CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 415
field far enough from the molecule can be regarded as caused by one dipole determined by a
combination of charges.
A homonuclear diatomic molecule (a molecule comprised of a pair of identical atoms) ex-
hibits a symmetrical charge distribution and therefore its dipole moment is zero. These sub-
stances are called non-polar (N 2 , Cl 2 ). If it is comprised of different atoms (HCl), the charge
distribution is asymmetrical, the molecule has a nonzero dipole moment, and the substance is
polar. In more complex molecules, the dipole moment depends on the type of atoms and their
mutual orientation. The overall dipole moment is the vector sum of the dipole moments of
individual bonds.
Example
Explain why CO 2 has a zero dipole moment while H 2 O is polar.Solution
Both the bonds C–O and O–H are polar and have a nonzero dipole moment. However, the
CO 2 molecule is linear and the dipole moments of both bonds cancel. The molecule of water is
nonlinear and its total dipole moment is the vector sum of the dipole moments of the two bonds.Note:Also nonpolar molecules create a certain electric field around them, which is usually
characterized by higher multipole moments (quadrupole, octupole, etc.).12.1.4 Polarizability
In the presence of an electric field, the charge distribution in the molecule changes so that even
nonpolar molecules acquire a nonzero dipole moment, i.e. they polarize. The magnitude of
thisinduceddipole moment (~μ∗) is proportional^2 to the intensity of the surrounding electric
field (E~). The proportionality constant is called polarizability (α),
μ∗=αE. (12.7)(^2) There are two simplifications here: The field is not too strong and the molecule is approximately rotationally
symmetric so that~μ∗has the same direction as~E