Radioactive substances undergo nuclear decay, emitting different types of radiation. Explain what happens to the nucleus of an atom when it undergoes alpha decay, and describe two properties of alpha radiation that make it the least penetrating of the three main types of nuclear radiation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
The nucleus emits a helium nucleus (α‑particle) consisting of 2 protons and 2 neutrons. Consequently the atomic number decreases by 2 and the mass number decreases by 4.
Alpha particles are large and have a high mass, so they interact strongly with matter. Their strong ionising power causes them to lose energy rapidly and they are stopped by only a few centimetres of air or a thin sheet of paper.
Alpha particles are large and have a high mass, so they interact strongly with matter. Their strong ionising power causes them to lose energy rapidly and they are stopped by only a few centimetres of air or a thin sheet of paper.
Examiner tips
- Use the phrase ‘emits a helium nucleus (α‑particle)’ to show you know what is lost.
- State both the atomic number and mass number changes.
- Mention the size/mass and ionising power as the two key properties.
- Keep the answer concise and use the exact terminology.
Common mistakes
- Confusing alpha decay with beta decay (missing the 2p+2n loss).
- Not stating both the atomic number and mass number changes.
- Describing alpha radiation as ‘highly penetrating’ instead of ‘least penetrating’.
Mark scheme (5 marks)
- The nucleus loses 2 protons and 2 neutrons (emits a helium nucleus / alpha particle)
- The proton number (atomic number) decreases by 2
- The mass number (nucleon number) decreases by 4
- Alpha particles are relatively large / have a large mass (compared to beta or gamma)
- Alpha particles are strongly ionising / interact strongly with matter, so they lose energy quickly and are stopped by a few centimetres of air or a thin sheet of paper
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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