// CHEMISTRY L4 — REDOX
Follow the electrons.
Rusting, burning, batteries, respiration — all the same deal underneath: electrons moving from one atom to another. Oxidation numbers are the bookkeeping that tracks them. Tap a compound to audit it.
// FIG. 01 — OXIDATION-NUMBER AUDITOR
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SUM OF OXIDATION NUMBERS = {{ sumLabel }} · (must equal the overall charge)
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// OIL RIG — THE ONE RULE TO REMEMBER
OIL — Oxidation Is Loss
A species is oxidised when it loses electrons. Its oxidation number goes up. Magnesium burning: Mg → Mg²⁺ + 2e⁻ (0 → +2).
RIG — Reduction Is Gain
A species is reduced when it gains electrons. Its oxidation number goes down. Oxygen: O₂ + 4e⁻ → 2O²⁻ (0 → −2).
The two always happen together — electrons lost by one species are gained by another — which is why we call it redox (REDuction + OXidation). The species that causes oxidation by grabbing the electrons is the oxidising agent (and is itself reduced); vice versa for the reducing agent.
// THE RULES FOR ASSIGNING OXIDATION NUMBERS
// HALF-EQUATIONS
A redox reaction splits neatly into two half-equations — one showing the oxidation (electrons on the right, being lost), one the reduction (electrons on the left, being gained). Balance the electrons in each, then add them so the electrons cancel. For zinc displacing copper: Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu combine to Zn + Cu²⁺ → Zn²⁺ + Cu. The electrons never appear in the overall equation — they've simply changed owner.
// REFERENCES & FURTHER READING
Cross-check against your own exam board's specification (AQA, Edexcel, OCR, SQA).