Iron reacts with chlorine gas to form iron(III) chloride, FeCl₃. Using your knowledge of ions and ionic compounds, explain why the formula of iron(III) chloride is FeCl₃ and not FeCl or FeCl₂.

Eduqas A-Level Chemistry — 3.1 Introduction to organic chemistry · Explain · 5 marks · View as Markdown

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

Iron(III) chloride is an ionic compound formed when iron reacts with chlorine gas. Iron(III) ions have a charge of 3+. Chloride ions are formed from chlorine atoms.

Model answer (5 marks)

Iron atoms lose three electrons to form Fe³⁺ ions.
Chlorine atoms gain one electron to form Cl⁻ ions.
In an ionic compound the total positive charge must equal the total negative charge.
One Fe³⁺ ion (3+) requires three Cl⁻ ions (3×1−) to balance the charge.
Therefore the empirical formula is FeCl₃, not FeCl or FeCl₂.

Examiner tips

  • Show the charge of Fe³⁺ and Cl⁻ explicitly. Explain the charge‑balance requirement. State that three Cl⁻ ions are needed to neutralise one Fe³⁺ ion.

Common mistakes

  • Assuming FeCl₂ because iron can form a +2 ion. Forgetting that chlorine gains one electron. Not showing the charge‑balance calculation.

Mark scheme (5 marks)

  1. Chlorine atoms gain one electron to form chloride ions with a charge of 1−
  2. Chlorine is in group 7, so it gains one electron to achieve a full outer shell
  3. The iron(III) ion has a charge of 3+
  4. In an ionic compound the charges of the ions must balance / equal zero overall
  5. Three Cl⁻ ions (3 × 1−) are needed to balance one Fe³⁺ ion (3+), giving the formula FeCl₃

Key terms in this question

ion · ionic compound

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