Oxidation-reduction or redox reactions involve the transfer of electrons from one molecule or atom to another. When an atom gains an electron, another atom must lose an electron, meaning oxidation and reduction must occur together. Since the redox occurs in pairs, the atom that gets oxidized is also called the reducing agent or reductant, and the atom that is reduced is also called the oxidizing agent or oxidant. A straightforward way to remember the definitions of oxidation and reduction is through the phrase OIL-RIG, which stands for: Oxidation Is Losing– Reduction Is Gaining (electrons).

Oxidation states

A redox reaction changes the atom's oxidation states. The oxidation state or number refers to the charge an atom would have if each of its bonds to other elements were purely ionic. Determining whether there are changes in the oxidation numbers of atoms in one or more reactants can help determine whether a reaction is an oxidation-reduction reaction.

In the reaction between sodium and chlorine to yield sodium chloride, sodium becomes oxidized, and chlorine becomes reduced,

It is helpful to view the process that happens to each reactant separately, that is, to represent the fate of each reactant in the form of an equation called a half-reaction:

These equations show that Na atoms lose electrons, going from an oxidation number of 0 to 1+, while Cl atoms (in the Cl₂ molecule) gain electrons, going from an oxidation number of 0 to 1-.

Incomplete Transfer of Electrons

A redox reaction can occur not only when an electron gets transferred but also when there is a change in the sharing of the electrons in a covalent bond. For example, methane and oxygen can react to form carbon dioxide and water. The electrons in methane are shared equally between carbon and hydrogen, but carbon dioxide's carbon is partially positive since oxygen attracts electrons more than carbon. Carbon dioxide is losing electrons, becoming oxidized, and oxygen is gaining electrons, becoming reduced.

This text is adapted from Openstax, Chemistry 2e, Section 4.2: Classifying Chemical Reactions.