The third law of thermodynamics states that the entropy of a system in a perfect crystalline state is zero as the temperature approaches zero Kelvin.

Molecular motion decreases with temperature and a pure substance may form a crystal with an uninterrupted lattice structure as , that is, a crystal with a unique configuration. The equation for statistical entropy is:

where is the number of ways to achieve the configuration of the system.

Since there is only one way to arrange the configuration of a perfect crystal, and