Iodine is a simple molecular covalent substance that exists as diatomic molecules, I₂. Explain why iodine is a solid at room temperature but has a much lower melting point than diamond, which is also a covalent substance.

Pearson Edexcel International GCSE Chemistry (4CH1) — 1.7 Covalent substances · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Iodine is a solid at room temperature because the I₂ molecules are held together by weak London dispersion forces. These forces are easily overcome, so only a small amount of energy is required to melt the solid.

Diamond, on the other hand, is a giant covalent network. Every carbon atom is bonded to four others by very strong covalent bonds. Breaking this extensive network requires a large amount of energy, giving diamond a very high melting point.

Examiner tips

  • Use the word "London dispersion forces" to show specific knowledge of intermolecular forces.
  • Explain that melting requires overcoming these forces, not breaking covalent bonds.
  • Contrast with diamond’s giant covalent network and the energy needed to break it.
  • Mention the difference in energy required for melting.

Mark scheme (4 marks)

  1. Iodine has (weak) intermolecular forces between its I₂ molecules
  2. Only a small amount of energy is needed to overcome these intermolecular forces, so the melting point is low
  3. Diamond has a giant covalent structure with many strong covalent bonds throughout the structure
  4. A large amount of energy is needed to break the many strong covalent bonds in diamond, giving it a very high melting point

Key terms in this question

simple molecular · melting point

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