Graphite and diamond are both giant covalent structures made entirely of carbon atoms, yet they have very different properties. Explain why graphite can conduct electricity whilst diamond cannot, and explain why both substances have high melting points.

OCR A-Level Chemistry A (H432) — 2.4 Electrons, bonding and structure · Explain · 5 marks · View as Markdown

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

Graphite is used as an electrode material in electrolysis cells. Diamond is used in cutting tools. Both are allotropes of carbon.

Model answer (5 marks)

In graphite each carbon atom is bonded to three others, leaving one valence electron that is not involved in a covalent bond. This electron is delocalised over the layers and can move freely, carrying charge and allowing graphite to conduct electricity. In diamond each carbon atom is bonded to four others, forming a rigid tetrahedral network; all valence electrons are used in covalent bonds, so there are no free electrons and diamond is an insulator. Both graphite and diamond are giant covalent solids with a large number of strong covalent bonds throughout the structure. Breaking these bonds requires a large amount of energy, which gives both materials very high melting points.

Examiner tips

  • Mention delocalised electrons in graphite for conductivity
  • State that diamond has no delocalised electrons
  • Explain strong covalent bonds give high melting points
  • Use correct terminology: covalent, delocalised, giant covalent

Common mistakes

  • Confusing graphite with diamond for conductivity
  • Forgetting that diamond has no free electrons
  • Not linking strong covalent bonds to high melting points

Mark scheme (5 marks)

  1. In graphite, each carbon atom forms three covalent bonds (with three other carbon atoms), leaving one electron per carbon atom that is not used in bonding.
  2. In graphite, this electron is delocalised (free to move) and can carry charge, allowing graphite to conduct electricity.
  3. In diamond, each carbon atom forms four covalent bonds with four other carbon atoms, so there are no delocalised electrons.
  4. Both substances have many strong covalent bonds throughout the giant covalent structure.
  5. A large amount of energy is needed to break these strong covalent bonds, which is why both substances have high melting points.

Key terms in this question

giant covalent structure · graphite · diamond

Related

More Electrons, bonding and structure questions

▶ Try answering this question with AI marking (free) →