Valence, << VAY luhns, >> also called valency, is a number that indicates the ability of a chemical element to combine with other elements. In the past, valence had several slightly different meanings. The term is gradually being replaced by more precise chemical descriptions.
Valence was first defined as the number of hydrogen atoms that can combine with each atom of an element. For example, each atom of oxygen can combine with two hydrogen atoms to form water. Therefore, oxygen has a valence of two. A second definition of valence is based on the charges of ionized atoms. Sodium ions generally have one positive charge, so the valence of sodium is one. A third definition is based on the number of bonds (chemical links) that an atom forms with other atoms. Because carbon atoms usually form four bonds, carbon is said to have a valence of four. Many elements can combine in so many ways that they have several valences. For example, sulfur has common valences of 2, 4, and 6.
