The virial expansion of a gas is a correction to the ideal gas law that is usually discussed in advanced courses in statistical mechanics. In this note we outline this derivation in a manner suitable for advanced undergraduate and introductory graduate classroom presentations. We introduce a physically meaningful interpretation of the virial expansion that has heretofore escaped attention, by showing that the virial series is actually an expansion in a parameter that is the ratio of the effective volume of a molecule to its mean volume. Using this interpretation we show why under normal conditions ordinary gases such as O_2 and N_2 can be regarded as ideal gases.
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