The Foundations of Chemistry

The longest arrow indicates the largest dipole, or greatest separation of electron density
in the molecule (see Table 7-3). For comparison, the (EN) values for some typical 1:1
ionic compounds are RbCl, 2.1; NaF, 3.0; and KCl, 2.1

EXAMPLE 7-10 Polar Bonds

Each halogen can form single covalent bonds with other halogens, to form compounds called
interhalogens;some examples are ClF and BrF. Use the electronegativity values in Table 6-3 to
rank the following single bonds from most polar to least polar: FXF, FXCl, FXBr, ClXBr,
ClXI, and ClXCl.

Plan

The bond polarity decreases as the electronegativity difference between the two atoms
decreases. We can calculate (EN) for each bond, and then arrange them according to
decreasing (EN) value.

Solution

We know that two F atoms have the same electronegativity, so (EN) for FXF must be zero,
and the FXF bond is nonpolar; the same reasoning applies to ClXCl. We use the values from
Table 6-3 to calculate (EN) for each of the other pairs, always subtracting the smaller from
the larger value:

Elements (EN)

F, Cl 4.03.01.0

F, Br 4.02.81.2

Cl, Br 3.02.80.2

Cl, I 3.02.50.5

The bonds, arranged from most polar to least polar, are:

FXBr FXCl ClXIClXBr ClXCl FXF

EN: 4.0 2.8 4.0 3.0 3.0 2.5 3.0 2.8 3.0 3.0 4.0 4.0

(EN): 1.2 1.0 0.5 0.2 0 0

You should now work Exercise 74.

DIPOLE MOMENTS

It is convenient to express bond polarities on a numerical scale. We indicate the polarity
of a molecule by its dipole moment, which measures the separation of charge within the
molecule. The dipole moment, , is defined as the product of the distance, d, separating
charges of equal magnitude and opposite sign, and the magnitude of the charge, q. A
dipole moment is measured by placing a sample of the substance between two plates and
applying a voltage. This causes a small shift in electron density of any molecule, so the
applied voltage is diminished very slightly. Diatomic molecules that contain polar bonds,
however, such as HF, HCl, and CO, tend to orient themselves in the electric field (Figure
7-5). This causes the measured voltage between the plates to decrease more markedly for
these substances, and we say that these molecules are polar.Molecules such as F 2 or N 2

7-9

d q

7-9 Dipole Moment s297

     

TABLE 7-3 
(EN) Values

and Dipole

Moments for

Some Pure

(Gaseous)

Substances

Dipole

Substance 
(EN) Moment ()*

HF 1.9 1.91 D

HCl 0.9 1.03 D

HBr 0.7 0.79 D

HI 0.4 0.38 D

HXH0.0 0 .00D

*The magnitude of a dipole moment is given

by the product of charge distance of separa-

tion. Molecular dipole moments are usually

expressed in debyes (D).