Explain fission with a balanced supplied nuclear equation
Describe splitting a heavy nucleus and account qualitatively for reduced rest mass and released energy.
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
- Use nuclide notation.
- Conserve stated particle-number totals.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens in nuclear fission?
What you will learn
- Describe splitting a heavy nucleus and account qualitatively for reduced rest mass and released energy.
- Describe fission, balance a supplied nuclide equation and distinguish nucleon count from mass-energy accounting.
Read a supplied splitting channel
Nuclear fission splits a heavy nucleus into smaller nuclei. In a supplied neutron-induced uranium-235 example, ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n. The outgoing neutrons belong in the nucleon account: 235 + 1 = 141 + 92 + 3. Proton numbers balance as 92 = 56 + 36.
This is one supplied fission channel, not the only possible pair of products. The fragments and other products carry released energy, chiefly through kinetic energy in this simple account. A neutron can initiate the supplied process; this does not mean every encountered nucleus must split.
Separate particle counts from exact mass
For an energy-releasing fission, the total rest mass of products is less than that of the initial system. The decrease corresponds to energy released; mass-energy accounting is conserved. No numerical mass defect or energy calculation is required here. Integer nucleon numbers still balance because they count particles rather than exact rest masses.
Fission is not chemical breaking of molecules, and it differs from an ordinary alpha decay even though both can produce smaller nuclei. This lesson interprets a supplied equation and qualitative energy transfer; it does not give procedures for acquiring fuel or operating a reactor.
Pause and explain
If nucleon totals balance, must initial and final exact rest masses be equal?
Worked example
Check the nucleon and proton-number totals in the supplied uranium-235 fission channel.
Show the worked solution
- Left A total is 235 + 1 = 236.
- Right A total is 141 + 92 + 3 = 236.
- Right Z total is 56 + 36 = 92.
Answer A totals 236; Z totals 92 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.
Complete the outgoing neutron count for U-235 plus one neutron producing Ba-141 and Kr-92.
Give me a hint
Subtract fragment nucleon totals from the initial total.
Compare my reasoning
- Initial A total is 236.
- Fragments total 233.
- Three neutrons supply the missing three.
3 neutrons.
Look for these in your work
- Count the incident neutron.
- Check Z = 92 separately.
Explain the qualitative mass and energy change in an energy-releasing fission.
Give me a hint
Compare total rest masses, not A labels.
Compare my reasoning
- Product rest mass is lower.
- The difference corresponds to released energy.
- Fragment motion and radiation carry that energy.
Lower total product rest mass with energy released.
Look for these in your work
- Use the complete reaction system.
- Do not calculate an unstated mass defect.
A report treats a balanced fission equation as the unique possible product pair. Evaluate it.
Give me a hint
Balance is necessary but does not establish uniqueness.
Compare my reasoning
- The supplied channel balances.
- Other allowed product combinations can also occur.
- The written example does not specify every possible outcome.
The balanced supplied channel is one possible example, not a uniqueness claim.
Look for these in your work
- Retain the given channel.
- Distinguish consistency from exclusivity.
Common mistakes
- Include all emitted neutrons.
- A counts are not exact rest masses.
- The supplied channel is not unique.
What can you explain now?
Does an uncharged neutron contribute to nucleon number?
Compare with the explanation
Yes
A neutron has A = 1 and Z = 0, so it contributes one nucleon despite zero electric charge.
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Make it stick
Describe fission, balance a supplied nuclide equation and distinguish nucleon count from mass-energy accounting.
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.