According to the third law of thermodynamics, what entropy does a perfect crystal possess at absolute zero (0 K)?

Core Concept

The third law fixes the floor of the entropy scale: a perfect crystal at 0 K has no thermal motion and exactly one accessible microstate, giving S = 0. This absolute footing explains why tabulated S° values are measured directly, unlike ΔH°f numbers, which are defined relative to elements. Heating from 0 K then accumulates the positive S° listed at 298 K.