It is shown that the chemist's concept of valency can be put in quantum mechanical language. Such a quantification of valency opens up possibilities of new applications which are briefly reviewed. Valency is related to the equilibrium geometry of the molecule through a maximum valency principle. This principle, combined with the idea of molecular orbital valencies, provides a theoretical foundation for the well known Walsh diagrams. Valency can be used as an index of chemical reactivity and valency changes in a reaction can be used to determine the radical nature of intermediates and transition states as well as to indicate the allowed or forbidden nature of reaction paths. Molecular strain may be related to the loss of sigma valency.

A simple method, based on the concept of molecular valency and making use of the virtual orbitals of the ground state, is proposed to predict the geometry of molecules in various excited and ionised states. The method is found to be accurate for bond angle predictions.

A numerical correlation between quantum-chemically calculated bond valency and experimental heat of atomisation of molecules is reported. It is shown that bond valency is a reliable measure of bond multiplicity in a variety of molecules including cases where these are ambiguous classically.