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We may classify all the elements in nine groups, according to their valencies. The same element may fall into several groups, since the valency is not a constant for an element but may vary according to the nature of the other element with which it is combined, e.g., with hydrogen (HCl, H2S), or with oxygen (Cl2O7, SO3). These groups are as follows:
- Zero-valent elements: inactive gases, radioactive emanations.
- Univalent elements: hydrogen, halogens, nitrogen in nitrous oxide, N2O, alkali-metals, silver, mercury in mercurous compounds (HgCl, Hg2O), copper in cuprous compounds (CuCl, Cu2O), gold in AuCl and Au2O.
- Bivalent elements: oxygen, nitrogen in nitric oxide, NO, sulphur in H2S, alkaline-earth metals (Ca, Sr, Ba), magnesium, zinc, cadmium, mercury in mercuric compounds (HgCl2, HgO), copper in cupric com pounds (CuCl2, CuO), tin in stannous compounds (SnCl2, SnO), lead in plumbous compounds (PbCl2, PbO), iron in ferrous compounds (FeCl2, FeO), platinum in platinous compounds. A large number of elements are bivalent.
- Tervalent elements: boron, nitrogen in NH3 and NCl3, phos phorus in PH3 and PCl3, arsenic in AsH3, AsCl3, As2O3, aluminium, iron in ferric compounds (FeCl3, Fe2O3), antimony, bismuth, gold in AuCl3.
- Quadrivalent elements: carbon, chlorine in ClO2, silicon, nitrogen in nitrogen dioxide, NO2, sulphur in SO2, lead in plumbic compounds (PbCl4, PbO2), tin in stannic compounds (SnCl4, SnO3), platinum in platinic compounds.
- Quinquevalent elements: phosphorus, arsenic, and antimony in higher halogen or oxygen compounds (PCl5, As2O5, SbCl5), chlorine in chloric acid, HClO3, and chlorates.
- Sexivalent elements: sulphur in SF6 and SO3, tungsten in WCl6, manganese in manganese trioxide, MnO3, and manganates, K2MnO4.
- Septavalent elements: chlorine in Cl2O7, iodine in periodates, KIO4, manganese in Mn2O7 and permanganates, KMnO4.
- Octovalent elements: osmium in OsO4 and OsF2, ruthenium in RuO4.
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