CalculateQuestion 1 · Recall
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By the end you can assign an oxidation number to any element in a compound or ion using the standard rules, and check your answer by making the numbers add up to the overall charge.
In an ionic compound, the ions really do carry charges: and in sodium chloride.
In a covalent compound, nobody has fully lost or gained an electron. Yet chemists still need to track where the electrons have moved.
So we pretend. We take every shared pair of electrons and give it to the more electronegative atom, the one that pulls harder. The charge the atom would then have is its oxidation number.
It is a bookkeeping device, but a very useful one: it turns "electrons have shifted" into simple arithmetic.
Oxidation number
The oxidation number (also called the oxidation state) of an element is the charge that its atom would have if the electrons in each bond were assigned to the more electronegative atom.
Oxidation numbers are written with the sign first: the iron in iron(III) chloride has oxidation number , written , not .
Learn these in order. When two rules disagree, the earlier one wins.
| Situation | Oxidation number |
|---|---|
| An uncombined element, e.g. , , | |
| A monatomic ion, e.g. , | equal to the ion's charge |
| Fluorine in a compound | always |
| Oxygen in a compound | (exceptions: peroxides , ) |
| Hydrogen in a compound | (exception: metal hydrides ) |
| Group 1 metal in a compound | |
| Group 2 metal in a compound | |
| Aluminium in a compound |
Everything else is found by subtraction: make the sum equal the overall charge.
Rules for oxygen and hydrogen come after fluorine because fluorine is the most electronegative element, so it always wins the electron tug-of-war.
Worked example: Worked example: sulfur in sulfuric acid
Find the oxidation number of sulfur in .
Worked example: Worked example: manganese in a permanganate ion
Find the oxidation number of manganese in the permanganate ion, .