Oxidation states track how many electrons an atom has effectively gained or lost in a compound. This reference lists the common and notable oxidation states for elements across the periodic table, with the principal state highlighted and an example compound for each.
Assignment rules: the priority order
Assigning oxidation states uses a fixed hierarchy of rules applied in order:
- Free elements are always 0 — Fe(s), O₂(g), Cl₂(g) are all zero.
- Monatomic ions equal the ion’s charge — Na⁺ is +1, Ca²⁺ is +2, Cl⁻ is −1.
- Fluorine is always −1 in compounds (it is the most electronegative element).
- Oxygen is usually −2; exceptions are peroxides (−1, as in H₂O₂), superoxides (−½), and compounds with fluorine such as OF₂ (+2).
- Hydrogen is usually +1 with non-metals and −1 in metal hydrides such as NaH or CaH₂.
- Group 1 metals are always +1 in compounds; group 2 metals are always +2.
- The sum of all oxidation states equals the overall charge of the species (0 for neutral compounds, the ion charge for polyatomic ions).
Apply these in order and solve algebraically for any unknown.
Worked examples
Sulfuric acid, H₂SO₄ (neutral):
- H is +1 × 2 = +2
- O is −2 × 4 = −8
- Sum must be 0, so S = +6
Permanganate ion, MnO₄⁻:
- O is −2 × 4 = −8
- Sum must be −1, so Mn = +7
Iron(III) chloride, FeCl₃ (neutral):
- Cl is −1 × 3 = −3
- Sum must be 0, so Fe = +3
Transition metals: multiple common states
Transition metals are unique in having several commonly accessible oxidation states because their d electrons are close in energy and can be removed selectively. The most important pairs to know:
| Element | Common states | Diagnostic example |
|---|---|---|
| Iron (Fe) | +2, +3 | FeSO₄ (+2), Fe₂O₃ (+3) |
| Copper (Cu) | +1, +2 | Cu₂O (+1), CuSO₄ (+2) |
| Manganese (Mn) | +2, +4, +7 | MnCl₂, MnO₂, KMnO₄ |
| Chromium (Cr) | +3, +6 | Cr₂O₃, K₂Cr₂O₇ |
| Vanadium (V) | +2, +3, +4, +5 | All appear in the vanadium flow cell |
Why this matters for redox reactions
Oxidation states are a bookkeeping tool for tracking electron transfer in redox reactions. When manganese goes from +7 in permanganate to +2 in Mn²⁺, it gains 5 electrons — it is reduced. The substance losing those electrons is oxidised and its oxidation state rises. Balancing a redox equation requires that total electrons gained equal electrons lost, and you cannot do that without correct oxidation-state assignments. Search for an element above, note its principal state, and use the rules above to verify assignments in any compound you encounter.