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Polar Covalent Bonds across a row increases, negatively charged electrons are held

In molecules such as hydrogen (H ) and oxygen (O ) gases, more tightly to the nucleus. This principle helps to explain
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electrons are shared equally by the atoms. In many covalent why O is more electronegative than N and why N is more
bonds, however, the electrons are not shared equally. A nota- electronegative than either C or H.

ble example, shown in FIGURE 1.8 , is the water molecule You can think of electronegativity as the “greed” of an
(H O) . A water molecule consists of two hydrogen atoms, atom for electrons. Oxygen (O) is greedier for electrons

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each of which is covalently bound to a single oxygen atom. than is nitrogen (N), carbon (C), or hydrogen (H). Note, too,
In a molecule of water, the region around the oxygen atom that carbon, hydrogen, nitrogen, and oxygen vary in their

has a partial negative charge, while the area around each of electronegativity. Carbon and nitrogen O(C N) and carbon

the two hydrogen atoms has a partial positive charge. In the and oxygen O(C O) each form polar covalent bonds because
+

figure, charges are shown using the symbol δ for a partial electrons are not shared equally between two atoms. Atoms

−
positive charge near the H atoms, and the δ symbol for that are closer together in electronegativity, such as carbon

a partial negative charge near the O atom. and hydrogen, form covalent bonds that are not polar. When
Electrons are shared unequally because of a difference a covalent bond is established between two atoms of the
in the ability of the atoms to attract electrons, a property same type, for instance, between two H atoms or two O

known as electronegativity. Oxygen is more electroneg- atoms, both of the atoms have the same degree of electro-
ative than hydrogen. As a result, in a molecule of water, negativity and hence the electrons are shared equally.
oxygen has a partial negative charge, while the two hydro-
gen atoms have a partial positive charge. When electrons PREP FOR THE AP EXAM
®
are shared unequally between two atoms, the interaction is ®
described as a polar covalent bond . AP EXAM TIP

By contrast, a covalent bond where atoms are shared Nonpolar and polar covalent bonds govern many of the
equally is sometimes referred to as a nonpolar covalent properties of organic molecules. Knowing these concepts

bond . The molecules of hydrogen gas (H ) , oxygen gas (O ) , will help you understand how these molecules function,

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®
which will help you succeed on the AP Biology Exam.

and nitrogen gas (N ) all have nonpolar covalent bonds. If
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two different kinds of atoms have similar electronegativities,
then the covalent bonds between them also tend to be non-
polar because the electrons are shared equally, or nearly Ionic Bonds
equally, by the atoms. Of the atoms commonly found in In a molecule of water, the difference in electronegativity
organic molecules, C and H frequently form nonpolar cova- between the oxygen and hydrogen atoms leads to unequal

lent bonds. For example, methane gas (CH) is a nonpolar sharing of electrons. In more extreme cases, when an atom
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compound. of high electronegativity is paired with an atom of low elec-
Electronegativity tends to increase across a row in the tronegativity, the difference in electronegativity may be so
periodic table. As the number of positively charged protons great that the electronegative atom “steals” the electron from
its less electronegative partner. This creates an electrically
charged atom, which you may recall is known as an ion. The
Chemical formula H O atom with the extra electron contains more electrons than
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protons, which gives it a negative charge. The atom that lost
+ + the electron has a positive charge because it now has more
H H

Space- lling model protons than electrons. The two ions form an ionic bond,
O O
a chemical bond in which two ions with opposite electrical
– charges associate with each other because of the differences
FIGURE 1.8 A polar covalent bond in charge. Some atoms may gain or lose more than one elec-
tron when they form an ionic bond. For instance, the cal-
In a polar covalent bond, the two atoms do not share the electrons cium ion, which is often used by cells, may be either a single
equally. In water, the shared electrons spend more time near the + ++

O atom than either of the H atoms. The result is that the O atom has (Ca) or a double (Ca ) positively charged ion.
−

a partial negative charge, written as δ , while the H atoms have partial Sodium chloride (NaCl), which is common table salt,

+
positive charges, noted as δ . is an example of a compound formed by the attraction of

34 UNIT 1 CHEMISTRY OF LIFE
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