# Sodium reacts with chlorine to form sodium chloride according to the balanced equation: 2Na + Cl₂ → 2NaCl. Explain, using the law of conservation of mass and your knowledge of ions, why the charges in sodium chloride balance and why the total mass of sodium chloride produced equals the total mass of sodium and chlorine that reacted.

> WJEC A-Level Chemistry (Wales) — 3.1 Introduction to organic chemistry · Explain · 5 marks

> Sodium is a Group 1 metal and chlorine is a Group 7 non-metal. When they react, an ionic compound is formed.

## Mark scheme (5 marks)

1. No atoms are lost or made during a chemical reaction (law of conservation of mass), so the mass of the products equals the mass of the reactants.
2. Sodium is in Group 1 and loses one electron to form a Na⁺ ion (a positive ion).
3. Chlorine is in Group 7 and gains one electron to form a Cl⁻ ion (a negative ion).
4. The +1 charge on Na⁺ and the −1 charge on Cl⁻ cancel/balance, so the overall charge of sodium chloride is zero.
5. Atoms form ions to achieve a full outer shell of 8 electrons, which is the most stable arrangement.

## Key terms

- [conservation of mass](https://www.gradenine.co.uk/glossary/conservation-of-mass)
- [ion](https://www.gradenine.co.uk/glossary/ion)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/sodium-reacts-with-chlorine-to-form-67ac0ea2) · Published by Druglandscape Ltd.