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2.1.1Organic Chemistry

Oxidation Number — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 1 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Part 1 of 6The Rules

2.1.1 Oxidation Number
Elements
  • Atoms in an uncombined element, such as O₂, Fe or S₈, have an oxidation number of 0.
Simple ions
  • The oxidation number of a monatomic ion equals its charge, for example Mg²⁺ is +2.
Sum rule
  • The oxidation numbers in a neutral compound add up to 0, and in an ion add up to the charge on the ion.
Fixed values
  • Group 1 is +1, group 2 is +2 and aluminium is +3 in compounds.

Part 2 of 6Common Exceptions

Common Exceptions
Hydrogen
  • Usually +1, but −1 in metal hydrides such as NaH.
Oxygen
  • Usually −2, but −1 in peroxides such as H₂O₂ and +2 in OF₂.
Halogens
  • Usually −1 in compounds, but positive when bonded to oxygen or a more electronegative halogen.
Fluorine
  • Always −1 in compounds because it is the most electronegative element.
SpeciesElementOxidation number
H₂SO₄S+6
MnO₄⁻Mn+7
Cr₂O₇²⁻Cr+6
ClO₃⁻Cl+5
H₂O₂O−1

Part 3 of 6Working Out an Oxidation Number

1
Write down the known oxidation numbers, such as oxygen −2 and hydrogen +1.
2
Let the unknown oxidation number be x.
3
Multiply each oxidation number by the number of atoms of that element in the formula.
4
Set the sum equal to the overall charge of the species: zero for a compound, the ion charge for an ion.
5
Solve for x and then include the sign with the answer.

Part 4 of 6Naming and Numerals

Roman numerals
  • The oxidation number of the element is shown in brackets, as in iron(III) oxide for Fe₂O₃.
Oxoanions
  • Names such as chlorate(V) and sulfate(VI) include the oxidation number of the central atom.
Variable oxidation states
  • Transition metals can form ions with different oxidation numbers, such as Fe²⁺ and Fe³⁺.
Sign
  • Always write the sign, for example +3, to avoid confusion with an ion charge written as 3+.

Part 5 of 6Worked Example

Question Find the oxidation number of manganese in the permanganate ion MnO₄⁻ and of chlorine in ClO₃⁻.

  1. For MnO₄⁻, let Mn = xx + 4(−2) = −1.
  2. x − 8 = −1, so x = +7.
  3. For ClO₃⁻, let Cl = yy + 3(−2) = −1.
  4. y − 6 = −1, so y = +5.

Answer Mn is +7 in MnO₄⁻ and Cl is +5 in ClO₃⁻

Part 6 of 6Exam Checklist & Tips

2.1.1 – Key Points for the Exam

  • Oxidation number The charge an atom would have if its bonds were fully ionic.
  • Oxidation state Another term for oxidation number.
  • Oxoanion A negative ion containing oxygen and another element.
  • Peroxide A compound containing the O–O group where oxygen is −1.

Common Mistakes

  • Forgetting that the oxidation numbers must sum to the charge of the ion.
  • Giving oxygen −2 in peroxides.
  • Leaving out the sign of the oxidation number.
  • Confusing the oxidation number with the number of bonds formed.

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