction of the opposite electric charges of the ions. When

immersed in water, com-pounds held together by ionic

bonds tend to separate or dissociate into their constituent

ions because of the attraction of the water molecule (which

we shall discuss later in this chapter). Many of the

substances required by human cells exist in nature in ionic

form. Some examples are the mineral salts such as sodium,

chloride, potassium, calcium, and phosphate.

A second type of bond found in many molecules is the

covalent bond. In this type of bond, the atoms share

electrons to fill their outermost levels. Molecules

containing covalent bonds do not dissociate when im-

mersed in water. Four of the most important elements

found in cells form this type of bond. They are carbon (C),

oxygen (O), hydrogen (H), and nitrogen (N). They

constitute about 95% of the materials found in cells. All of

the cell’s larger molecules, and many of its smaller ones,

contain such bonds; for example, the formation of the

covalent bond between two hydrogen atoms forms the

compound hydrogen gas (H 2 ) (Figure 2-4).

Another type of bond is the hydrogen bond. Hydro-

gen bonds are very weak bonds and help hold water

1

Na Cl

Hydrogen (H) Hydrogen (H)

Sodium atom Chlorine atom

Na+ Cl-

• _^

Sodium ion^ Chloride ion^
Figure 2- 3 The formation of an ionic bond
between -sodium and chloride forms Na^1 Cl^2
(sodium chloride or table salt).

(^) ®
Learning
Cengage ©
(^)
®^
(^) or
Learning^

H H H H

Cengage^

©^

Figure 2- 4 The formation of compounds through

covalent bonding.