Atomic Valence States and Chemical Binding

A very general theory of the electronic structure of molecules is presented, and it is shown how particular instances of it represent the main sources of chemical binding. A method of describing the properties of an atom in a specific molecule is outlined. Particular attention is then paid to the electron-pairing theory of chemical binding, due to Slater and Pauling. It is demonstrated how an atom engaged in the formation of electron-pair bonds may be regarded as being in a non-stationary state. This is Van Vleck's valence state. The importance of the valence state concept is illustrated with reference to certain simple molecules. For example, the part played by the promotional energy of the oxygen atom's valence state in thermochemistry is outlined: whenever the bond additivity assumption is used, bond energies must be referred to atomic valence states rather than to their ground states. This is also important in assessing the excitation energies of certain molecular states, as is exemplified by the NH radical. Tables of valence states are drawn up which may be used in order to derive their promotional energies. The effect of orbital hybridization on the properties of atoms in molecules is illustrated. Finally, Mulliken's electronegativity scale is introduced and it is shown how this too depends on the energies of atomic valence states.