Explain why lithium fluoride (LiF) is described as an ionic compound and outline the process by which it forms from its constituent elements.

IB DP Chemistry Standard 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)

LiF is an ionic compound because it is made up of oppositely charged ions, Li⁺ and F⁻, which are held together by electrostatic attraction.

Lithium, a metal, transfers one electron to fluorine, a non‑metal.

By losing this electron lithium attains the electron configuration of helium (1s²), giving a full outer shell and forming Li⁺.

Fluorine gains the electron to achieve the electron configuration of neon (2,8), giving a full outer shell and forming F⁻.

Examiner tips

  • Use the word "ionic" and mention electrostatic attraction; link electron transfer to the formation of ions; state the resulting electron configurations; keep the answer concise and to the point.

Common mistakes

  • Confusing covalent with ionic bonding; not specifying the electron transfer; omitting the full outer shell configurations; using incorrect electron configurations for Li⁺ or F⁻.

Mark scheme (4 marks)

  1. LiF consists of oppositely charged ions (Li⁺ and F⁻) held together by electrostatic attraction
  2. Lithium (a metal) transfers/loses one electron to fluorine (a non-metal)
  3. Lithium achieves the electron configuration of helium (1s²) / a full outer shell, forming Li⁺
  4. Fluorine achieves the electron configuration of neon (2,8) / a full outer shell, forming F⁻

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