Radioactive decay is a random and spontaneous process. Explain what is meant by the terms 'random' and 'spontaneous' as applied to radioactive decay, and describe how the count rate from a radioactive source changes over time.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Random means it is impossible to predict which particular nucleus will decay next.
Spontaneous means the decay is not triggered or caused by any external factor or conditions.
The count rate (or activity) decreases over time.
The decrease follows a regular pattern, with the same fraction of nuclei decaying in each equal time interval – the half‑life.
This occurs because there are fewer undecayed nuclei remaining, so fewer decays occur per second.
Spontaneous means the decay is not triggered or caused by any external factor or conditions.
The count rate (or activity) decreases over time.
The decrease follows a regular pattern, with the same fraction of nuclei decaying in each equal time interval – the half‑life.
This occurs because there are fewer undecayed nuclei remaining, so fewer decays occur per second.
Examiner tips
- Use the exact terms ‘random’ and ‘spontaneous’ as defined in the mark scheme.
- Show the link between fewer undecayed nuclei and the decreasing activity.
- Mention the regular pattern/half‑life to hit the pattern point.
Common mistakes
- Confusing ‘random’ with ‘unpredictable’ in a general sense rather than the specific nuclear context.
- Forgetting to mention that the decrease is exponential/regular (half‑life).
- Using ‘spontaneous’ to mean ‘no external trigger’ but not explicitly stating it isn’t caused by external conditions.
Mark scheme (5 marks)
- Random means it is impossible to predict which particular nucleus will decay next
- Spontaneous means the decay is not triggered or caused by any external factor / conditions
- The count rate (or activity) decreases over time
- The count rate decreases in a regular pattern / by the same fraction in equal time intervals (half-life)
- This is because there are fewer undecayed nuclei remaining over time, so fewer decays occur per second
Key terms in this question
random · spontaneous · count rate · radioactive decay
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Particles and nuclear structure questions
- Explain what is meant by the term 'isotopes' and describe how the structure of t…
- Explain how scientists' model of the atom changed as a result of the alpha parti…
- Describe the structure of an atom, including the location and relative charge of…
- Explain what happens to the nucleus of an atom during alpha decay, including how…