How to Find Oxidation Numbers: Rules and Worked Examples

Oxidation numbers are a bookkeeping system. They don’t describe real charges on atoms (in most molecules, no atom carries a full charge). Instead they answer a what-if question: if every bond in this compound were completely ionic, what charge would each atom have?

That fiction turns out to be enormously useful. It’s how chemists track electrons in oxidation-reduction reactions, name compounds like iron(III) chloride, and balance complicated redox equations.

The rules, in priority order

Apply these top to bottom. When two rules conflict, the higher one wins.

1. An uncombined element is 0. Fe, O₂, S₈, Cl₂ — every atom is 0.
2. A monatomic ion equals its charge. Na⁺ is +1, Fe³⁺ is +3, Cl⁻ is −1.
3. Fluorine is always −1 in compounds. It’s the most electronegative element, so it always “wins” the electrons.
4. Group 1 metals are +1, group 2 metals are +2 in compounds.
5. Hydrogen is +1 when bonded to nonmetals, but −1 when bonded to metals (in metal hydrides like NaH).
6. Oxygen is −2, except in peroxides (−1), superoxides (−½) and when bonded to fluorine (positive).
7. Chlorine, bromine and iodine are usually −1, except when bonded to oxygen or a more electronegative halogen.
8. The sum of all oxidation numbers equals the overall charge: zero for a neutral compound, the ion’s charge for a polyatomic ion.

Rule 8 is the one that solves most problems: assign everything you can from rules 1–7, then find the last element by making the total come out right.

Worked examples

Carbon dioxide, CO₂. O is −2, two of them make −4. The molecule is neutral, so C = +4.

Methane, CH₄. H is +1 (bonded to a nonmetal), four of them make +4. So C = −4.

Carbon goes from −4 in methane to +4 in CO₂ when methane burns — that’s an oxidation, and it’s why burning is a redox reaction.

Sulfuric acid, H₂SO₄. H: 2 × (+1) = +2. O: 4 × (−2) = −8. Total must be 0, so S + 2 − 8 = 0, and S = +6.

Potassium permanganate, KMnO₄. K: +1. O: 4 × (−2) = −8. So Mn = 0 − 1 + 8 = +7.

Dichromate ion, Cr₂O₇²⁻. O: 7 × (−2) = −14. The total must be −2, so the two chromiums together are +12, and each Cr = +6.

Ammonium ion, NH₄⁺. H: 4 × (+1) = +4. Total must be +1, so N = −3.

Nitrate ion, NO₃⁻. O: 3 × (−2) = −6. Total −1, so N = +5.

Nitrogen covers an enormous range of oxidation states, from −3 in ammonia to +5 in nitrate, which is part of why nitrogen chemistry (and the nitrogen cycle) is so rich.

The tricky cases

Hydrogen peroxide, H₂O₂. If you insisted oxygen is −2, you’d get 2(+1) + 2(−2) = −2, not 0. The hydrogen rule has priority, so hydrogen stays +1 and oxygen gives way: the two oxygens share −2, so each is −1. That’s what makes it a peroxide.

Oxygen difluoride, OF₂. Fluorine outranks oxygen. F is −1, so O = +2 — one of the few compounds where oxygen is positive.

Sodium hydride, NaH. Sodium is +1 (rule 4 outranks rule 5), so hydrogen = −1. In metal hydrides hydrogen behaves like a halide ion.

Potassium superoxide, KO₂. K is +1, so the two oxygens share −1: each is −½.

Fractional oxidation numbers

Magnetite, Fe₃O₄. O: 4 × (−2) = −8. Three irons share +8, so the average is +8/3.

No single iron atom is +2.67. In reality, one iron is +2 and two are +3 (2 + 3 + 3 = 8). A fractional oxidation number is always an average over atoms in different states. The same thing happens with sulfur in the tetrathionate ion, S₄O₆²⁻, where the average is +2.5.

Using oxidation numbers to name compounds

Transition metals often have more than one common oxidation state, so their compound names include it in Roman numerals: FeCl₂ is iron(II) chloride (Fe is +2), and FeCl₃ is iron(III) chloride (Fe is +3). Working out the oxidation number is exactly how you get the name right. See how to name ionic compounds.

Oxidation number vs. formal charge vs. real charge

• Oxidation number assumes every bond is fully ionic, electrons going to the more electronegative atom.
• Formal charge (used in Lewis structures) assumes every bond is fully covalent, electrons shared equally.
• Real partial charges are somewhere in between and have to be calculated or measured.

They answer different questions, so don’t be surprised when they disagree.

Quick answers

Can oxidation numbers be positive for nonmetals? Yes — any time a nonmetal is bonded to a more electronegative element. Chlorine is +7 in perchlorate, ClO₄⁻.

What’s the highest possible oxidation number? +8 is well established, for example in osmium tetroxide, OsO₄. There have been reports of +9 for iridium in an ion made under extreme conditions.

Is oxidation state the same as oxidation number? In everyday use, yes. Some textbooks distinguish them slightly, but the values you calculate are the same.

Check your answers

The oxidation number calculator applies these rules in priority order, handles peroxides, hydrides and polyatomic ions, shows the rule behind each value, and gives exact fractions for mixed-valence compounds.