The word oxidation originally meant combining a substance with oxygen, because early chemists studied reactions like rusting and burning. Chemists now define oxidation more broadly, based on electron transfer rather than oxygen alone. When a species loses electrons, its oxidation state increases, becoming more positive.
The substance that takes those electrons is called the oxidizing agent. In gaining electrons, the oxidizing agent itself gets reduced. Oxidation reactions drive many familiar processes.
Burning fuel releases energy as carbon and hydrogen atoms lose electrons to oxygen. Metal corrosion, like rust forming on a bike chain, is a slow oxidation reaction. Inside living cells, oxidation reactions break down glucose to release energy the body can use, a process called cellular respiration.
Chemists track oxidation using oxidation numbers, which assign a charge-like value to each atom in a compound based on electronegativity rules. An increase in oxidation number signals that oxidation has occurred, even in reactions that do not involve simple ion formation. Redox reactions also power batteries, where a spontaneous oxidation-reduction reaction at separate electrodes generates a flow of electrons through an external circuit.
Balancing redox equations often requires tracking electrons separately in two half-reactions, one for oxidation and one for reduction, before combining them into a single balanced equation.