In chemistry, valence, also known as valency or valence number, is the number of valence bonds a given atom has formed, or can form, with one or more other atoms. For most elements the number of bonds can vary. The IUPAC definition limits valence to the maximum number of univalent atoms that may combine with the atom, that is the maximum number of valence bonds that is possible for the given element.
The valence of an element depends on the number of valence electrons that may be involved in the forming of valence bonds. A univalent (monovalent) atom, ion or group has a valence of one and thus can form one covalent bond. A divalent molecular entity has a valence of two and can form two sigma bonds to two different atoms or one sigma bond plus one pi bond to a single atom. Alkyl groups and hydroxyl ions are univalent examples; oxo ligands are divalent.
Over the last century, the concept of valence evolved into a range of approaches for describing the chemical bond, including Lewis structures (1916), valence bond theory (1927), molecular orbitals (1928), valence shell electron pair repulsion theory (1958) and all the advanced methods of quantum chemistry.
|In cyclohexanonoxime (image left), the nitrogen atom has three valence bonds and therefore a valence of three.|