Calcium fluoride is an ionic compound containing calcium ions and fluoride ions. Describe how a calcium atom and two fluorine atoms form ions, and explain what holds the ions together in calcium fluoride.

Cambridge International IGCSE Chemistry (0620) — 2.4 Ions and ionic bonds · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A calcium atom loses two electrons to give a Ca²⁺ ion.
Each fluorine atom gains one electron to give an F⁻ ion.
The ions now have full outer shells, a stable noble‑gas configuration.
Oppositely charged Ca²⁺ and F⁻ ions attract each other; the electrostatic attraction holds the crystal together.

Examiner tips

  • Use the exact wording Ca²⁺ and F⁻; mention electron loss/gain.
  • Show that the ions achieve a noble‑gas configuration.
  • Explain that electrostatic attraction holds the lattice together.
  • Keep the answer concise – 4 marks only.

Common mistakes

  • Writing Ca⁺ instead of Ca²⁺; confusing electron loss/gain.
  • Omitting the noble‑gas configuration or the electrostatic attraction.
  • Using vague terms like ‘bond’ instead of ‘electrostatic attraction’.

Mark scheme (4 marks)

  1. Calcium atom loses two electrons (to form a 2+ ion)
  2. Each fluorine atom gains one electron (to form a fluoride/F⁻ ion)
  3. The ions formed have full outer electron shells / achieve a stable noble gas configuration
  4. Oppositely charged ions attract each other / electrostatic attraction between Ca²⁺ and F⁻ ions holds them together

Key terms in this question

ion

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