Oxidation number or oxidation state is the number of electrons that an atom appears to have gained or lost when it is combined with other atoms.

Rules for Assigning Oxidation Numbers

Rule 1: The oxidation number of all elements in free state is zero. This rule is also applied for diatomic or polyatomic elements. Example: The oxidation number of Na = 0, Cu = 0, Cl in Cl₂ = 0, O in O₃ = 0, S in S₈ = 0.

Rule 2: The oxidation number of a monatomic ion is equal to the charge on the ion. Example: Na⁺ = +1, Mg²⁺ = +2, S^2– = –2.

Rule 3: The oxidation number of oxygen in a compound is usually –2 except in the following cases:

Exceptions

The oxidation number of oxygen in:

(i) peroxides is –1.                                       Example: Na₂O₂
(ii) superoxides is –1/2.                                Example: KO₂
(iii) oxygen diflouride is +2.                           Example: OF₂

Rule 4: The oxidation number of hydrogen in its entire compounds is +1 except in metal hydrides, (like NaH, CaH₂ and AlH₃), where its oxidation number is –1.

Rule 5: Elements of group IA have +1 and group IIA have +2 oxidation states in all of their compounds.

Rule 6: In a compound, the more electronegative element is assigned a negative oxidation number, and the less electronegative element is assigned a positive oxidation number.