A student says: 'When a nucleus emits a beta particle, the nucleus loses a particle so it must become lighter and its atomic number must decrease.' Explain why the student is incorrect, describing what actually happens to the nucleus during beta decay.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A beta particle is a fast‑moving electron emitted from the nucleus.
During beta decay a neutron in the nucleus changes into a proton, and the beta particle is emitted.
The atomic (proton) number therefore increases by 1, not decreases.
The mass or nucleon number stays the same; the nucleus does not become lighter.
The student is wrong that the nucleus becomes lighter because the number of nucleons is conserved – a neutron is replaced by a proton, so the total count of particles in the nucleus remains the same.
During beta decay a neutron in the nucleus changes into a proton, and the beta particle is emitted.
The atomic (proton) number therefore increases by 1, not decreases.
The mass or nucleon number stays the same; the nucleus does not become lighter.
The student is wrong that the nucleus becomes lighter because the number of nucleons is conserved – a neutron is replaced by a proton, so the total count of particles in the nucleus remains the same.
Examiner tips
- Use the exact terms ‘beta particle’, ‘neutron → proton’, ‘atomic number increases’, ‘mass number unchanged’.
- Show the chain of events: neutron → proton + beta‑particle + antineutrino.
- Highlight the conservation of nucleon number to justify the mass number point.
Common mistakes
- Confusing the beta particle with a positron (wrong charge).
- Claiming the atomic number decreases instead of increases.
- Saying the nucleus becomes lighter without recognising nucleon conservation.
Mark scheme (5 marks)
- A beta particle is a fast-moving electron emitted from the nucleus
- During beta decay a neutron in the nucleus changes into a proton (and a beta particle is emitted)
- The atomic/proton number increases by 1 (not decreases)
- The mass/nucleon number stays the same (nucleus does not become lighter)
- The student is wrong that the nucleus becomes lighter because the number of nucleons is conserved — a neutron is replaced by a proton, so the total count of particles in the nucleus remains the same
Key terms in this question
beta particle · atomic number · nucleus
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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