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If we examine a series of oxygen compounds: Na2O (Ca2O2) Al2O3 (C2O4) P2O5 (S2O6) Cl2O7 (Os2O8) we see that two additional higher valencies, 7 and 8, appear. Chlorine is septavalent and osmium octovalent, in their highest oxides. (The formulae of CaO, CO2, SO3, and OsO4 have been doubled for clearness.) The number of atoms of oxygen combining with two atoms of an element is a measure of the valency of the latter, since oxygen is bivalent. The valency of 8, shown in the oxygen series, and there only in the compounds osmium and ruthenium tetroxides, OsO4, and RuO4, is the highest value ever exhibited. The inactive gases argon, helium, etc., form no compounds with any elements, and their valency is zero. We have therefore, in all, eight valencies shown by various elements, viz., 1, 2, 3, 4, 5, 6, 7, and 8.
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