The number of effective charge present on an element of atom, ion or molecule in called oxidation number of that element. If oxidation number of an element on ion, atom or molecule increases, then oxidation takes place. Whereas if the oxidation number of element decreases, reduction takes place. The following steps are used for balancing equation by oxidation number method:
Step I First write the oxidation number of each element above its symbol.
Step II Identify the oxidation number of each element above its symbol.
Step III Identify the elements which undergo change in oxidation number.
Step IV Calculate the increases in oxidation number per atom. If more than .one atom of the same element is involved, find out the total number of increase or decrease in oxidation number.
Step V Equate the increase in oxidation number with decrease in oxidation number on reactant side by multiplying the formula of oxidizing and reducing agent.
Step VI Balance all other atoms except H and O.