Explain spontaneous changes in unstable nuclei
Distinguish unpredictable nuclear decay from electron transfer and chemical change.
Section 5.2.3 · Radioactive decay
Exam scope and assessment notes
Core and Extended · Physics 0972, 2026–2028. Use labelled nuclides, supplied count intervals and stated background conditions. Model drawings are not to scale. Written practice is self-checked. Human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0972 has the same subject content as IGCSE 0625, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Distinguish nucleus and electrons.
- Understand a mean over many events.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Does an individual spontaneous decay need an external trigger?
What you will learn
- Distinguish unpredictable nuclear decay from electron transfer and chemical change.
- Explain spontaneous random nuclear changes without confusing them with electron transfer.
Describe an event
An unstable nucleus can change spontaneously and emit radiation. No external trigger is needed for each decay, and the exact time of an individual event is random. A large collection can have a predictable average decrease even though no particular nucleus has a known decay deadline.
A daughter nucleus is the nucleus after the change. It may be more stable but still radioactive. One emission does not guarantee that a decay chain has finished.
Identify the changing object
Radioactive decay changes a nucleus. Ion formation changes electron number without changing the nucleus. Ordinary chemical rearrangement also does not mean that every nucleus has changed. Keep these accounts separate when describing what was observed.
A detector records a sample of emissions under its operating conditions. A quiet short interval does not prove that an unstable source has become stable, and regular-looking ticks do not prove regular individual decay times.
Pause and explain
Must every daughter nucleus be stable?
Worked example
A source gives several ticks followed by a quiet interval. Does that establish when the next nucleus will decay?
Show the worked solution
- Individual events are random.
- Detector counts sample emissions.
- A short interval gives no exact next-event deadline.
Answer No exact next-event time is established.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why a mean decay pattern cannot identify the next decaying nucleus.
Give me a hint
Separate sample averages from individual events.
Compare my reasoning
- The mean concerns many nuclei.
- Individual times remain random.
- A sample trend does not select the next nucleus.
The average pattern does not predict a particular next nucleus.
Look for these in your work
- Name the two scales.
- Retain individual randomness.
An atom loses an orbital electron while its nucleus stays unchanged. Is this radioactive decay?
Give me a hint
Identify what changed.
Compare my reasoning
- Only an electron leaves.
- The nucleus has not changed.
- This is ion formation, not nuclear decay.
No; it is ion formation.
Look for these in your work
- Locate the change.
- Use nuclear change as the distinction.
A report says a daughter must be stable because one emission was detected. Correct it.
Give me a hint
Check whether one event proves the entire chain.
Compare my reasoning
- One emission establishes one detected event.
- A daughter can remain unstable.
- Further decay may follow.
One emission does not prove a stable daughter.
Look for these in your work
- Allow a decay chain.
- Avoid an unsupported stability claim.
Common mistakes
- Random does not mean no average pattern.
- A daughter may remain radioactive.
- Ionisation is not nuclear decay.
What can you explain now?
What changes during radioactive decay?
Compare with the explanation
A nucleus
The unstable nucleus changes; electron transfer alone is a different process.
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Make it stick
Explain spontaneous random nuclear changes without confusing them with electron transfer.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.