Oxidation Numbers

A bookkeeping trick that tells you, at a glance, which atoms gained electrons and which lost them.

High schoolIntro Gen ChemUni Year 1
⏱️ About 16 min

Rust, batteries, respiration, bleach — they all come down to electrons hopping from one atom to another. But electrons are invisible and hard to track. Oxidation numbers are a simple accounting system that lets you follow every electron through a reaction, without ever seeing one.

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The big idea: An oxidation number is a charge we assign to each atom by a fixed set of rules, pretending every bond is fully ionic. Once each atom has a number, a reaction reveals itself instantly: an atom whose number goes UP was oxidized (lost electrons); one whose number goes DOWN was reduced (gained electrons).
🎯 By the end, you'll be able to
  • State the oxidation-number rules in priority order
  • Assign an oxidation number to any atom in a compound or ion
  • Identify which element is oxidized and which is reduced in a reaction
  • Connect rising/falling oxidation number to loss/gain of electrons (OIL RIG)
📎 Helpful to know first

Redox is electron transfer

A redox reaction is any reaction in which electrons move from one substance to another. Two things always happen together:

  • Oxidation = loss of electrons.
  • Reduction = gain of electrons.

The classic memory hook is OIL RIG: Oxidation Is Loss, Reduction Is Gain. One substance can't give electrons away unless another accepts them, so oxidation and reduction are two halves of the same event — you never get one without the other.

⚠️ The name is backwards from the meaning
It feels natural to guess that 'oxidation' means gaining something. It does not. Oxidation is losing electrons. The word comes from reactions with oxygen, but the modern definition is all about electrons — and it is loss, not gain. Burn this in now; it trips up more students than any other point.

What an oxidation number really is

An oxidation number (or oxidation state) is the charge an atom would have if we handed every shared electron to the more electronegative atom in each bond. It's a deliberate pretend: we treat every bond as if it were fully ionic, just so we can count. The number isn't a real charge — it's a bookkeeping label that makes electron transfer visible.

🔑 The rules, in priority order
Apply these top-down; if two rules ever conflict, the higher one wins.
  • A free element is 0 (e.g. Na, O₂, P₄, Fe).
  • A monatomic ion equals its charge (Na⁺ is +1, S²⁻ is −2).
  • Fluorine is always −1 in compounds.
  • Oxygen is usually −2 (exceptions: peroxides −1, and with F it's positive).
  • Hydrogen is usually +1 (but −1 in metal hydrides like NaH).
  • The oxidation numbers in a neutral compound sum to 0; in a polyatomic ion they sum to the ion's charge.
\[ \sum (\text{oxidation numbers}) = \text{overall charge} \]
For a neutral molecule the sum is 0; for an ion it equals the ion's charge. This is the equation you actually solve.
📝 Worked example: Find the oxidation number of sulfur in sulfate, SO₄²⁻.
  1. Oxygen is −2 (the usual rule). There are 4 oxygens: 4 × (−2) = −8.
  2. Let sulfur be x. The atoms must sum to the ion's charge, −2: x + (−8) = −2.
  3. Solve: x = −2 + 8 = +6.
✓ Sulfur is +6 in sulfate.
✨ Reading a reaction in one glance
Assign numbers to both sides, then compare. In Zn + Cu²⁺ → Zn²⁺ + Cu: zinc goes 0 → +2 (number UP → oxidized, lost electrons), while copper goes +2 → 0 (number DOWN → reduced, gained electrons). The atom that is oxidized is the reducing agent; the one reduced is the oxidizing agent.
📝 Worked example: In the reaction 2Na + Cl₂ → 2NaCl, which element is oxidized and which is reduced?
  1. Reactants are free elements, so Na is 0 and Cl is 0.
  2. In NaCl, sodium is +1 and chlorine is −1.
  3. Sodium 0 → +1: number rose, so sodium is oxidized (lost an electron).
  4. Chlorine 0 → −1: number fell, so chlorine is reduced (gained an electron).
✓ Sodium is oxidized; chlorine is reduced. (Na is the reducing agent, Cl₂ the oxidizing agent.)
✏️ Practice: What is the oxidation number of manganese in the permanganate ion, MnO₄⁻? (Oxygen is −2; the four oxygens plus Mn must total the −1 charge.)
(i.e. +7)
Solution
  1. Four oxygens: 4 × (−2) = −8.
  2. Let Mn = x. Sum equals the ion charge: x + (−8) = −1.
  3. x = −1 + 8 = +7. (Enter 7.)

Check your understanding

1. Oxidation is best defined as:
Oxidation Is Loss of electrons (OIL RIG). It shows up as an INCREASE in oxidation number. Gaining electrons is reduction.
2. What is the oxidation number of chromium in Cr₂O₇²⁻ (dichromate)?
Seven O at −2 give −14. For two Cr: 2x + (−14) = −2, so 2x = +12 and x = +6 each.
3. In O₂, the oxidation number of each oxygen atom is:
O₂ is a free element, and any free element has an oxidation number of 0 — the '−2 for oxygen' rule only applies inside compounds.
🔎 Related lessons
Next: Balancing Redox Reactions Galvanic Cells & Cell Potential
✅ Key takeaways
  • Redox = electron transfer; OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons.
  • Oxidation numbers are assigned by fixed rules: element = 0, monatomic ion = charge, O usually −2, H usually +1.
  • The oxidation numbers in a species sum to its overall charge — that's the equation you solve.
  • Number goes UP = oxidized (lost e⁻); number goes DOWN = reduced (gained e⁻).
  • The species oxidized is the reducing agent; the species reduced is the oxidizing agent.
➡️ Now that you can spot who gained and lost electrons, the next challenge is making the electrons balance in a full equation. The half-reaction method turns that into a tidy, step-by-step recipe — coming up next.
Want to test yourself on this? Try the Chemistry practice test →