Neon (Ne) is a simple molecular substance that exists as individual atoms. It is a gas at room temperature with a boiling point of –246 °C. Krypton (Kr) is also a simple molecular substance that exists as individual atoms. It has a boiling point of –153 °C. Explain why both neon and krypton have very low boiling points, and why krypton has a higher boiling point than neon.

Edexcel A-Level Chemistry (9CH0) — 2.4 Forces between molecules · Explain · 5 marks · View as Markdown

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

Neon has a relative atomic mass of 20. Krypton has a relative atomic mass of 84. Both are noble gases that exist as simple molecules.

Model answer (5 marks)

Both neon and krypton are monoatomic gases, so the only forces between them are London dispersion forces, which are very weak. Because these forces are weak, only a small amount of energy is required to overcome them, giving very low boiling points.

Krypton has a higher boiling point than neon because it has a larger number of electrons and a greater relative atomic mass. The larger electron cloud in krypton produces stronger London dispersion forces, so more energy is needed to break the intermolecular forces, raising its boiling point.

Examiner tips

  • Mention London dispersion forces explicitly; link weak forces to low boiling point. Explain that larger atoms have more electrons → stronger dispersion forces → higher boiling point. Use correct terminology and UK spelling.

Common mistakes

  • Saying the gases are covalently bonded; they are monoatomic. Forgetting to mention London dispersion forces. Using “stronger bonds” instead of “stronger dispersion forces”.

Mark scheme (5 marks)

  1. Both neon and krypton have low boiling points because the forces between molecules (intermolecular forces) are weak
  2. Only a small amount of energy is needed to overcome these weak intermolecular forces
  3. Krypton has a higher boiling point than neon because krypton has more electrons / a greater relative atomic mass
  4. More electrons means stronger intermolecular forces in krypton
  5. More energy is needed to overcome the stronger intermolecular forces in krypton, so it has a higher boiling point

Key terms in this question

simple molecular substance · boiling point

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