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The oxidation number of an element indicates the number of electrons lost, gained, or shared as a result of chemical bonding. The change in the oxidation state of a species helps us to know if it has undergone oxidation or reduction.
There are several rules for assigning the oxidation number to an element.
1.The oxidation number of an atom in the elemental state is zero.
2.The oxidation number of a mono atomic ion is equal to its charge.
3.The algebraic sum of the oxidation numbers in the formula of a compound is zero.
4.The oxidation number of hydrogen in a compound is 1+, except when hydrogen forms compounds called hydrides with active metals, and then it is 1-.
5.The oxidation number of oxygen in a compound is 2-, except in peroxides when it is 1-, and when combined with fluorine. Then it is 2+.
6.The algebraic sum of the oxidation numbers in the formula for a polyatomic ion is equal to the charge on that ion.