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(August 2011)| 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