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
Part 1 of 5What an Oxidation Number Tells You
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
- Purpose Oxidation numbers show whether an atom has been oxidised or reduced and help balance redox equations.
Simple ions
- Simple ions In a simple monatomic ion the oxidation number equals the charge, so Mg²⁺ is +2 and Cl⁻ is -1.
Elements
- Elements An atom in an uncombined element, such as O₂, Fe or S₈, has an oxidation number of 0.
Sign convention
- 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
- 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 and 2 Group 1 metals are always +1 and Group 2 metals are always +2 in their compounds.
Fluorine
- Fluorine Fluorine is -1 in all its compounds because it is the most electronegative element.
Hydrogen
- Hydrogen Hydrogen is +1 with non-metals but -1 in metal hydrides such as NaH.
Oxygen
- Oxygen Oxygen is -2 in most compounds, but -1 in peroxides such as H₂O₂.
| Species | Atom | Oxidation number |
|---|---|---|
| NaH | H | -1 |
| H₂O₂ | O | -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⁻.
- Thiosulfateoxygen is -2, so three oxygen atoms give -6.
- The two sulfur atoms must total -2 - (-6) = +4, so each has an average of +2.
- Dichromateseven oxygen atoms give 7 × (-2) = -14.
- 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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