Toward a definition of valence as a quantity (IUPAC Technical Report)

Abstract

Valence has a rich history in chemistry, as a bonding concept, in terms of quantitative context, and as a true quantity. In the latter, a survey preceding this project revealed differing perceptions of valence values and helped formulate candidate definitions. This IUPAC task group evaluated nine quantities behind eight alternative definitions on 39 chemical entities of 48 bonding formulas, each giving a set of meaningful values with mutual relationships. Given the reflection principle of IUPAC normative work, 15 comparative examples with high variation of these alternative valences for an element were selected, and chemistry articles in English searched for valence-termed quantities of the compared compounds to imply the definition behind the stated valence value, the frequency of such use, and the chemistry field. Summarized preferences for the alternative definitions show two main areas of use. Organic and physical chemists count valence as a number of two-electron bonds at the atom. Inorganic chemists working with semi-metallic and metallic elements use n-valent as an adjective for oxidation state. The diverse yet infrequent use cannot be covered by a single definition of the valence quantity. Clarity in articles that use valence as a quantity is essential and achievable by stating the intended context.