# 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

> 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.

## 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

- [ion](https://www.gradenine.co.uk/glossary/ion)
- [ionic compound](https://www.gradenine.co.uk/glossary/ionic-compound)

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Source: [GradeNine](https://www.gradenine.co.uk/q/iron-reacts-with-chlorine-gas-to-66a1600a) · Published by Druglandscape Ltd.