Explain electronically that oxidation and reduction always occur simultaneously and balance the given reaction

edox reactions are comprised of two parts, a reduced half and an oxidized half, that always occur together.

The reduced half gains electrons and the oxidation number decreases, while the oxidized half loses electrons and the oxidation number increases.

this happens because for oxidation to take place the electrons lost by the substance that is oxidized must be accepted by atoms or ions of another substance. 
Also, for reduction to occur the electrons gained by the substance that is being reduced must be transferred from the atoms of ions of another substance. 
Therefore, oxidation and reduction are complementary processes: that is to say that oxidation cannot occur without reduction and visa versus.