Explain excess-neutron and heavy-nucleus instability
Explain the syllabus models of instability without assuming every isotope or daughter is radioactive.
Section 5.2.3 · Radioactive decay
Extended only (Supplement); Physics 0625, 2026–2028. Use labelled nuclides, supplied count intervals and stated background conditions. Model drawings are not to scale. Written practice is self-checked.
Before you begin
- Calculate neutron number.
- Know alpha and beta-minus changes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which change reduces neutron excess in beta-minus decay?
What you will learn
- Explain the syllabus models of instability without assuming every isotope or daughter is radioactive.
- Use neutron excess and heavy-nucleus models while retaining isotope and daughter limits.
Use two models of instability
A nucleus may be unstable because it has too many neutrons relative to its protons or because it is very heavy. In beta-minus decay a neutron becomes a proton, reducing neutron excess. Alpha emission reduces a very heavy nucleus by removing two protons and two neutrons.
These explanations do not mean every nucleus with many neutrons is unstable. The proton-neutron balance and specific nuclide matter. Different isotopes of one element can have different stability, and isotopes are not automatically radioactive.
Do not promise the end of a chain
Decay can move a nucleus toward greater stability, but a daughter may still be unstable. An alpha-emitting heavy parent can be followed by further emissions. A sequence requires the supplied nuclides or decay information; particle counts alone do not give a measured half-life.
The qualitative explanation does not determine an exact decay date, energy or detector rate. Use only the information supplied in the problem and keep the random timing of individual events.
Pause and explain
Does alpha decay guarantee a stable daughter?
Worked example
A neutron-rich nucleus undergoes beta-minus decay. Explain the change in its neutron-proton balance.
Show the worked solution
- One neutron is changed into a proton.
- Neutron number falls by one and proton number rises by one.
- The neutron excess is reduced.
Answer Neutron excess decreases through neutron-to-proton conversion.
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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 beta-minus can reduce instability from neutron excess.
Give me a hint
Track both nucleon types.
Compare my reasoning
- One neutron changes.
- It becomes a proton.
- The neutron-proton balance changes toward less neutron excess.
It reduces neutron excess by neutron-to-proton conversion.
Look for these in your work
- Track neutron and proton counts.
- Retain one-event scope.
Explain how alpha emission reduces a very heavy nucleus.
Give me a hint
Identify the emitted nucleons.
Compare my reasoning
- Alpha contains two protons and two neutrons.
- Four nucleons leave the parent.
- The daughter is a smaller nucleus.
The parent loses four nucleons in an alpha particle.
Look for these in your work
- Name both nucleon types.
- Do not promise final stability.
Two isotopes are said to have equal half-lives because they share proton number. Evaluate it.
Give me a hint
Compare isotope identity and stability evidence.
Compare my reasoning
- Their neutron numbers differ.
- Nuclear stability and decay properties can differ.
- Equal proton number alone does not establish equal half-life.
The claim is unsupported without decay data.
Look for these in your work
- Separate element and isotope properties.
- Name missing decay evidence.
Common mistakes
- Isotope does not mean radioactive.
- A daughter may still be unstable.
- Particle counts do not supply a half-life.
What can you explain now?
Are all isotopes radioactive?
Compare with the explanation
No
Isotopes share proton number but may have different nuclear stability.
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Make it stick
Use neutron excess and heavy-nucleus models while retaining isotope and daughter limits.
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.