Sodium chloride is an ionic compound. Explain how sodium chloride is formed from its elements, and why it has a high melting point.

Edexcel A-Level Chemistry (9CH0) — 2.1 Ionic bonding · Explain · 5 marks · View as Markdown

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

Sodium is a metal in Group 1 of the periodic table. Chlorine is a non-metal in Group 7 of the periodic table. When they react together, they form the ionic compound sodium chloride.

Model answer (5 marks)

Sodium loses one electron to give a Na⁺ ion. Chlorine gains one electron to give a Cl⁻ ion. Both ions now have a full outer shell and a stable electronic configuration. The Na⁺ and Cl⁻ ions arrange themselves into a giant ionic lattice, where strong electrostatic forces of attraction act between the oppositely charged ions. A large amount of energy is required to overcome these forces, so the melting point of NaCl is high.

Examiner tips

  • Use the word ‘lose’ for sodium and ‘gain’ for chlorine; include the ion symbols Na⁺ and Cl⁻. Mention the stable electronic configuration. Explain that the lattice gives strong electrostatic attraction and that this requires a lot of energy to break, giving a high melting point. Keep the answer concise and to the points.

Mark scheme (5 marks)

  1. Sodium loses one electron (to form a positive ion / Na⁺)
  2. Chlorine gains one electron (to form a negative ion / Cl⁻)
  3. Both ions achieve a full outer shell / stable electronic configuration
  4. Sodium chloride forms a giant ionic lattice in which there are strong electrostatic forces of attraction between oppositely charged ions
  5. A large amount of energy is needed to overcome these strong electrostatic forces, so the melting point is high

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