Oxidation Numbers: The Rules and How to Assign Them

Oxidation numbers look intimidating but follow a short priority list — and the last unknown always falls out by balancing. Fix the elements that have rules (group 1 is +1, oxygen is −2, and so on), then solve the one that’s left so everything sums to the overall charge. For KMnO₄ that instantly gives Mn = +7.

The rules, in priority order

The rules are a priority list — when two could apply, the higher one wins:

1. A free element is 0 (e.g. O₂, Na metal, S₈).
2. A monatomic ion equals its charge (Na⁺ is +1, Cl⁻ is −1).
3. Group 1 metals are +1; group 2 metals are +2.
4. Fluorine is always −1.
5. Hydrogen is +1 — except in metal hydrides, where it’s −1.
6. Oxygen is −2 — except in peroxides (−1) and with fluorine (+2).
7. The oxidation numbers sum to the overall charge (0 for a neutral compound; the ion’s charge otherwise).

The trick: solve the last one by balancing

Rule 7 is the workhorse. Once every element with a rule is fixed, the remaining element is whatever makes the sum come out right. Take KMnO₄:

• K (group 1) = +1
• O = −2, and there are four of them → −8
• Everything must sum to 0 (neutral compound): +1 + Mn + (−8) = 0
• So Mn = +7.

No memorising manganese’s oxidation states — it drops out of the balance. The oxidation number calculator does exactly this: it applies the rules and solves the unknown for you.

More worked examples

Compound	Fixed	Solve	Answer
H₂SO₄	H = +1, O = −2	S	+6
K₂Cr₂O₇	K = +1, O = −2	Cr (×2)	+6 each
HNO₃	H = +1, O = −2	N	+5
Fe₂O₃	O = −2	Fe (×2)	+3 each

For polyatomic ions, set the overall charge to the ion’s charge instead of 0 — e.g. in the sulfate ion SO₄²⁻, S + 4(−2) = −2 gives S = +6.

Mind the exceptions

Three cases break the default rules:

• Peroxides — oxygen is −1 (as in hydrogen peroxide, H₂O₂).
• Metal hydrides — hydrogen is −1 (as in NaH, CaH₂).
• Oxygen difluoride, OF₂ — oxygen is +2, because fluorine is more electronegative.

When you spot a peroxide or a metal hydride, override the default before balancing.

Why bother?

Oxidation numbers are how chemists track electron transfer. If an element’s oxidation number goes up, it was oxidised (lost electrons); if it goes down, it was reduced (gained electrons). That’s the foundation of balancing redox reactions — the same reactions the equation balancer handles — and of electrochemistry.

Quick summary

Assign oxidation numbers by applying the rules in priority order (free element 0; group 1 +1; group 2 +2; F −1; H +1; O −2), then solve the last element so everything sums to the overall charge — KMnO₄ gives Mn = +7. Watch the peroxide, metal-hydride and OF₂ exceptions. Let the oxidation number calculator do the balancing for you.

Sources: standard rules for assigning oxidation numbers as taught in general chemistry. Educational information.