Oxidation Number Rules
Assign oxidation states systematically
Priority Rules (Apply in Order)
- Free elements: Oxidation number = 0 (e.g., Na, O₂, Cl₂)
- Monatomic ions: Oxidation number = ion charge (e.g., Na⁺ = +1, Cl⁻ = -1)
- Fluorine: Always -1 in compounds
- Oxygen: Usually -2 (exceptions: peroxides -1, OF₂ +2)
- Hydrogen: +1 with nonmetals, -1 with metals (hydrides)
- Group 1 metals: Always +1
- Group 2 metals: Always +2
- Sum rule: Sum of all oxidation numbers = overall charge
Example: H₂SO₄
Compound is neutral, so sum = 0.
H: +1 (rule 5) → 2 H = +2 total
O: -2 (rule 4) → 4 O = -8 total
Let S = x: 2(+1) + x + 4(-2) = 0
2 + x - 8 = 0 → x = +6
Answer: S has oxidation number +6
Common Mistakes
- Forgetting peroxide exception (H₂O₂: O is -1, not -2).
- Not accounting for overall charge in polyatomic ions.
- Applying rules out of priority order.