The quantivalence or valency of the atoms; the number of combination bonds, or bonds of affinity, possessed by the atoms of any substance. Thus two atoms of hydrogen combine with one atom of oxygen, and three of oxygen with one of sulphur, forming saturated compounds. Therefore, taking hydrogen as of single atomicity or a monad, oxygen is of double atomicity or a dyad, and sulphur is of six-fold atomicity, or a hexad. The elements are thus classified into seven orders of atomicities, thus:   1, Monads or Univalent elements,   Hydrogen, etc.
 2, Dyads or Bivalent        "      Oxygen, etc.
 3, Triads or Trivalent      "      Nitrogen, etc.
 4, Tetrads or Quadrivalent  "      Lead, etc.
 5, Pentads or Quinquivalent "      Phosphorous, etc.
 6, Hexads or Sexivalent     "      Chromium, etc.
 7, Heptads or Septivalent   "      Chromium, etc. The same element often possesses several atomicities. Barium is generally a dyad, sometimes a tetrad; nitrogen acts as a monad, dyad, triad, tetrad and pentad. The familiar electrolysis of water, giving two volumes of hydrogen to one of oxygen, is one of the illustrations of the theory indicating that two atoms of hydrogen are combined with one of oxygen.