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2.3.1Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Oxidation Numbers – Introduction — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 21 of 53 Unit quiz

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Part 1 of 5What an Oxidation Number Tells You

2.3.1 Oxidation Numbers - Introduction

An oxidation number is the charge an atom would carry if its bonds were treated as fully ionic.

What an Oxidation Number Tells You
Purpose
  • Oxidation numbers show whether an atom has been oxidised or reduced and help balance redox equations.
Simple ions
  • In a simple monatomic ion the oxidation number equals the charge, so Mg²⁺ is +2 and Cl⁻ is -1.
Elements
  • An atom in an uncombined element, such as O₂, Fe or S₈, has an oxidation number of 0.
Sign convention
  • Write the sign before the number, as in +3 or -2, to distinguish oxidation numbers from ionic charges such as 3+.

Part 2 of 5The Rules

Apply these in order, remembering that the more electronegative atom takes the negative value.

Total
  • The oxidation numbers in a neutral compound add up to zero, and in an ion they add up to the charge on the ion.
Group 1 and 2
  • Group 1 metals are always +1 and Group 2 metals are always +2 in their compounds.
Fluorine
  • Fluorine is -1 in all its compounds because it is the most electronegative element.
Hydrogen
  • Hydrogen is +1 with non-metals but -1 in metal hydrides such as NaH.
Oxygen
  • Oxygen is -2 in most compounds, but -1 in peroxides such as H₂O₂.
SpeciesAtomOxidation number
NaHH-1
H₂O₂O-1
SO₄²⁻S+6
NO₃⁻N+5
MnO₄⁻Mn+7

Part 3 of 5Names and Stock Notation

Elements with variable oxidation numbers carry a Roman numeral in the name of the compound.

1
Decide which element has a variable oxidation number, which is usually a transition metal or a non-metal such as sulfur or chlorine.
2
Work out its oxidation number from the formula using the known values for the other atoms and the overall charge.
3
Write that number as a Roman numeral in brackets straight after the element, such as iron(II) oxide for FeO.
4
For oxyanions the numeral follows the central atom, giving manganate(VII) for MnO₄⁻ and chromate(VI) for CrO₄²⁻.
5
Use Roman numerals only when needed; Na₂SO₄ is simply sodium sulfate, since sodium has one possible oxidation number.

Part 4 of 5Worked Example

Question Deduce the oxidation number of sulfur in the thiosulfate ion, S₂O₃²⁻, and of chromium in the dichromate ion, Cr₂O₇²⁻.

  1. Thiosulfateoxygen is -2, so three oxygen atoms give -6.
  2. The two sulfur atoms must total -2 - (-6) = +4, so each has an average of +2.
  3. Dichromateseven oxygen atoms give 7 × (-2) = -14.
  4. The two chromium atoms must total -2 - (-14) = +12, so each is +6.

Answer Sulfur is +2 in S₂O₃²⁻ and chromium is +6 in Cr₂O₇²⁻, so the second is named dichromate(VI).

Part 5 of 5Exam Checklist & Tips

2.3.1 – Key Points for the Exam

  • Oxidation number The charge an atom would have if the compound were completely ionic.
  • Stock notation A Roman numeral in brackets showing the oxidation number of an element in a name.
  • Peroxide A compound containing O-O, in which oxygen has an oxidation number of -1.
  • Metal hydride A compound of hydrogen with a metal in which hydrogen has the oxidation number -1.
  • Average oxidation number A fractional value found when atoms of one element occupy different environments.

Common Mistakes

  • Writing the sign after the number or forgetting the sign altogether, so +2 becomes 2.
  • Giving oxygen -2 in every compound, including peroxides where it is -1.
  • Mixing up charge on the ion with the oxidation number of one atom within the ion.
  • Forgetting that the sum must equal the overall charge, not always zero.

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