Redox is about electrons moving, but in a covalent molecule nobody owns them outright. Oxidation numbers are a bookkeeping system that pretends every bond is ionic, and that lets you track oxidation and reduction in any reaction.
An oxidation number is the charge an atom would have if all its bonds were ionic. Elements are 0; a simple ion has its charge; the numbers in a compound add up to 0, and in an ion to the ion's charge. Oxygen is usually −2, hydrogen +1 and fluorine always −1. Oxidation is loss of electrons, an increase in oxidation number; reduction is gain of electrons, a decrease. An oxidising agent is itself reduced; a reducing agent is itself oxidised.
Pick species in the simulator and assign each atom's oxidation number before revealing it. Apply the rules in order: the fixed values first, then solve for the unknown atom.
| Rule | Example |
|---|---|
| uncombined element = 0 | Na, O2, S8: 0 |
| simple ion = its charge | Fe3+ +3, Cl− −1 |
| sum = 0 (compound) or charge (ion) | SO42−: S + 4(−2) = −2 |
| F is always −1; Group 1 +1; Group 2 +2 | NaF, MgCl2 |
| O is −2 (except peroxides −1, OF2 +2) | H2O2: O = −1 |
| H is +1 (except metal hydrides −1) | NaH: H = −1 |
Write the equation for the sum and solve it. With practice you will do these in your head.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
What is the oxidation number of sulfur in SO42−?
Oxidation is:
What is the oxidation number of hydrogen in sodium hydride, NaH?
What is the formula of iron(III) oxide?
Use the tracker to follow each element across an equation. Any element whose oxidation number changes has been oxidised or reduced.
The oxidising agent is the species that is reduced, because it takes electrons from something else. Students often name the species that was oxidised instead. Also remember that oxidation numbers are written with the sign first (+3), while ionic charges are written with the sign last (3+).
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
What is the oxidation number of chromium in Cr2O72−?
In the reaction 2Fe3+ + 2I− → 2Fe2+ + I2, which statement is correct?
Which is a disproportionation reaction?
What is the oxidation number of chlorine in sodium chlorate(V)?
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