📘 IB Understanding
An oxidation number is a hypothetical charge assigned to an atom assuming complete electron transfer to the more electronegative atom. Oxidation numbers track electron movement in redox reactions.
Rules for Assigning Oxidation Numbers
| Rule | Oxidation Number | Example |
|---|---|---|
| Uncombined elements | 0 | O2, Zn, Na |
| Monatomic ions | = ionic charge | Na+ = +1, Cl- = -1 |
| Fluorine | Always -1 | HF, NaF |
| Oxygen | -2 (except peroxides: -1) | H2O, H2O2 |
| Hydrogen | +1 (except metal hydrides: -1) | HCl, NaH |
| Neutral compound | Sum = 0 | H2SO4 |
| Polyatomic ion | Sum = ion charge | MnO4- |
Worked Example
Q: Find the oxidation state of Mn in MnO4-.
O = -2, and there are 4 oxygens: 4 x (-2) = -8
Overall charge = -1, so: Mn + (-8) = -1
Mn = +7
Stock Notation
Transition metals with variable oxidation states use Roman numerals:
- CuSO4 = copper(II) sulfate
- FeCl3 = iron(III) chloride
- MnO2 = manganese(IV) oxide
⚠️ Exam Tip
Write the sign before the number for oxidation states (+2), not after (2+). This distinguishes oxidation states from ionic charges.