Oxidation Number

Oxidation number is the apparent charge which an atom appears to have when each pair of electrons is counted with more electronegative atom. Oxidation number is always assigned to an atom. It is a number written with +ve or –ve sign.

The number indicates the number of electrons that has been shifted from an atom towards a more electronegative atom. The +ve sign is for the atom shifting its electron away from itself and –ve is given to more electronegative atom.

Rules for Assigning Oxidation Number

There are certain rules that are followed for computing the oxidation number of an atom in a molecule or ion.

  1. Oxidation number is taken as zero if atoms are present in elemental form. For example, O2, Na, P4 are elemental forms. They have oxidation number zero.
  2. The oxidation number of a mono-atomic ion is the same as the charge present on it. For example, Na+, Mg2+, Al3+, Cl–, S2– have oxidation number of +1, +2, +3, –1, –2 respectively.
  3. The oxidation number of oxygen is –2 in almost all the compounds except in peroxides (Na2O2, H2O2; oxidation number is –1) and super oxides (KO2; oxidation number is -½).
  4. The oxidation number of H is +1 when combined with non-metal and is –1 when combined with metal. For example, in HCl the Oxidation Number of H is +1 but in CaH2, it is –1.
  5. The Oxidation Number of alkali metal is +1 in its compounds.
  6. In a compound made up of different elements, the more electronegative element will have negative oxidation number and less electronegative atoms will have positive oxidation number. For example, in NCl3, N has +3 oxidation number and Cl has –1 oxidation number.
  7. The sum of the oxidation numbers of all the atoms in a neutral compound is zero.
  8. In a poly-atomic ion, the sum of the oxidation numbers of all the atoms is equal to the charge on the ion. For example, in CO32-, the sum of oxidation Number of carbon and oxygen is –2.