Explain fusion with a balanced supplied nuclear equation
Describe light nuclei joining and trace qualitative rest-mass reduction and energy release.
Section 5.1.2 · The nucleus
Exam scope and assessment notes
Extended only (Supplement) · 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
- Read hydrogen-isotope notation.
- Distinguish fission from electron transfer.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which description identifies fusion?
What you will learn
- Describe light nuclei joining and trace qualitative rest-mass reduction and energy release.
- Describe fusion and use balanced nuclide numbers with qualitative mass-energy changes.
Read the supplied joining process
Nuclear fusion joins light nuclei to form a heavier nucleus. A supplied example is ²₁H + ³₁H → ⁴₂He + ¹₀n. Nucleon totals are 2 + 3 = 4 + 1 = 5, and proton-number totals are 1 + 1 = 2 + 0 = 2. The emitted neutron must be retained in the complete equation.
Both initial nuclei are positively charged and repel as they approach. Suitable conditions must allow them to approach closely enough for the nuclear interaction; ordinary mixing is not fusion. The supplied equation explains a nuclear reaction, not a procedure to make it occur.
Account for the energy qualitatively
For this energy-releasing reaction, total product rest mass is lower than total initial rest mass. Released energy appears in product kinetic energies and can later be transferred to surroundings. Nucleon and charge counts balance while exact rest mass differs; the mass decrease corresponds to the energy release.
This particular hydrogen-isotope example is not a claim that it is the only fusion process or the exact overall reaction in every star. Fusion joins light nuclei; fission splits a heavy one. Neither is ordinary electron rearrangement in a chemical reaction. Numerical mass-energy values lie beyond this lesson’s stated requirement.
Pause and explain
Which particle completes the supplied hydrogen-2 plus hydrogen-3 reaction yielding helium-4?
Worked example
Check ²₁H + ³₁H → ⁴₂He + ¹₀n.
Show the worked solution
- Left A totals five.
- Right A totals five.
- Both sides have total Z = two.
Answer A total 5 and Z total 2 on both sides.
Try a topic test
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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.
Identify the missing product in ²₁H + ³₁H → ⁴₂He + ?.
Give me a hint
Balance A and Z independently.
Compare my reasoning
- Initial A is five and Z is two.
- Helium contributes four and two.
- The missing product has A = one and Z = zero.
¹₀n.
Look for these in your work
- Check both totals.
- Name the neutron.
Explain why ordinary mixing of positively charged light nuclei does not guarantee fusion.
Give me a hint
Consider their approach before a close nuclear interaction.
Compare my reasoning
- The nuclei repel electrically.
- They must approach sufficiently closely.
- Suitable physical conditions are required.
Positive nuclei repel; close approach under suitable conditions is needed.
Look for these in your work
- Separate a reaction equation from an apparatus procedure.
- Name the repulsive interaction.
A report says balanced A proves no energy can be released in fusion. Correct it.
Give me a hint
Separate count totals from exact rest masses.
Compare my reasoning
- A counts nucleons.
- Products may have lower total rest mass.
- The mass decrease corresponds to released energy.
Balanced nucleon counts are compatible with rest-mass reduction and energy release.
Look for these in your work
- Retain both conservation accounts.
- Avoid treating A as an exact mass.
Common mistakes
- Include the emitted neutron.
- Joining is nuclear, not chemical mixing.
- Mass number is not exact rest mass.
What can you explain now?
What is the charge relationship of the initial light nuclei?
Compare with the explanation
Both are positive
Their positive charges create repulsion that must be overcome sufficiently for a close nuclear interaction.
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Make it stick
Describe fusion and use balanced nuclide numbers with qualitative mass-energy changes.
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.