Describe what happens to a large nucleus during nuclear fission and explain how the energy released in this process originates.

Eduqas GCSE Physics — 9.4 Nuclear fission and fusion · Describe and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A large nucleus captures a neutron, becoming unstable. It then splits into two smaller daughter nuclei and releases additional neutrons. The total mass of the fission products is less than the mass of the original nucleus – a mass defect. This lost mass is converted into energy according to Einstein’s mass–energy equivalence, E=mc².

Examiner tips

Use the exact sequence: neutron absorption → instability → split into two nuclei + neutrons → mass defect → E=mc²
Mention the mass defect explicitly
Show the energy origin with Einstein’s equation

Common mistakes

Forgetting to state that neutrons are released
Not mentioning the mass defect or Einstein’s equation
Using vague terms like ‘energy is released’ without explaining the mass–energy link

Mark scheme (4 marks)

A large/heavy nucleus absorbs a neutron (and becomes unstable)
The nucleus splits into two smaller/daughter nuclei (and releases neutrons)
The total mass of the products is less than the mass of the original nucleus (mass defect / mass is lost)
The lost mass is converted into energy (reference to Einstein's mass–energy equivalence or E = mc²)

Key terms in this question

nuclear fission