valency Definition

valency
valence
noun valencies, valences

1. chem.
A positive or negative whole number that denotes the combining power of an atom of a particular element, equal to the number of hydrogen atoms or their equivalent with which it could combine to form a compound. For example, in water (H2O), oxygen has a valency of two.

Etymology: Late 19c in this sense; 17c in obsolete sense ‘power' or ‘strength', from Latin valentia strength or capacity.