Chemical reactions · Topic 10 of 15

Oxidation-Reduction

Tap any line to see why it matters

Chemistry · redox

Oxidation-Reduction

Oxidation is losing electrons, reduction is gaining them, and they always happen together; the reducing agent is oxidized and the oxidizing agent is reduced.

01 · OxidationOxidation = losing electrons

02 · ReductionReduction = gaining electrons

03 · AgentsReducing agent = the one oxidized

What it is

Oxidation = loss of electrons (oxidation number goes up) must know

Reduction = gain of electrons (oxidation number goes down) must know

Oxidation and reduction always happen together must know

Oxidation number = the charge an atom would have if fully ionic

The rules

Reducing agent is oxidized; oxidizing agent is reduced must know

Uncombined element = 0 (Fe, O₂, Cl₂)

Monatomic ion = its charge; group 1 = +1, group 2 = +2

Oxygen usually −2 (peroxide −1); hydrogen usually +1

Sum = 0 in a neutral compound; sum = the charge in an ion

Worked examples

KMnO₄: +1 + Mn + 4(−2) = 0 → Mn = +7

Zn + Cu²⁺ → Zn²⁺ + Cu: Zn oxidized, Cu²⁺ reduced

NO₃⁻: N + 3(−2) = −1 → N = +5

Oxidized or reduced?

Commonly confused

	Oxidation	Reduction
Electrons	Lost	Gained
Oxidation number	Goes up	Goes down
The substance acts as the…	Reducing agent	Oxidizing agent
Mnemonic	OIL · LEO	RIG · GER
In Zn + Cu²⁺ → Zn²⁺ + Cu	Zn (0 → +2)	Cu²⁺ (+2 → 0)

Common mistakes

Reduced = gained electrons (its oxidation number is reduced)

The oxidizing agent oxidizes something else and is itself reduced

Also know

OIL RIG / LEO GER: oxidation loses e⁻, reduction gains e⁻

More detail

Show 5 more facts

Half-reactions: one shows e⁻ lost, the other e⁻ gained

Rusting, burning and cellular respiration are redox reactions

F always −1; Cl, Br, I usually −1 (not with O or another halogen)

Hydrogen is −1 with metals (NaH)

Oxidation number +2; ion charge 2+ (sign placement differs)

Pearl

OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).

Sources · OpenStax Chemistry 2e §4.2 Classifying Chemical Reactions (oxidation-reduction, oxidation numbers) · LibreTexts Introductory Chemistry: Atoms First (Fresno City College) 10.7 Oxidation and Reduction — Some Definitions · Union Test Prep — HESI Chemistry study guide, page 3 (redox, oxidation numbers) · NurseHub — HESI A2 Chemistry course outline (Oxidation and Reduction lesson) · TSTC Library — HESI A2 guide (Chemistry: identify oxidation/reduction reactions) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

Practice this topic, then test the whole section.

Start HESI prep free

HESI and HESI A2 are registered trademarks of Elsevier. NurseSavvy is not affiliated with or endorsed by Elsevier.