What is oxidation number?
The charge left on an atom in a molecule or ion, when all the bonding electrons are broken, with the charge assigned to the most electronegative atom.
Oxidation number and oxidation state are fictitious entities. They are a method of electron counting, and have no real physical significance. The use of oxidation numbers can be very helpful when balancing chemical equations, which I suppose is why it is still included on a chemistry syllabus.
Let's take a very simple example: the oxidation of iron metal by concentrated hydrochloric acid. Note, that there is nothing fundamental in what I am doing. The method of redox equations will NOT help predict reactivity; it is a way of ACCOUNTING for OBSERVED reactivity. Chemistry is still an experimental science, and the experiment is always done first.
If iron filings are placed in hydrochloric acid, the filings will foam and, eventually, will go up into solution. We say that the iron is oxidized up to
Iron metal is zerovalent: it is neutral with an oxidation number of
In the final redox equation we add (i) and (ii) so that electrons (which are after all conceptual entities) do not appear in the final redox equation:
Equally, I could have written:
Importantly, in all the reactions given, both MASS and CHARGE were balanced, as they must be in any chemical equation.
Try here for more examples.