Explain how the discovery of the neutron changed scientists' understanding of atomic structure, and explain why neutrons are important for the existence of stable nuclei.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Before the discovery of the neutron, the mass of a nucleus could not be explained by the number of protons alone – the mass number was larger than the proton number.
The neutron is a neutral particle found in the nucleus.
Its mass is about 1 u, almost equal to that of a proton, so it accounts for the missing mass.
Because neutrons carry no charge they do not add to the electrostatic repulsion between protons.
They contribute to the strong nuclear force, helping to bind protons together and stabilise the nucleus.
The neutron is a neutral particle found in the nucleus.
Its mass is about 1 u, almost equal to that of a proton, so it accounts for the missing mass.
Because neutrons carry no charge they do not add to the electrostatic repulsion between protons.
They contribute to the strong nuclear force, helping to bind protons together and stabilise the nucleus.
Examiner tips
- Use the exact terms ‘neutron’, ‘neutral’, ‘strong nuclear force’ and explain the mass discrepancy first.
- Show the logical sequence: mass problem → discovery → properties → role in stability.
- Keep each point brief but complete to hit all 5 marks.
Common mistakes
- Confusing the neutron with the electron or giving it a charge.
- Failing to mention the neutron’s mass or its role in reducing proton–proton repulsion.
Mark scheme (5 marks)
- Before the discovery of the neutron, scientists could not account for the extra mass in the nucleus (the mass number was greater than the proton number alone could explain).
- The neutron is a particle found in the nucleus with no charge (electrically neutral).
- The neutron has a relative mass of 1 (approximately equal to that of a proton), which accounts for the missing mass in the nucleus.
- Without neutrons, the repulsion between positively charged protons in the nucleus would cause the nucleus to break apart.
- Neutrons help hold the nucleus together / contribute to the strong nuclear force holding the nucleus stable.
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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