Oxidation and Reduction
Redox reactions in which electrons transfer between species and oxidation states change — the basis of batteries, corrosion, and metabolism.
Oxidation and reduction — redox, for short — are the complementary halves of reactions in which electrons are transferred between species. Oxidation is the loss of electrons; reduction is the gain of electrons (mnemonic: OIL RIG). The two always occur together: an electron donor (the reducing agent) is itself oxidized, and an electron acceptor (the oxidizing agent) is reduced. The concept extends beyond literal electron transfer to changes in oxidation state, a bookkeeping number assigned to each atom that tracks its share of electrons.
Oxidation states are assigned by rules — elements in their standard state have oxidation state 0, monatomic ions carry their charge, and oxygen is usually −2. In Fe₂O₃, iron is +3 and oxygen is −2; in the reaction 2Fe₂O₃ + 3C → 4Fe + 3CO₂, iron is reduced (+3 → 0) while carbon is oxidized (0 → +4). Tracking these numbers reveals which atoms change — even in reactions, such as the combustion of methane, where no free electrons are ever visible.
Redox reactions power technology and life. Batteries and fuel cells separate the oxidation and reduction half-reactions into two electrodes; electrons travel through the external circuit, producing current — the same principles that drive electrical circuits. Corrosion is unwanted redox: iron oxidizes to rust in the presence of oxygen and water. Electroplating, metal extraction from ores, and electrolysis of water all rely on driving redox reactions with electricity.
Biology is built on redox. Cellular respiration oxidizes glucose, passing its electrons through a chain of carriers to oxygen; photosynthesis runs the reverse: it uses light energy to force electrons onto carbon dioxide, reducing it to sugar. In both cases, energy is released or stored in proportion to the electron-transfer steps. Redox is therefore one of the most consequential concepts in chemistry — bridging chemical bonding, electrochemistry, geochemistry, and biochemistry.
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chemistry electrochemistry electrons redox