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This paper opens with a comparison of the electron shell model of Langmuir with the assignment of quantum numbers made by Bohr and slightly modified by Stoner in the years immediately preceding quantum mechanics. A discussion is given of the role of spin in the development of quantum mechanics, culminating in the relativistic Dirac equations. Because of the exclusion principle spin is the great indicator of valence behaviour. This is shown particularly clearly by a vector-model formula first derived by Dirac. The Heitler—London-Slater—Pauling and Hund—Mulliken approaches are compared. The two methods each have their difficulties but there is often comforting agreement in their conclusions. The results are displayed of some early calculations of binding energies of the hydrogen molecule and of resonance energies. The final portion of the paper discusses the development of the crystal field theory of the paramagnetic susceptibilities of salts of the iron group during the first decade of quantum mechanics, and how this theory furnishes the key to the theoretical inorganic chemistry underlying the diverse behaviour of the various ions in this group.

Spin, the great indicator of valence behaviour