Explain why the boiling points of the halogens increase going down Group VII from fluorine to iodine.

Cambridge International IGCSE Chemistry (0620) — 8.3 Group VII properties · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The halogen molecules become larger and contain more electrons as you move down Group VII.

This increase in size and electron number strengthens the London dispersion forces between the molecules.

Stronger dispersion forces mean more energy is required to separate the molecules.

Therefore a higher temperature (more heat) is needed to boil the halogen, so the boiling point rises down the group.

Examiner tips

Use the word ‘increase’ to link size/electrons with stronger forces; mention London dispersion forces explicitly.
Show the logical chain: larger → stronger forces → more energy needed → higher boiling point.
Keep each point concise to fit the 4‑mark structure.

Common mistakes

Confusing boiling point with melting point or citing dipole–dipole forces for halogens.
Failing to mention that the halogens are non‑polar and only London forces apply.
Writing a long paragraph instead of the required short, point‑wise answer.

Mark scheme (4 marks)

The molecules become larger / have more electrons going down the group
Stronger intermolecular forces / van der Waals forces between molecules going down the group
More energy is needed to overcome these intermolecular forces
Therefore a higher temperature / more heat is required to boil the halogen, so boiling point increases

Key terms in this question

boiling point · Group VII