Any process in which an element loses electrons during a chemical reaction is called oxidation. A reactant is oxidized if it loses electrons. Note that the modern definition of oxidation is much broader than the original meaning. Oxygen doesn't always have to be present in order for an element to lose electrons. For example, when sodium reacts with chlorine, each neutral sodium atom loses one electron and becomes a sodium ion, Na⁺.

Reduction As calcium atoms lose electrons during the synthesis of calcium oxide, the oxygen atoms gain electrons. As each neutral oxygen atom gains two electrons, it becomes an ion with a charge of 2−.

$\mathrm{O} + 2{\mathrm{e}}^{- }\to {\mathrm{O}}^{2-}$

The process in which an element gains electrons during a chemical reaction is called reduction. A reactant is said to be reduced if it gains electrons.

Oxidation and reduction always occur together. When one element loses electrons, another element must gain electrons. Note that oxidation-reduction reactions do not always involve complete transfers of electrons. For example, in the synthesis of water, hydrogen is oxidized as it partially loses electrons. Oxygen is reduced as it partially gains electrons.

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