In order to keep track of electron transference during oxidation-reduction reactions, and to help us identify the substances that are oxidized and reduced, we use the concept of oxidation number. The oxidation number is a measure of the extent of oxidation of an element in its compounds and is assigned by using a set of simple rules:

(i)    An element in any of its allotropic forms is assigned an oxidation number of zero. For examples Oxidation No. of P in white phosphorus or red phosphorus is zero. Oxidation No. of S in monoclinic, rhombic, plastic or colloidal sulphur is zero.

(ii)    The oxidation number of a monatomic ion is equal to its charge.
The Na⁺ is assigned an oxidation number + 1 and magnesium ion, Mg²⁺ is assigned a number equal to + 2 Singly charged monatomic, negative ions such as CI^- are  assigned an oxidation number of -1 and doubly charged negative ions such as O^{2 -}  have an oxidation number of -2.

(iii)    In any neutral molecule the sum of the oxidation numbers is zero: in an ion sum of the oxidation number is equal to the charge on the ion.

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