Explain why beta decay causes a change in the atomic number of a nucleus but does not cause a change in the mass number.

AQA GCSE Physics (8463) — 4.4.2 Atoms and nuclear radiation · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

In beta decay a neutron in the nucleus changes into a proton, emitting an electron (the beta particle). The proton number therefore increases by one, so the atomic number rises. The neutron that was converted is no longer present, but a proton replaces it, keeping the total number of nucleons (protons + neutrons) unchanged – the mass number stays the same. The emitted electron is not a nucleon and is not counted in the mass number.

Examiner tips

  • Show the neutron→proton conversion first, then state the effect on Z and A; keep the answer concise and use the terms ‘atomic number’ and ‘mass number’.
  • Use the word ‘increase’ for Z and ‘remain unchanged’ for A to match the scheme.

Common mistakes

  • Confusing the beta particle with a nucleon and claiming it changes the mass number.
  • Saying the mass number increases because a proton is added, ignoring that a neutron is lost.

Mark scheme (4 marks)

  1. In beta decay, a neutron converts into a proton (and an electron is emitted)
  2. The number of protons increases by one, so the atomic number increases
  3. The total number of protons and neutrons (mass number) stays the same because one neutron is replaced by one proton
  4. The emitted beta particle (electron) comes from the nucleus and is not counted in the mass number

Key terms in this question

atomic number · mass number

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