Topic 8 · page 1 of 29

Oxidation number and the rules

Oxidation number is a bookkeeping device that tracks electrons in ionic and covalent substances alike. Eight short rules, applied in order, give every atom its number.

11 min read8.1

Why we need oxidation numbers 8.1

Magnesium burns in chlorine to give magnesium chloride. Hydrogen burns in chlorine to give hydrogen chloride.

Mg(s)+ClX2(g)→MgClX2(s) \ce{Mg(s) + Cl2(g) -> MgCl2(s)}
HX2(g)+ClX2(g)→2 HCl(g) \ce{H2(g) + Cl2(g) -> 2HCl(g)}

Both are redox reactions, but the electrons behave differently. In MgCl₂ they are handed over: the substance is made of Mg²⁺ and Cl⁻ ions. In HCl they are only shared: HCl is a covalent molecule and no ions form. Chemists want one system that follows the electrons in both cases. That system is the oxidation number.

In MgCl₂ the model is real: Mg is +2 and each Cl is −1. In HCl the model is a pretend: chlorine is more electronegative than hydrogen, so we give it both electrons of the H–Cl bond. Chlorine becomes −1 and hydrogen +1, even though HCl is not made of ions. The numbers are not real charges. They are a way of counting electrons.

The rules 8.1

Apply the rules from the top of the list down. If two rules disagree, the one higher in the list wins.

Rule Oxidation number Example
1. An uncombined element 0 Na, O₂, Cl₂, S₈ and P₄ are all 0
2. The numbers in a species add up to its overall charge 0 for a compound; the ion charge for an ion in SO₄²⁻ the total is −2
3. A simple (monatomic) ion the charge on the ion Mg²⁺ is +2; Cl⁻ is −1
4. Group 1, Group 2 and aluminium in compounds +1, +2, +3 Na in NaCl is +1; Al in Al₂O₃ is +3
5. Fluorine in compounds −1 F in HF is −1
6. Hydrogen +1 H in H₂O is +1 (exception: metal hydrides, −1)
7. Oxygen −2 O in CO₂ is −2 (exception: peroxides, −1)
8. Chlorine, bromine and iodine −1 Cl in MgCl₂ is −1 (exception: when combined with O or F)

The exceptions in rules 6 and 7 are on the next page. Rule 8 has an exception that matters later: in ClO⁻ and ClO₃⁻ the chlorine is bonded to oxygen, which is more electronegative, so chlorine is positive.

Worked exampleReading the rules

Give the oxidation number of every element in water, H₂O, and in carbon dioxide, CO₂.

  1. H₂O. Rule 6: H is +1. Rule 7: O is −2. Check with rule 2: 2 × (+1) + (−2) = 0. ✓
  2. CO₂. Rule 7: O is −2, so the two oxygens give 2 × (−2) = −4.
  3. Rule 2: the total is 0, so C + (−4) = 0. Carbon is +4.

Writing oxidation numbers correctly 8.1

An oxidation number has its sign first: +2, −1, +7. An ion charge has its sign after: 2+, 1−, 7+. They are different things, and a mark scheme will check which you wrote.

Charge on the ion Oxidation number of the metal
Magnesium ion Mg²⁺ (2+) Mg is +2
Iron(III) ion Fe³⁺ (3+) Fe is +3
Manganate(VII) ion MnO₄⁻ (1−) Mn is +7

Check yourself

  1. State the oxidation number of the underlined element in each: (a) Cl in Cl₂, (b) Na in NaF, (c) Cl in AlCl₃, (d) H in HCl, (e) S in H₂S, (f) C in CO₂.
  2. Explain the difference between the charge on the Mg²⁺ ion and the oxidation number of magnesium in MgCl₂.
  3. Explain why the oxidation number of chlorine in HCl is −1 and that of hydrogen is +1, although HCl is covalent.
  4. What must the oxidation numbers of all the atoms in (a) CH₄ and (b) SO₄²⁻ add up to?
  5. Correct this statement: "The oxidation number of sodium in sodium chloride is 1+."
  6. In oxygen difluoride, OF₂, oxygen is bonded to fluorine. Use the order of the rules to work out the oxidation number of each element.

Answers cover these until you have tried the questions

  1. (a) Cl in Cl₂: 0 (uncombined element). (b) Na in NaF: +1 (Group 1). (c) Cl in AlCl₃: −1 (rule 8; check Al +3 and 3 × (−1) = 0). (d) H in HCl: +1 (rule 6). (e) S in H₂S: 2 × (+1) + S = 0, so S is −2. (f) C in CO₂: C + 2 × (−2) = 0, so C is +4.
  2. The charge on the ion, Mg²⁺, is a real charge of 2+. The oxidation number of Mg in MgCl₂ is +2. In an ionic compound the two are the same size, but the oxidation number is written sign first and the charge sign last. Oxidation numbers also apply to covalent compounds where there are no ions.
  3. Chlorine is more electronegative than hydrogen, so in the H–Cl bond the shared electrons are assigned to Cl. Chlorine then has one more electron than a neutral atom, so it is −1. Hydrogen has lost its electron in this bookkeeping, so it is +1.
  4. (a) 0, because CH₄ is a neutral compound. (b) −2, the charge on the ion.
  5. "The oxidation number of sodium in sodium chloride is +1." (Sign first for oxidation numbers; 1+ is how you write a charge.)
  6. The fluorine rule (rule 5) is higher in the list than the oxygen rule (rule 7), so fluorine is −1 in each F atom. The total is 0, so O + 2 × (−1) = 0 and oxygen is +2. Fluorine is the most electronegative element, so it always takes the electrons, even from oxygen.