Talk:Molar refractivity
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It is definitely not a measure of the volume of a substance, but somehow related to its polarizability at optical ("very high") frequencies. In crude models, the polarizability of an atom is proportional to its volume. But this is a very simplified view and only holds for simple systems such as single atoms or mono-atomic ions.
The statement: "Therefore, the molar refractivity is the volume of the substance (in cubic meters) taken up by each mole of that substance." is absolutely wrong.
Note that polarizabilities alpha are very often referred to by alpha/(4 Pi epsilon), where epsilon is the vacuum permittivity. alpha/(4 Pi epsilon) has also units of volume (m^3).