# Explain why simple molecular substances, such as iodine and bromine, have low melting points and why melting points increase as the size of the molecule increases.

> Edexcel A-Level Chemistry (9CH0) — 2.4 Forces between molecules · Explain · 5 marks

> Simple molecular substances are made up of molecules held together by intermolecular forces. Iodine is a solid at room temperature, bromine is a liquid, and chlorine is a gas.

## Mark scheme (5 marks)

1. Simple molecular substances have weak intermolecular forces (between molecules)
2. Only a small amount of energy is needed to overcome these intermolecular forces
3. Therefore the melting point is low
4. As the size of the molecule increases, the intermolecular forces become stronger
5. More energy is needed to overcome the stronger intermolecular forces, so the melting point is higher

## Key terms

- [simple molecular substances](https://www.gradenine.co.uk/glossary/simple-molecular-substances)
- [melting point](https://www.gradenine.co.uk/glossary/melting-point)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](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/explain-why-simple-molecular-substances-such-5b2f47d1) · Published by Druglandscape Ltd.