Oxidation and Reduction in Terms of Electrons

Section 1 of 8Electron definitions (HT only)

Oxidation

• Oxidation A substance loses electrons when it is oxidised, so its atoms or ions become more positive.
• For example, when calcium atoms form Ca²⁺ ions they lose two electrons each.

Reduction

• Reduction A substance gains electrons when it is reduced, so its atoms or ions become less positive or more negative.
• For example, when chlorine molecules form chloride ions each chlorine atom gains one electron.

Remembering it

• Remembering it OIL RIG: Oxidation Is Loss, Reduction Is Gain, of electrons.
• Metals usually lose electrons to form positive ions, so they are normally oxidised.

Link to oxygen

• Link to oxygen Magnesium gaining oxygen to form MgO is also loss of electrons, because each Mg atom becomes an Mg²⁺ ion.
  • The two definitions describe the same change.

Section 2 of 8Iron displacing copper (HT only)

1

Word equation: iron + copper sulfate → iron(II) sulfate + copper.

2

Full ionic equation: Fe + Cu²⁺ + SO₄²⁻ → Fe²⁺ + SO₄²⁻ + Cu.

3

Cancel the sulfate ions, which appear unchanged on both sides; these are spectator ions.

4

Ionic equation: Fe + Cu²⁺ → Fe²⁺ + Cu.

5

Split into half equations: Fe → Fe²⁺ + 2e⁻ (oxidation) and Cu²⁺ + 2e⁻ → Cu (reduction).

Section 3 of 8Redox in electrolysis (HT only)

Electrode	Ions attracted	Process	Example
Cathode (negative)	Positive ions (cations)	Reduction: gain of electrons	Pb2+ + 2e− → Pb
Anode (positive)	Negative ions (anions)	Oxidation: loss of electrons	2Br− → Br2 + 2e−

Section 4 of 8Reading a worked question

1Compare charges

• Compare charges Look at each species before and after, such as V³⁺ becoming V⁴⁺.

2Count electrons

• Count electrons A higher positive charge means electrons were lost;
  • a lower charge means electrons were gained.

3Apply OIL RIG

• Apply OIL RIG V³⁺ to V⁴⁺ is loss of an electron, so it is oxidation.

4Check the partner

• Check the partner Fe³⁺ to Fe²⁺ gains an electron and is reduced, completing the redox pair.

Section 5 of 8Worked examples of displacement (HT only)

Reaction	Oxidised	Reduced
Zn + Cu2+ → Zn2+ + Cu	Zn → Zn2+ + 2e−	Cu2+ + 2e− → Cu
Mg + 2Ag+ → Mg2+ + 2Ag	Mg → Mg2+ + 2e−	Ag+ + e− → Ag (twice)
Cl2 + 2Br− → 2Cl− + Br2	2Br− → Br2 + 2e−	Cl2 + 2e− → 2Cl−

Section 6 of 8Applications and exam context (HT only)

Redox with electrons

Reactivity

• Reactivity A more reactive metal loses electrons more easily, so it displaces a less reactive metal from its salt solution.
• The bigger the difference in reactivity, the more vigorous the displacement reaction.

Halogen displacement

• Halogen displacement A more reactive halogen gains electrons more easily and takes electrons from the halide ions of a less reactive halogen.

Exam technique

• Exam technique Name the species oxidised and reduced, quote the half equation, and state electron loss or gain to earn full marks.

Section 7 of 8Key terms

Term	Meaning
Oxidation	Loss of electrons by an atom, ion or molecule.
Reduction	Gain of electrons by an atom, ion or molecule.
Half equation	An equation showing only the oxidation or only the reduction, with electrons included.
Spectator ion	An ion present in a reaction mixture that does not change and is left out of the ionic equation.
Ionic equation	An equation showing only the particles that actually take part in a reaction.
Displacement	A reaction in which a more reactive element takes the place of a less reactive element in a compound.

Section 8 of 8Exam technique

1

Mixing up the directionoxidation is loss of electrons, even though oxidation in terms of oxygen is gain.

2

Leaving spectator ions in an ionic equation.

3

Forgetting that half equations must balance in charge as well as atoms.

4

Writing electrons on the wrong sidethey appear on the right for oxidation and on the left for reduction.