The Foundations of Chemistry

In the case of the angular arrangement, the two equal dipoles do not cancel,but add to give

a dipole moment greater than zero. The angular molecular arrangement represents a polar

molecule.

If the electronegativity differences were reversed in this BXAXB molecule—that is, if A

were more electronegative than B—the directions of all bond polarities would be reversed.

But the bond polarities would still cancel in the linear arrangement to give a nonpolar

molecule. In the angular arrangement, bond polarities would still add to give a polar mole-

cule, but with the net dipole pointing in the opposite direction from that described earlier.

We can make similar arguments based on addition of bond dipoles for other arrange-

ments. As we will see in Section 8-8, lone pairs on the central atom can also affect the

direction and the magnitude of the net molecular dipole, so the presence of lone pairs on

the central atom must always be taken into account.

For a molecule to be polar, twoconditions must be met:

1.There must be at least one polar bond or one lone (unshared) pair on the central

atom.

and

2. a.The polar bonds, if there are more than one, must not be arranged so that
   their polarities (bond dipoles) cancel.
   or
   b.If there are two or more lone (unshared) pairs on the central atom, they must
   not be arranged so that their polarities cancel.

Put another way, if there are no polar bonds or unshared pairs of electrons on the central

atom, the molecule cannotbe polar. Even if polar bonds or unshared pairs are present,

they may be arranged so that their polarities cancel one another, resulting in a nonpolar

molecule.

Carbon dioxide, CO 2 , is a three-atom molecule in which each carbon–oxygen bond is

polarbecause of the electronegativity difference between C and O. But the molecule as a

wholeis shown by experiment (dipole moment measurement) to be nonpolar. This tells

us that the polar bonds are arranged in such a way that the bond polarities cancel. Water,

H 2 O, on the other hand, is a very polar molecule; this tells us that the HXO bond polar-

ities do not cancel one another. Molecular shapes clearly play a crucial role in determining

molecular dipole moments. We will develop a better understanding of molecular shapes

in order to understand molecular polarities.

The logic used in deducing whether a molecule is polar or nonpolar is outlined in

Figure 8-1. The approach described in this section will be applied to various electronic

and molecular geometries in parts B of Sections 8-5 through 8-12.

B A

B

Net dipole  0

(polar molecule)

B A B

Net dipole  0

(nonpolar molecule)

H O

H

angular molecule;

bond dipoles do not cancel;

molecule is polar

O C O

linear molecule;

bond dipoles cancel;

molecule is nonpolar

312 CHAPTER 8: Molecular Structure and Covalent Bonding Theories