In a uniform electric field a molecule gets polarized due to the distortion of its charge distribution. This distortion can be measured by the induced dipole moment (to be distinguished from the permanent dipole moment) which is defined as

(10.11)

where

(x,y,z) is the polarizability. For small fields,

is independent of

. Since the charge distribution in molecules is different in different directions (such as along a bond, perpendicular to a bond, etc), the value of

depends on the direction of the field as well as the direction in which we are considering the polarization. Therefore, we have two subscripts for

i.e.,

_xx,

_xyand so on. The first subscript of

is taken as the direction of

and the second subscript, the direction of

. The unit of

is volume. The value of

, Q and

for a few systems are given in Table 10.2

Atom / Molecule	Dipole Moment (Debyes)	Quadrupole Moment (10^-16 cm²)	Polarizability (10^-24 cm³)
H	0	0	0.6668
Xe	0	0	4.04
H₂	0	0.616	0.819
O₂	0	- 0.4	1.60
CO	0.112	- 2.0	1.98
HCl	1.109	3.7	2.63
NH₃	1.47	1.51	2.22
H₂O	1.86	2.6, - 1.9, - 0.1	1.48

Table 10.2 Values of dipole moments, quadrupole moments and polarizabilities for a few systems. 

The three values of the quadrupole moments of water refer to Q_xx Q_yy and Q_zz, with the molecule placed in the xz plane and the z axis being the molecular axis.

The ion-induced dipole (id) interaction is given by

U _ion-id( r ) = -

q²_i / 2r⁴

(10.12)

Here q_i is the ionic charge. The separation of a ion from the other species which is polarized (whose polarizability is given by

) is denoted by r. This interaction is attractive because a +ve ion pulls the negative charge density (induced by it) towards itself.