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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
Simple molecular substances are held together only by weak intermolecular forces such as London dispersion forces.
Because these forces are weak, only a small amount of energy is required to break them, so the melting point is low.
When the size of the molecule increases, the surface area and polarizability increase, making the London forces stronger.
Stronger forces mean more energy is needed to separate the molecules, so the melting point rises.
Because these forces are weak, only a small amount of energy is required to break them, so the melting point is low.
When the size of the molecule increases, the surface area and polarizability increase, making the London forces stronger.
Stronger forces mean more energy is needed to separate the molecules, so the melting point rises.
Examiner tips
- Use the term ‘London dispersion forces’ to show understanding of the force type.
- Explain that larger molecules have greater surface area and polarizability.
- Show the logical link: weaker forces → low melting point; stronger forces → higher melting point.
- Keep the answer concise and directly address each point in the mark scheme.
Common mistakes
- Confusing intermolecular forces with covalent bonds.
- Failing to mention that larger molecules have more surface area and polarizability.
- Giving a generic statement about ‘more energy needed’ without linking it to force strength.
Mark scheme (5 marks)
- Simple molecular substances have weak intermolecular forces (between molecules)
- Only a small amount of energy is needed to overcome these intermolecular forces
- Therefore the melting point is low
- As the size of the molecule increases, the intermolecular forces become stronger
- More energy is needed to overcome the stronger intermolecular forces, so the melting point is higher
Key terms in this question
simple molecular substances · melting point
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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