Potassium iodide is an ionic compound. Describe how a potassium atom and an iodine atom form a potassium ion and an iodide ion, and state the type of bond that holds these ions together in the solid compound.

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

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

Model answer (4 marks)

A potassium atom loses one electron to give a K⁺ ion with a 1+ charge.
An iodine atom gains one electron to give an I⁻ ion with a 1− charge.
Both ions now have a full outer shell, a stable noble‑gas configuration.
In the solid, the K⁺ and I⁻ ions are held together by an ionic bond – electrostatic attraction between opposite charges.

Examiner tips

  • Use the exact wording ‘loses one electron’ and ‘gains one electron’ to show the transfer of charge.
  • Mention the 1+ and 1− charges explicitly.
  • State that the ions achieve a noble‑gas configuration.
  • Identify the bond as an ionic bond (electrostatic attraction).

Common mistakes

  • Writing ‘gains two electrons’ or ‘loses two electrons’ – incorrect charge transfer.
  • Forgetting to state the charges or the noble‑gas configuration.
  • Calling the bond a covalent bond instead of ionic.

Mark scheme (4 marks)

  1. Potassium atom loses one electron (to form a positive ion / potassium ion with a 1+ charge)
  2. Iodine atom gains one electron (to form a negative ion / iodide ion with a 1− charge)
  3. Both ions achieve a stable (full outer shell / noble gas) electron configuration
  4. The ions are held together by an ionic bond / electrostatic attraction between oppositely charged ions

Key terms in this question

ion

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