Originally, oxidation was simply regarded as a chemical reaction with oxygen. The reverse process – loss of oxygen – was called reduction. Reaction with hydrogen also came to be regarded as reduction. Later, a more general idea of oxidation and reduction was developed in which oxidation was loss of electrons and reduction was gain of electrons. This wider definition covered the original one. For example, in the reaction
the sodium atoms lose electrons to give Na+ ions and are oxidized. At the same time, the oxygen atoms gain electrons and are reduced. These definitions of oxidation and reduction also apply to reactions that do not involve oxygen. For instance in
the sodium is oxidized and the chlorine reduced. Oxidation and reduction also occurs at the electrodes in cells.
This definition of oxidation and reduction applies only to reactions in which electron transfer occurs – i.e. to reactions involving ions. It can be extended to reactions between covalent compounds by using the concept of oxidation number (or state). This is a measure of the electron control that an atom has in a compound compared to the atom in the pure element. An oxidation number consists of two parts:
The change of electron control may be complete (in ionic compounds) or partial (in covalent compounds). For example, in SO2 the sulphur has an oxidation number +4, having lost partial control over 4 electrons compared to sulphur atoms in pure sulphur. The oxygen has an oxidation number –2, each oxygen having gained partial control over 2 electrons compared to oxygen atoms in gaseous oxygen. Oxidation is a reaction involving an increase in oxidation number and reduction involves a decrease. Thus in
the hydrogen in water is +1 and the oxygen –2. The hydrogen is oxidized and the oxygen is reduced.
The oxidation number is used in naming inorganic compounds. Thus in H2SO4, sulphuric(VI) acid, the sulphur has an oxidation number of +6. Compounds that tend to undergo reduction readily are oxidizing agents; those that undergo oxidation are reducing agents.