One of the many significant scientific achievements of the 20th century was the clarification of the relationship between chemistry, statistical thermodynamics, and quantum mechanics, which came through work associated primarily with the names J.W. Gibbs and Linus Pauling. I think most people are vaguely aware that the boundary between chemistry and physics is hazy and more a matter of content than concept, but from a purely theoretical point of view, we now know that chemistry is reducible to quantum mechanics. In practice, the disciplines remain distinct because molecules of any appreciable size, such as nearly every molecule involved in biology, are much too complicated to be modeled from quantum mechanical first principles. Nevertheless, molecules and the electrons in chemical bonds are described by wave functions, and through quantum mechanics and statistical thermodynamics, we understand why a chemical bond forms.