Oxidation–Reduction (Redox) Basics
- Assign oxidation numbers using the standard rules
- Identify which species is oxidized and which is reduced in a redox reaction
- Recognize the oxidizing agent and the reducing agent
Oxidation numbers track electrons
An oxidation number is the charge an atom would have if every bond were fully ionic — a bookkeeping tool for electrons. The rules, in priority order: a free element is 0 (e.g. Zn, O₂); a monatomic ion equals its charge (Na⁺ is +1); oxygen is usually −2 (except −1 in peroxides); hydrogen is +1 with nonmetals (−1 with metals); group 1 is +1, group 2 is +2, fluorine is −1. Finally, the oxidation numbers in a neutral compound sum to 0, and in a polyatomic ion they sum to the ion’s charge.
OIL RIG: who loses, who gains
Oxidation Is Loss of electrons (oxidation number goes up); Reduction Is Gain of electrons (oxidation number goes down). The two always happen together — electrons freed by one species are grabbed by another. The species that gets reduced is the oxidizing agent (it oxidizes its partner); the species that gets oxidized is the reducing agent. Agents cause the opposite of what happens to them.
What is the oxidation number of manganese in the permanganate ion, MnO₄⁻?
- 1.Assign the known atom first: each O is −2, and there are 4 of them, contributing 4 × (−2) = −8.
- 2.The whole ion has a charge of −1, so all oxidation numbers must sum to −1.
- 3.Set up: Mn + (−8) = −1.
- 4.Solve: Mn = −1 + 8 = +7.
Assign the atoms you are sure of first (O = −2, H = +1, group 1/2), then let the sum rule solve for the mystery atom. Transition metals like Mn, Cr, and Fe take variable oxidation numbers, so they are almost always the unknown you solve for.
What is the oxidation number of sulfur in the sulfate ion, SO₄²⁻?
In the reaction Zn + Cu²⁺ → Zn²⁺ + Cu, which species is oxidized?
To name agents, first find what is oxidized and what is reduced, then flip the labels: the reduced species is the oxidizing agent, the oxidized species is the reducing agent. Mixing these up is the single most common redox error on the AP exam.
Answer the 2 checkpoints as you read.
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