IB Understanding
Concept of Oxidation Numbers
An oxidation number is the hypothetical charge an atom would have if all bonds to different atoms were 100% ionic. It tracks the distribution of electrons in chemical species.
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
Assigning Oxidation States in Oxyanions
Problem: Determine the oxidation state of sulfur in \(\text{SO}_4^{2-}\) and \(\text{SO}_3^{2-}\).
- \(\text{SO}_4^{2-}: x + 4(-2) = -2 \implies x = \mathbf{+6}\) → Sulfate(VI)
- \(\text{SO}_3^{2-}: x + 3(-2) = -2 \implies x = \mathbf{+4}\) → Sulfite(IV)
Stock Notation
Transition metals with variable oxidation states use Roman numerals:
- CuSO4 = copper(II) sulfate
- FeCl3 = iron(III) chloride
- MnO2 = manganese(IV) oxide
Examiner Trap
Sign Notation: +2 vs 2+
Sign placement: Oxidation states are written with the sign before the number (\(+2, -1\)); ion charges are written with the sign after the number (\(2+, 1-\)).
AQA GCSE & IB Chemistry
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