Explain why lithium fluoride (LiF) has a significantly higher melting point than lithium iodide (LiI), even though both compounds contain a singly charged cation and a singly charged anion.

IB DP Chemistry Higher Level (2023 syllabus) — S2.1 The ionic model · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Both LiF and LiI are ionic, so melting requires breaking the electrostatic attraction between Li⁺ and the halide ion.
1. In LiF the F⁻ ion is much smaller than the I⁻ ion.
2. The smaller radius gives a shorter Li⁺–F⁻ distance than the Li⁺–I⁻ distance.
3. A shorter inter‑ionic distance increases the electrostatic force of attraction.
4. The stronger attraction (higher lattice enthalpy) means more energy is needed to melt LiF, giving it a higher melting point than LiI.

Examiner tips

  • Use the term ‘lattice enthalpy’ to link ion size to melting point.
  • Show the chain: smaller ion → shorter distance → stronger attraction → higher melting point.
  • Mention both compounds are ionic to justify the need to overcome electrostatic forces.

Common mistakes

  • Confusing ionic radius with atomic radius; students may say ‘F is smaller than I’ without linking to lattice enthalpy.
  • Failing to explain that the stronger attraction is due to the shorter inter‑ionic distance rather than just ‘smaller ion’ alone.
  • Using vague terms like ‘stronger bonds’ without specifying electrostatic attraction or lattice enthalpy.

Mark scheme (4 marks)

  1. Both compounds are ionic, so melting requires overcoming electrostatic attractions between oppositely charged ions.
  2. The fluoride ion (F⁻) has a much smaller ionic radius than the iodide ion (I⁻).
  3. A smaller ionic radius leads to a shorter interionic distance (smaller Li⁺–F⁻ distance compared to Li⁺–I⁻ distance).
  4. A shorter interionic distance results in a stronger electrostatic attraction (greater lattice enthalpy), so more energy is required to melt LiF.

Key terms in this question

melting point

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