PHYSICAL CHEMISTRY IN BRIEF

CHAP. 12: BASIC TERMS OF CHEMICAL PHYSICS [CONTENTS] 414

12.1.2 Molar polarization and refraction

Molar polarizationis another quantity derived from relative permittivity:

Pm=

εr− 1

εr+ 2

Vm=

εr− 1

εr+ 2

M

ρ

, (12.3)

whereVmis the molar volume [see2.1.1].
U Main unit:m^3 mol−^1.
Molar refractionis defined in a similar way:

Rm=

n^2 − 1

n^2 + 2

Vm (12.4)

wherenis the refractive index of the given substance.

In terms of the electric and magnetic properties of substances, electromagnetic radiation

can be described as electromagnetic field oscillations. Since non-polar substances, i.e. those

without permanent dipole moments [see12.1.3] in the range of higher wavelengths, have to

satisfy theMaxwell relation^1

εr=n^2 , (12.5)

their molar polarization and refraction acquire the same values.

12.1.3 Dipole moment

A pair of opposite electric charges of magnitudeQat a distanceLconstitutes an electric dipole.

The measure of its magnitude is the dipole moment defined by the relation

~μ=Q~L , (12.6)

where vector~Lpoints from−QtoQ.
U Main unit:C m.
Older unit:debye, 1 D = 3. 33564 × 10 −^30 C m.
In terms of the electric charge distribution, a molecule represent a much more complicated
system containing positive charges (atomic nuclei) and negative charges (electrons). The electric

(^1) More preciselyεrμr=n (^2) (see (12.11)), whereμr= 1..