Oxidation, any chemical reaction in which an atom of an element loses one or more of its electrons to an atom of a different element. Originally, the term was applied only to a reaction in which oxygen combines with another element or group of elements to form a compound called an oxide.

Familiar examples of oxidation include the burning of coal, which is rapid oxidation; and the rusting of iron, which is slow oxidation. Oxidation also occurs in animals and plants in the process of respiration. The carbon dioxide exhaled from the lungs is formed during this process.

The element whose atoms lose electrons during oxidation is said to be oxidized. The other element, whose atoms gain electrons, is called the oxidizing agent. The oxidizing agent is said to be reduced, and the process of gaining electrons is called reduction. Oxidation and reduction always occur simultaneously, and therefore chemists often use the term oxidation-reduction (or redox) when referring to this type of reaction.

The oxygen in the air serves as the oxidizing agent in most oxidation-reduction reactions. When iron rusts, for example, oxygen in the air combines chemically with the iron to form a coating of iron oxide on the surface of the iron. In this reaction, the iron atoms give up electrons to the oxygen atoms. The iron is oxidized and the oxygen is reduced.

The halogen elements (fluorine, chlorine, bromine, iodine, and astatine) also serve as oxidizing agents. When chlorine combines with sodium to form sodium chloride, or common salt, the sodium atoms give up electrons to the chlorine atoms. In this reaction, the sodium is oxidized and the chlorine is reduced.