Stoichiometric valence and structural valence--two different sides of the same coin: "bonding power"

Recent studies use the term valence to describe two distinct aspects of the phenomenon bonding power of an atom. Measured in valence units, one valence term, the classical chemical valence, has integer values and is derived solely from the composition of a compound. The second one, used mainly by solid-state physicists and crystallographers, has non-integer values. It is determined from structure data, which are derived from diffraction experiments, spectroscopy, or quantum-chemical calculations. To distinguish clearly between these two types of valencies, the descriptive terms stoichiometric valence and structural valence and the respective symbols (stoich)V and (struct)V should be used. For the majority of crystalline structures, values of (stoich)V and (struct)V, both measured in valence units, differ by less than 5%. However, for p-block atoms with one lone electron pair, differences between (stoich)V and (struct)V of up to 30% have been reported.