The rules, in the order to apply them
- An element on its own is 0: Na, O₂, P₄, S₈.
- The sum is the charge: 0 for a neutral compound, or the ion's charge.
- Fluorine is always −1.
- Group 1 metals are +1, group 2 metals +2, and aluminium +3.
- Hydrogen is +1, except in metal hydrides like NaH and CaH₂, where it's −1.
- Oxygen is −2, except in peroxides (−1, as in H₂O₂ and Na₂O₂), superoxides (−½, as in KO₂) and with fluorine (+2 in OF₂).
- Chlorine, bromine and iodine are −1, unless they're bonded to oxygen or a more electronegative halogen.
Then solve for the one element left. In KMnO₄: (+1) + Mn + 4(−2) = 0, so Mn = +7.
Fractional answers are averages. Fe₃O₄ gives Fe = +8/3 because it's really FeO·Fe₂O₃, with one Fe²⁺ and two Fe³⁺. In S₄O₆²⁻ the average is +2.5, but the two middle sulphurs are 0 and the two end ones +5. Structure-based questions like these, and CrO₅ (Cr is +6, with two peroxide groups), need the structure, not just the formula.
Balancing a redox reaction? Use the equation balancer. For colours and ores, see the inorganic facts sheet.
Worked examples
Tap one to see it solved step by step:
| Element in compound | Oxidation number |
|---|---|
| Mn in KMnO4 | +7 |
| Cr in K2Cr2O7 | +6 |
| Cr in Cr2O72− | +6 |
| Cr in CrO42− | +6 |
| Mn in MnO42− | +6 |
| Mn in MnO2 | +4 |
| S in H2SO4 | +6 |
| S in SO2 | +4 |
| S in SO3 | +6 |
| S in H2S | −2 |
| S in Na2S2O3 | +2 |
| S in S4O62− | +5/2 |
| S in H2SO5 | +6 |
| N in HNO3 | +5 |
| N in NH3 | −3 |
| N in NH4+ | −3 |
| N in NO3− | +5 |
| N in NO2 | +4 |
| N in HNO2 | +3 |
| N in N2O | +1 |
| N in N2O5 | +5 |
| N in N2H4 | −2 |
| N in NH2OH | −1 |
| N in HN3 | −1/3 |
| P in H3PO4 | +5 |
| P in H3PO3 | +3 |
| P in H3PO2 | +1 |
| Cl in HClO4 | +7 |
| Cl in HClO | +1 |
| Cl in KClO3 | +5 |
| Cl in CaOCl2 | 0 |
| Fe in Fe3O4 | +8/3 |
| Fe in K4[Fe(CN)6] | +2 |
| Fe in K3[Fe(CN)6] | +3 |
| Cr in CrO5 | +6 |
| O in H2O2 | −1 |
| O in OF2 | +2 |
| O in KO2 | −1/2 |
| O in Na2O2 | −1 |
| H in NaH | −1 |
| Pb in Pb3O4 | +8/3 |
| C in CO2 | +4 |
| C in CO | +2 |
| C in CH4 | −4 |
| C in C6H12O6 | 0 |
| C in HCOOH | +2 |
| Ni in Ni(CO)4 | 0 |
| Cu in [Cu(NH3)4]2+ | +2 |
| Co in [Co(NH3)6]Cl3 | +3 |
| Xe in XeF6 | +6 |
| Os in OsO4 | +8 |
| Al in LiAlH4 | +3 |
Questions students ask
What is the oxidation number of Mn in KMnO₄?
+7. Potassium is +1 and each oxygen is −2, so 1 + Mn − 8 = 0.
What is the oxidation number of Cr in K₂Cr₂O₇?
+6. 2(+1) + 2Cr + 7(−2) = 0 gives 2Cr = 12.
What is the oxidation number of oxygen in H₂O₂?
−1, because it's a peroxide. The O–O bond means each oxygen only takes one electron's worth from hydrogen.
What is the oxidation number of S in Na₂S₂O₃?
+2 on average. Structurally, one S is +6 (central) and one is −2 (terminal) in the common NCERT view, or +5 and −1 in another convention.