A nucleus of uranium-238 undergoes alpha decay to form a nucleus of thorium-234. Explain what an alpha particle is and describe the changes that occur to the nucleus of uranium-238 during this decay.

WJEC A-Level Physics (Wales) — 3.5 Nuclear decay · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

An alpha particle is a helium nucleus, consisting of 2 protons and 2 neutrons.
During alpha decay the uranium‑238 nucleus emits this alpha particle. Consequently:
1. The mass number of the nucleus decreases by 4 (238 → 234).
2. The atomic number decreases by 2 (92 → 90).
The loss of the 2 protons and 2 neutrons forms the new thorium‑234 nucleus.

Examiner tips

  • Use the term "helium nucleus" to show you know the identity of an alpha particle.
  • State both the decrease in mass number and atomic number explicitly.
  • Mention that the alpha particle is emitted and carries away the 2 protons and 2 neutrons.

Common mistakes

  • Confusing the alpha particle with a neutron or a proton.
  • Failing to state the change in both mass and atomic numbers.
  • Not recognising the alpha particle as a helium nucleus.

Mark scheme (5 marks)

  1. An alpha particle consists of 2 protons and 2 neutrons
  2. The alpha particle is the same as a helium nucleus (or has the same structure)
  3. The mass number (nucleon number) of the nucleus decreases by 4
  4. The atomic number (proton number) of the nucleus decreases by 2
  5. The nucleus emits the alpha particle, which carries away the 2 protons and 2 neutrons / the nucleus loses those particles to form the new (thorium) nucleus

Key terms in this question

alpha particle · nucleus

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