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.