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# Molar refractivity

Molar refractivity is a measure of the volume occupied by an atom or group and is dependent on the temperature, the index of refraction, and the pressure.

One form of the Lorentz-Lorenz formula (also known as the Clausius-Mossotti equation) gives the molar refractivity of a dilute gas as $A = \frac{R T (n^2 - 1)}{3p}$

where

R is the universal gas constant
T is the temperature
n is the index of refraction
p is the pressure

In SI units, R has units of J mol-1 K-1, T has units K, n has no units, and p has units of Pa, so the units of A are m3 mol-1. Therefore, the molar refractivity is the volume of the substance (in cubic meters) taken up by each mole of that substance.