Explain why fullerene C₆₀ has a much lower melting point than diamond, despite both being composed entirely of carbon atoms.

IB DP Chemistry Standard Level (2023 syllabus) — S2.4 From models to materials · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Diamond has a giant covalent network structure, whereas C₆₀ fullerene consists of discrete molecules.

In diamond the melting point is high because all the strong C–C covalent bonds in the lattice must be broken, requiring a large amount of energy.

C₆₀ molecules are held together only by weak London dispersion forces. Only these weak intermolecular forces need to be overcome to melt C₆₀, so far less energy is required, giving a much lower melting point.

Examiner tips

  • State the structural difference first – network vs. discrete molecules. Explain that diamond requires breaking covalent bonds, C₆₀ only weak forces. Mention the energy difference and link to melting point.
  • common_mistakes
  • :
  • Confusing covalent bonds in C₆₀ with those in diamond. Forgetting to mention London dispersion forces. Using vague terms like "strong bonds" without specifying covalent.

Mark scheme (4 marks)

  1. Diamond has a giant covalent (network covalent / macromolecular) structure, whereas C₆₀ fullerene has a simple molecular structure.
  2. Melting diamond requires breaking strong covalent C–C bonds throughout the entire lattice, requiring very large amounts of energy.
  3. In C₆₀, the intermolecular forces between discrete molecules are weak London (dispersion) forces.
  4. Only these weak intermolecular forces need to be overcome to melt C₆₀, so much less energy is required, resulting in a lower melting point.

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