Essentials of Ecology

S40 SUPPLEMENT 6

Ionic and Covalent Bonds Hold
Compounds Together

Sodium chloride (NaCl) consists of a three-
dimensional network of oppositely charged
ions (Na and Cl) held together by the forces
of attraction between opposite charges (Fig-

Na+ Cl–

Figure 2 A solid
crystal of an ionic com-
pound such as sodium
chloride consists of
a three–dimensional
array of oppositely
charged ions held to-
gether by ionic bonds
resulting from the
strong forces of attrac-
tion between opposite
electrical charges. They
are formed when an
electron is transferred
from a metallic atom
such as sodium (Na) to
a nonmetallic element
such as chlorine (Cl).

Chloride ion

in solution

Sodium chloride

(NaCl) salt

Sodium ion

in solution

Na+

Na+

Na+

Na+

Na+

Na+

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Cl–

Water molecules

Figure 3 How a salt

dissolves in water.

Figure 4 Chemical formulas and shapes for some covalent compounds formed when atoms of one or
more nonmetallic elements combine with one another and share one or more pairs of their electrons.
The bonds between the atoms in such molecules are called covalent bonds.

S

N

N

N

C

C

C

S

S

O

OO

O

O

H

H

H

H H

H H

H H H

O Cl

OO

OO

OO

OO

OO

O

NO

nitric oxide

CO

carbon monoxide

HCI

hydrogen chloride

H 2 O

water

H 2

hydrogen

O 2

oxygen

N 2

nitrogen

CI 2

chlorine

NO 2

nitrogen dioxide

CO 2

carbon dioxide

SO 2

sulfur dioxide

O 3

ozone

CH 4

methane

NH 3

ammonia

SO 3

sulfur trioxide

H 2 S

hydrogen sulfide

HH

H H N N Cl Cl

ure 2). The strong forces of attraction between

such oppositely charged ions are called ionic

bonds. Because ionic compounds consist of ions

formed from atoms of metallic (positive ions)

and nonmetallic (negative ions) elements (Fig-

ure 1), they can be described as metal–nonmetal

compounds.

Sodium chloride and many other ionic com-

pounds tend to dissolve in water and break apart

into their individual ions (Figure 3).

NaCl Na  Cl

sodium chloride sodium ion  chloride ion

(in water)

Water, a covalent compound, consists of mol-

ecules made up of uncharged atoms of hydrogen

(H) and oxygen (O). Each water molecule con-

sists of two hydrogen atoms chemically bonded

to an oxygen atom, yielding H 2 O molecules.

The bonds between the atoms in such molecules

are called covalent bonds and form when the

atoms in the molecule share one or more pairs

of their electrons. Because they are formed from

atoms of nonmetallic elements (Figure 1), cova-

lent compounds can be described as nonmetal–

nonmetal compounds. Figure 4 shows the chemical

formulas and shapes of the molecules that are

the building blocks for several common covalent

compounds.

What Makes Solutions Acidic?

Hydrogen Ions and pH

The concentration, or number of hydrogen ions

(H) in a specifi ed volume of a solution (typi-

cally a liter), is a measure of its acidity. Pure

water (not tap water or rainwater) has an equal

number of hydrogen (H) and hydroxide (OH)

ions. It is called a neutral solution. An acidic

solution has more hydrogen ions than hydrox-

ide ions per liter. A basic solution has more

hydroxide ions than hydrogen ions per liter.

Scientists use pH as a measure of the acid-

ity of a solution based on its concentration of

hydrogen ions (H). By defi nition, a neutral

solution has a pH of 7, an acidic solution has a

pH of less than 7, and a basic solution has a pH

greater than 7.

Each single unit change in pH represents a

10-fold increase or decrease in the concentra-

tion of hydrogen ions per liter. For example, an

acidic solution with a pH of 3 is 10 times more

acidic than a solution with a pH of 4. Figure 5