3.1.6—Oxidation states
- Syllabus
- First assessment 2025
- Objective
- 3.1.6
- Level
- HL
Oxidation States
An oxidation state is the charge an atom would have if bonding electrons were assigned according to the ionic convention. It is not necessarily the physical charge on an atom in a covalent compound.
Use known oxidation-state rules and the overall charge to solve for the unknown state in compounds and ions.
| Required case | Oxidation state | Check |
|---|---|---|
| Uncombined element, e.g. Fe or ClX2 | 0 | no ionic charge separation is assigned within an uncombined element |
| Hydrogen in a metal hydride | -1 | exception to the usual +1 |
| Oxygen in a peroxide | -1 | exception to the usual -2 |
| Compound or ion | sum equals overall charge | write the charge-sum equation |
In MnO₄⁻, four O atoms contribute −8, so Mn must be +7 to give the overall −1 charge. Write the charge-sum equation explicitly and remember that +7 is an oxidation-state assignment, not a claim that manganese exists as a free Mn⁷⁺ ion in permanganate.
Calculating Oxidation States
Assessment in practice
Representative question
Question 1
[Maximum number: 1]
State the oxidation state of nitrogen in nitrous acid, HNO2.
+3
Marking guidance:
Accept (III).
Do not accept 3+ or 3.
The Periodic Table Summary
Retrieve the route: locate an element from configuration, explain periodic and group trends, write oxide/reaction and oxidation-state answers, then connect incomplete d-sublevels to transition properties, ion configurations, and colours.
Check that every trend explanation names its particle-level cause, every equation is balanced, every oxidation state is a formal charge convention, and every transition colour uses absorbed/observed complementarity.