Redox reactions

The word "oxidation" is a historical fossil. It was coined for reactions with oxygen, but the underlying process is electron loss, and oxygen is merely a common and enthusiastic electron acceptor. Sodium burning in chlorine, iron dissolving in acid and a battery discharging are all oxidation with no oxygen anywhere. The modern definition — oxidation is loss of electrons, reduction is gain — is the one that generalises.

Because electrons are neither created nor destroyed, the two halves are inseparable. Every electron lost by one species is gained by another, so oxidation and reduction occur together and in exactly matched amounts. It is often useful to write the two half-reactions separately and then scale them so the electrons cancel, which is also how redox equations get balanced.

The terminology inverts in a way that reliably catches people. The oxidising agent is the substance that causes oxidation in something else — which it does by taking electrons, so the oxidising agent is itself reduced. The reducing agent is oxidised. Agent names describe what a species does to its partner, not what happens to it. Keeping that straight matters because the whole of electrochemistry is built on it: in a battery, separating the two half-reactions and forcing the electrons to travel through an external wire is exactly what turns a chemical reaction into a usable current.