# 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

## 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

- [melting point](https://www.gradenine.co.uk/glossary/melting-point)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-lithium-fluoride-lif-has-5d8c95ff) · Published by Druglandscape Ltd.