The Role of Oxidation Numbers in Oxidation-Reduction Reactions Chemists eventually extended the idea of oxidation and reduction to reactions that do not formally involve the transfer of electrons. As can be seen in the figure below, the total number of electrons in the valence shell of each atom remains constant in this reaction. What changes in this reaction is the oxidation state of these atoms. Oxidation and reduction are therefore best defined as follows. Oxidation occurs when the oxidation number of an atom becomes larger.
Reduction occurs when the oxidation number of an atom becomes smaller.
Nothing happens to the oxidation number of the oxygen in this reaction, which means that oxygen is neither oxidized nor reduced. Hydrogen is therefore reduced in this reaction. The terms ionic and covalent describe the extremes of a continuum of bonding. There is some covalent character in even the most ionic compounds and vice versa. It is useful to think about the compounds of the main group metals as if they contained positive and negative ions.