  An historical overview is given of the relevant steps that allowed the
genesis of the quantum theory of the chemical bond, starting from the
appearance of the new quantum mechanics and following later developments till
approximately 1931. General ideas and some important details are discussed
concerning molecular spectroscopy, as well as quantum computations for simple
molecular systems performed within perturbative and variational approaches, for
which the Born-Oppenheimer method provided a quantitative theory accounting for
rotational, vibrational and electronic states. The novel concepts introduced by
the Heitler-London theory, complemented by those underlying the method of the
molecular orbitals, are critically analyzed along with some of their relevant
applications. Further improvements in the understanding of the nature of the
chemical bond are also considered, including the ideas of one-electron and
three-electron bonds introduced by Pauling, as well as the generalizations of
the Heitler-London theory firstly performed by Majorana, which allowed the
presence of ionic structures into homopolar compounds and provided the
theoretical proof of the stability of the helium molecular ion. The study of
intermolecular interactions, as developed by London, is finally examined.
