Explain why potassium reacts more vigorously with water than lithium does.

Cambridge International IGCSE Chemistry (0620) — 8.2 Group I properties · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Group I metals all react with water, but the violence of the reaction increases down the group.

Model answer (4 marks)

Potassium has a larger atomic radius than lithium because it has more electron shells.
The outermost electron in potassium is therefore further from the nucleus and is more shielded by inner electrons.
This makes the ionisation energy of potassium lower, so its outer electron is lost more easily.
Consequently potassium reacts more vigorously with water than lithium does.

Examiner tips

  • Use the command word ‘explain’ – give a clear cause–effect chain. Mention the larger radius, greater shielding and lower ionisation energy. Show the link to increased reactivity with water.

Common mistakes

  • Confusing ionisation energy with electronegativity. Forgetting to mention the number of electron shells. Using vague terms like ‘more reactive’ without explaining why.

Mark scheme (4 marks)

  1. Potassium has more electron shells (energy levels) than lithium
  2. The outer electron in potassium is further from the nucleus / is more shielded
  3. The outer electron is lost more easily in potassium
  4. Therefore potassium is more reactive / reacts more vigorously with water

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