Sodium is a metal that is soft and has a relatively low melting point, whilst tungsten is a metal used in light bulb filaments because it has a very high melting point. Using your knowledge of metallic bonding, explain why tungsten has a much higher melting point than sodium.

Edexcel A-Level Chemistry (9CH0) — 2.3 Metallic bonding · Explain · 5 marks · View as Markdown

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

Sodium has one electron in its outer shell and forms a 1+ ion. Tungsten has six electrons in its outer shell and forms ions with a higher charge. Both metals consist of a lattice of positive metal ions surrounded by a sea of delocalised electrons.

Model answer (5 marks)

Both metals consist of a giant lattice of positive metal ions surrounded by a sea of delocalised electrons.
Tungsten ions have a higher charge and more delocalised electrons per atom than sodium.
The electrostatic attraction between the positive ions and the delocalised electrons is therefore stronger in tungsten.
More energy is required to overcome these stronger attractions in tungsten.
Hence tungsten has a much higher melting point than sodium.

Examiner tips

  • Use the exact phrase "giant lattice of positive metal ions surrounded by a sea of delocalised electrons" to secure the first point. Show the charge difference (1+ vs 6+) and link it to stronger attraction. Finish with a clear, linked conclusion that states the higher melting point.
  • common_mistakes
  • :
  • Confusing the number of delocalised electrons with the number of valence electrons. Failing to mention the higher charge of tungsten ions. Giving a generic statement about metallic bonding without linking to melting point.

Mark scheme (5 marks)

  1. Both metals contain a giant lattice of positive metal ions surrounded by / attracted to a sea of delocalised electrons
  2. Tungsten ions have a higher charge / more electrons are delocalised per atom than in sodium
  3. The electrostatic attraction between the positive ions and the delocalised electrons is stronger in tungsten than in sodium
  4. More energy is needed to overcome / break these stronger attractions in tungsten
  5. Therefore tungsten has a higher melting point than sodium (linked conclusion)

Key terms in this question

metallic bonding · melting point

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