Lithium fluoride is an ionic compound formed when lithium reacts with fluorine. Explain how lithium fluoride 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).

Lithium is in Group 1 of the periodic table. Fluorine is in Group 7 of the periodic table. Lithium fluoride is used in specialist optical equipment because it can withstand very high temperatures.

Model answer (5 marks)

Lithium loses one electron to give a Li⁺ ion with a full outer shell (stable electronic configuration). Fluorine gains one electron to give an F⁻ ion with a full outer shell. The oppositely charged ions attract each other, forming a crystal lattice of Li⁺ and F⁻ ions. The lattice is giant, so many strong electrostatic attractions exist throughout the solid. A large amount of energy is required to break all these attractions, giving lithium fluoride a high melting point.

Examiner tips

  • Use the word ‘lose’ for Li and ‘gain’ for F to show electron transfer. Mention the stable electronic configuration (full outer shell). State that the lattice is giant and that many attractions must be overcome to melt. Keep the answer concise and use the exact terminology from the mark scheme.

Common mistakes

  • Writing ‘Li loses an electron to form Li⁺’ but forgetting to mention the full outer shell. Mixing up the charges (e.g., calling F⁻ a cation). Not explaining that the high melting point is due to the large lattice energy.

Mark scheme (5 marks)

  1. Lithium loses one electron (to form a Li⁺ ion with a full outer shell / stable electronic configuration)
  2. Fluorine gains one electron (to form an F⁻ ion with a full outer shell / stable electronic configuration)
  3. The oppositely charged ions attract each other / there is electrostatic attraction between Li⁺ and F⁻ ions
  4. Lithium fluoride has a giant ionic lattice structure with many strong electrostatic attractions throughout the lattice
  5. A large amount of energy is needed to overcome / break all of these strong electrostatic attractions, so the melting point is high

Key terms in this question

ion · melting point

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