Microscopic Scales for Liquids and Gases

One of the basic scales of gas and liquid systems is the size of the molecules. The size of simple molecule is of the order of few angstroms. A second important scale is the average distance between molecules. Let us consider the intermolecular distance of air. The volume occupied by a single air molecule can be written as 

The typical intermolecular distance of liquids is 0.3 nm, which is 10 times smaller than that of air. For liquids, the intermolecular distance is comparable to the size of the molecule. For gases, the intermolecular distance is much larger than the size of the molecule. 

The mean free path, A is another fundamental scale necessary for describing the dynamical properties of gases. Mean free path is defined as the average distance traveled by the molecule between two successive collisions. Kinetic theory of gases establishes the following two expressions for the mean free path of gases. 

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