The Genesis of the Quantum Theory of the Chemical Bond
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.