The basic event that occurs during a redox reaction is the exchange of electrons between two different elements or compounds. The chemicals that lose electrons during such reactions are said to be oxidized, while the chemicals that gain electrons are said to be reduced. Oxidation in one chemical never takes place without reduction in the other and vice versa, since electrons always move to a nearby receptor.
Redox reactions are extremely common in nature, especially in the oxidation of metals and other minerals. Oxidation reactions were originally named after the bonding of oxygen that occurred, which was the most typical observed reaction in which chemicals lost electrons. The term was soon extended to include any reaction in which electrons were lost by one chemical to another. The chemical that gains electrons is known as the oxidizing agent, while the chemical that loses electrons is known as the reducing agent.
Predicting a redox reaction depends on knowing the oxidation number of a given element or compound. The most frequent chemical is elemental oxygen, with an oxidation number of minus 2. To balance a redox reaction involving oxygen, the reducing agent must have a positive oxidation number equal to the sum of the oxidation numbers of any oxygen atoms in the reaction.