OIL RIG becomes oxidation states, and redox becomes relative

Oxidation is gain of oxygen or loss of electrons; reduction the reverse. OIL RIG. Applied mainly to metal displacement and simple electrolysis.

Oxidation state is a bookkeeping number assigned by rules based on electronegativity (Structure 3.1.6), which works even where no electrons are fully transferred (carbon in CH₄ is −4; in CO₂ it is +4). Redox is then any change in oxidation state. Half-equations (Reactivity 3.2.2) split the process and must be balanced for atoms and charge, in acid using H⁺ and H₂O. Oxidising/reducing agents are defined relationally: the oxidising agent is itself reduced. Disproportionation appears. Organic oxidation/reduction is folded in (3.2.9–3.2.11) — alcohol → aldehyde → carboxylic acid is a redox sequence, defined by loss of H / gain of O. And at HL, standard electrode potentials E⦵ quantify oxidising/reducing strength on a single scale (3.2.12–3.2.14).

Redox stops being about oxygen and stops being about literal electron transfer. It becomes a formal bookkeeping system that applies to covalent compounds. And it becomes comparative — nothing is an oxidising agent absolutely, only relative to a partner (RSC calls this the threshold concept for post-16 redox: "it's all relative").

Oxygen-based definitions are historically primary and intuitive; oxidation states require electronegativity.

Confusing oxidising agent with "the thing being oxidised" (RSC: "oxidising and reducing agents are often muddled up"); confusing halogen oxidising power (decreases down group) with halide reducing power (increases down group) — RSC flags this specifically; inability to assign oxidation states in polyatomic ions; inability to balance half-equations with H⁺/H₂O; not recognising alcohol oxidation as redox.