The concept of valency is refined and developed. Three types of valency are distinguished : primary or classical, coordinate, and nonclassical. The first relates to simple compounds and to the components of multiple ones, the second to coordination compounds, and the third to transition-metal carbonyls and similar species. Each type is defined, evaluated, and applied to the prediction of the atomic composition of compounds. Relations are established between valency and bond number, charge number, oxidation number, and number of d or f electrons.
The resulting theory has several advantages over electronic theories of bonding. It is simpler, it applies to transition-metal compounds as well as to main-group ones, and it enables chemical formulae to be predicted without reference to the nature of the bonding (e.g. it does not presuppose that metal oxides are ionic, or that atoms in hypervalent compounds do or do not use d orbitals). It also provides a classification of substances that avoids the artificiality involved in the current use of oxidation number, and brings together species that are presently classified separately [e.g. HgI2 and HgIICl2; Fe0(CO)5, FeII(CO)4Cl2, and Fe?II(CO)2(NO)2].
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