Explain what is meant by the term 'half-life' of a radioactive substance and explain why it is not possible to predict exactly when a particular nucleus will decay.

WJEC GCSE Physics (Wales) — 2.9 Nuclear decay and nuclear energy · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Half‑life is the time taken for half the radioactive nuclei in a sample to decay.
The half‑life is a constant for a given isotope and does not depend on external conditions.
Nuclear decay is a random process.
Because the decay of each nucleus is random, the exact moment any particular nucleus decays cannot be predicted – only the probability of decay can be stated.

Examiner tips

  • Use the definition of half‑life first, then state its constancy, explain randomness, and finish with the impossibility of predicting an exact decay time.
  • Show the logical sequence: definition → constancy → randomness → prediction limitation.
  • Use precise terminology: ‘half‑life’, ‘isotope’, ‘random process’, ‘probability’.
  • Keep the answer concise – 4 marks only.

Mark scheme (4 marks)

  1. Half-life is the time taken for half the (number of) radioactive nuclei in a sample to decay
  2. Half-life is constant / the same for a given isotope (regardless of external conditions)
  3. Nuclear decay is a random process
  4. Because decay is random, the exact moment any individual nucleus decays cannot be predicted / only probabilities can be stated

Key terms in this question

half-life · nucleus

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