do not reorient, so the change in voltage between the plates remains slight; we say that
these molecules are nonpolar.
Generally, as electronegativity differences increase in diatomic molecules, the measured
dipole moments increase. This can be seen clearly from the data for the hydrogen halides
(see Table 7-3).
Unfortunately, the dipole moments associated with individual bondscan be measured
only in simple diatomic molecules. Entire moleculesrather than selected pairs of atoms must
be subjected to measurement. Measured values of dipole moments reflect the overallpolar-
ities of molecules. For polyatomic molecules they are the result of all the bond dipoles in
the molecules. In Chapter 8, we will see that structural features, such as molecular geom-
etry and the presence of lone (unshared) pairs of electrons, also affect the polarity of a
molecule.THE CONTINUOUS RANGE OF BONDING TYPES
Let us now clarify our classification of bonding types. The degree of electron sharing or
transfer depends on the electronegativity difference between the bonding atoms. Nonpolar
covalent bonding (involving equal sharingof electron pairs) is one extreme, occurring when
the atoms are identical ((EN) is zero). Ionic bonding (involving complete transferof elec-
trons) represents the other extreme, and occurs when two elements with very different
electronegativities interact ((EN) is large).
Polar covalent bonds may be thought of as intermediate between pure (nonpolar) cova-
lent bonds and pure ionic bonds. In fact, bond polarity is sometimes described in terms
of partial ionic character.This usually increases with increasing difference in elec-
tronegativity between bonded atoms. Calculations based on the measured dipole moment
of gaseous HCl indicate about 17% “ionic character.”
When cations and anions interact strongly, some amount of electron sharing takes
place; in such cases we can consider the ionic compound as having some partial covalent
character.For instance, the high charge density of the very small Liion causes it to
distort large anions that it approaches. The distortion attracts electron density from the
anion to the region between it and the Liion, giving lithium compounds a higher degree
of covalent character than in other alkali metal compounds.7-10
Figure 7-5 If polar molecules, such
as HF, are subjected to an electric
field, they tend to line up very
slightly in a direction opposite to
that of the field. This minimizes the
electrostatic energy of the molecules.
Nonpolar molecules are not oriented
by an electric field. The effect is
greatly exaggerated in this drawing.
298 CHAPTER 7: Chemical Bonding
Field off Field on