Explain what happens to the nucleus of an atom during alpha decay, including what an alpha particle is and how the nucleus changes.

WJEC A-Level Physics (Wales) — 1.7 Particles and nuclear structure · Explain · 5 marks · View as Markdown

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

Unstable nuclei can emit radiation in a process called radioactive decay. One type of decay involves the emission of an alpha particle.

Model answer (5 marks)

An alpha particle is a helium nucleus, consisting of 2 protons and 2 neutrons. During alpha decay the parent nucleus emits this particle, so it loses 2 protons and 2 neutrons. Consequently the atomic number (Z) decreases by 2 and the mass number (A) decreases by 4. The remaining nucleus is therefore a different element with Z‑2 and A‑4. The decay occurs because the original nucleus is unstable and the emission of the alpha particle allows it to move towards a more stable configuration.

Examiner tips

  • Use the term ‘alpha particle’ and state it is a helium nucleus (2p+2n).
  • Show the change in Z and A explicitly (ΔZ=−2, ΔA=−4).
  • Explain that the element changes to Z‑2.
  • Mention that the decay is driven by instability toward stability.

Common mistakes

  • Calling the alpha particle a neutron or a proton instead of a helium nucleus.
  • Failing to state the decrease in both atomic and mass numbers.
  • Not recognising that the element changes (e.g., from U to Th).

Mark scheme (5 marks)

  1. An alpha particle consists of 2 protons and 2 neutrons (equivalent to a helium nucleus)
  2. The nucleus loses 2 protons, so the atomic/proton number decreases by 2
  3. The nucleus loses 2 neutrons, so the mass/nucleon number decreases by 4
  4. The nucleus of a different element is formed / the element changes
  5. Alpha decay occurs because the nucleus is unstable and emits the alpha particle to become more stable

Key terms in this question

alpha particle · nucleus

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