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How to balance a chemical reaction using oxidation number method?

charged mono atomic ion has oxidation number equal to its charge. Thus Na+ has oxidation number +1, Fe3+ has oxidation number +3, F- has oxidation number of -1 and S2- has oxidation number of -2.
Second rule says that the oxidation number of a free element is always 0. Thus oxidation number of solid, metallic Cu is 0, oxidation number of O in O2 is 0, the same holds for S in S8 and so on.
Oxygen in almost all compounds has oxidation number -2.
Hydrogen in almost all compounds has oxidation number +1.
Some elements usually have the same oxidation number in their compounds:

alkali metals - Li, Na, K, Rb, Cs - oxidation numbers are +1
alkaline earth metals - Be, Mg, Ca, Sr, Ba - oxidation numbers are +2
halogens (except when they form compounds with oxygen or one another) - oxidation numbers are -1 (always true for fluorine)

Last rule says that the charge of the ion or molecule equals sum of oxidation numbers of all atoms.

There are some exceptions to the rules 3 and 4 - for example oxygen in peroxides has oxidation number of -1, it is also not -2 in compounds with fluorine (where F is always -1), hydrogen in hydrides has oxidation number -1. what is oxidation number of sulfur in SO2? Particle is not charged, so oxidation number of sulfur must equal sum of oxidation numbers of oxygens, but with the opposite sign. Oxygen oxidation number is -2, there are two oxygens - that gives -4 together, so sulfur must have ON=+4.
What is oxidation number of atoms in CrO42-? Oxygen is -2 and there are 4 oxygens - that gives overall of -8, ion has charge of -2, so central atom must have ON=+6.
How do we use oxidation numbers for balancing? First of all, we have to understand that oxidation means increase of oxidation number, while reduction means decrease of oxidation number. In both cases change of oxidation number is due to electrons lost (oxidation) or gained (reduction). We calculate oxidation numbers for all atoms present in the reaction equation (note that it is not that hard as it sounds, as for most atoms oxidation numbers will not change) and we look for a ratio that makes the number of electrons lost equal to the number of electrons gained. That gives us additional information needed for reaction balancing.


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