\Va"lence\, n. [From L. valens, -entis, p. pr. of valere
to have power, to be strong. See {Valiant}.] (Chem.)
The degree of combining power of an atom (or radical) as
shown by the number of atoms of hydrogen (or of other monads,
as chlorine, sodium, etc.) with which it will combine, or for
which it can be substituted, or with which it can be
compared; thus, an atom of hydrogen is a monad, and has a
valence of one; the atoms of oxygen, nitrogen, and carbon are
respectively dyads, triads, and tetrads, and have a valence
respectively of two, three, and four.
Note: The valence of certain elements varies in different
      compounds. Valence in degree may extend as high as
      seven or eight, as in the cases of iodine and osmium
      respectively. The doctrine of valence has been of
      fundamental importance in distinguishing the
      equivalence from the atomic weight, and is an essential
      factor in explaining the chemical structures of
      compounds.