Explain why alpha decay causes a change in both the mass number and the atomic number of a nucleus, but gamma decay causes a change in neither.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
An alpha particle consists of two protons and two neutrons, so it has a mass number of 4. Emitting an alpha particle removes 2 protons from the nucleus, so the atomic number decreases by 2. Gamma radiation is electromagnetic radiation (not a particle), so no protons or neutrons are emitted. Because no protons or neutrons leave the nucleus during gamma decay, the mass number and atomic number both remain unchanged.
Examiner tips
- Use the exact terminology – ‘alpha particle’, ‘mass number’, ‘atomic number’, ‘gamma radiation’.
- Show the link between the particles emitted and the change in numbers.
- Remember that gamma decay is a photon, not a particle emission.
Mark scheme (4 marks)
- An alpha particle consists of two protons and two neutrons, so it has a mass number of 4.
- Emitting an alpha particle removes 2 protons from the nucleus, so the atomic number decreases by 2.
- Gamma radiation is electromagnetic radiation (not a particle), so no protons or neutrons are emitted.
- Because no protons or neutrons leave the nucleus during gamma decay, the mass number and atomic number both remain unchanged.
Key terms in this question
Related
- All AQA GCSE Physics (8463) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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