Oxidation numbers and redox equations

Oxidation number rules

• Uncombined elements: 0.
• The oxidation numbers in a compound add to 0; in an ion they add to the charge.
• Group 1 is +1, Group 2 is +2, F is -1.
• O is usually -2, but -1 in peroxides (H2O2) and +2 in OF2.
• H is usually +1, but -1 in metal hydrides (NaH).

Roman numerals show oxidation numbers in names: iron(III) chloride is FeCl3; sulfate(VI) is SO4²-; chlorate(V) is ClO3-.

Oxidation and reduction

Oxidation is loss of electrons, or an increase in oxidation number. Reduction is gain of electrons, or a decrease in oxidation number. An oxidising agent gains electrons (and is reduced); a reducing agent loses electrons (and is oxidised).

Half-equations

Balance the main atom, add H2O to balance O, add H+ to balance H, then add electrons to balance charge:

MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O

To combine two half-equations, multiply so the electrons cancel, then add:

MnO4- + 8H+ + 5Fe2+ -> Mn2+ + 5Fe3+ + 4H2O

Disproportionation

Disproportionation is when the same element is simultaneously oxidised and reduced. Example: Cl2 + 2NaOH -> NaCl + NaClO + H2O, in which chlorine goes from 0 to -1 and to +1.

Worked example

Find the oxidation number of chromium in Cr2O7²-.

1. Oxygen: 7 x (-2) = -14
2. 2Cr + (-14) = -2, so 2Cr = +12

Answer: Cr is +6

Check yourself

1. What is the oxidation number of manganese in MnO4-?
   Show answer

   +7. Mn + 4(-2) = -1, so Mn = +7.

2. What is the oxidation number of oxygen in H2O2?
   Show answer

   -1. 2(+1) + 2x = 0, so x = -1 (a peroxide).

3. What is the oxidation number of hydrogen in sodium hydride, NaH?
   Show answer

   -1. Na is +1, so H must be -1.

4. In Zn + Cu2+ -> Zn2+ + Cu, what is the oxidising agent?
   • Zn
   • Zn2+
   • Cu
   • Cu2+
   Show answer

   Cu2+. Cu2+ gains electrons (is reduced), so it oxidises the zinc.

5. Which reaction is a disproportionation?
   • Zn + 2HCl -> ZnCl2 + H2
   • 2H2O2 -> 2H2O + O2
   • NaOH + HCl -> NaCl + H2O
   • 2Mg + O2 -> 2MgO
   Show answer

   2H2O2 -> 2H2O + O2. Oxygen goes from -1 in H2O2 to -2 in H2O and 0 in O2.

6. How many electrons are needed to balance Cr2O7²- + 14H+ -> 2Cr3+ + 7H2O?
   Show answer

   6. Left charge: -2 + 14 = +12; right: +6. Add 6e- to the left.

7. What is the formula of iron(II) sulfate?
   • Fe2(SO4)3
   • FeSO4
   • Fe2SO4
   • FeSO3
   Show answer

   FeSO4. Iron(II) is Fe2+, sulfate is SO4²-: 1 : 1.

8. What is the oxidation number of chlorine in ClO3-?
   • +5
   • +3
   • -1
   • +7
   Show answer

   +5. Cl + 3(-2) = -1, so Cl = +5; the ion is chlorate(V).

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.