Oxidation Number

The electron-transfer picture in Section 6.1.2 describes redox reactions cleanly when the species involved are genuinely ionic, with a complete transfer of one or more whole electrons from one species to another. But many everyday reactions that still satisfy the classical (combination-with-oxygen-type) definition of oxidation involve only covalent bonds, where electrons are shared rather than fully transferred -- for example 2H2(g) + O2(g) → 2H2O(l) is an oxidation of hydrogen by combination with oxygen, and H2(g) + Cl2(g) → 2HCl(g) is likewise an oxidation of hydrogen by combination with the electronegative element chlorine, yet the products of both, H2O and HCl, are polar covalent molecules, not ionic compounds -- so there is only a partial shift of electron density toward the more electronegative atom (O or Cl), not a full transfer of a whole electron. To describe ionic and covalent redox reactions with one single, consistent bookkeeping method, chemists developed the oxidation-number method. It makes one deliberate simplifying assumption: the shared electron pair of a covalent bond is treated as if it belonged completely to the more electronegative atom of the two, exactly as if the bond were ionic. Under this assumption, the oxidation number of an element in a compound is …