Oxidation is losing electrons. Reduction is gaining them. They always happen together.
OIL RIG: Oxidation Is Loss, Reduction Is Gain. Electrons don't vanish, so if one species loses them another has to take them. Oxidation numbers are the ledger: a bookkeeping charge assigned to every atom so you can see who lost and who gained.
Oxidized (lost e⁻)
Oxidation number goes up. It's the reducing agent, because it gave electrons away to reduce something else.Reduced (gained e⁻)
Oxidation number goes down. It's the oxidizing agent, because it took electrons and oxidized something else.Assigning oxidation numbers
- An element by itself (Fe, O₂, Cl₂) is 0. A monatomic ion equals its charge (Na⁺ = +1).
- F is always −1. Group 1 metals are +1, Group 2 metals are +2, and Al is +3.
- H is +1 with nonmetals and −1 with metals (hydrides, like NaH).
- O is −2, except in peroxides (H₂O₂, where it's −1) and when bonded to F.
- Cl, Br, and I are −1, unless they're bonded to O or a more electronegative halogen.
- Everything adds up to the overall charge: 0 for a compound, the ion's charge for an ion. Solve for whatever's left.
Oxidation number calculator
Who got oxidized, who got reduced?
Will this metal react?
A metal can be oxidized by the ion of any metal below it in the series. Metals above H₂ react with acids to release hydrogen gas. Metals below H₂ won't.
Assign oxidation numbers
Activity series
Corrosion is unwanted redox.
Iron rusts because Fe → Fe²⁺/Fe³⁺ is favorable and O₂ is happy to take the electrons. Stainless steel resists it because its chromium oxidizes first, forming a thin Cr₂O₃ layer that seals the surface (passivation). Anything that strips or breaks that layer, like chlorides, strong oxidizers, or reducing acids, re-exposes the iron underneath.
The activity series also explains galvanic corrosion. When two different metals touch in a wet environment, the more active one gets oxidized first. That's why zinc coatings protect steel: zinc is higher in the series and sacrifices itself.
Common mistakes
- Mixing up oxidizing agent and the thing oxidized. The oxidizing agent is the one that gets reduced.
- Forgetting the overall charge on polyatomic ions. In SO₄²⁻, S is +6 (not +8) because the ion carries −2.
- Giving O a −2 in H₂O₂. It's a peroxide, so O is −1.
- Assuming fractional oxidation numbers are wrong. Fe₃O₄ averages +8/3 because it's a mix of Fe²⁺ and Fe³⁺.
- Calling every reaction redox. Precipitation and neutralization reactions don't change any oxidation numbers.