A Levels Physics (9702)•9702/12/M/J/23
Explanation
Conservation of Nucleon Number in Fission
Steps:
- Uranium-235 absorbs one neutron, forming uranium-236 with mass number 236.
- It splits into barium-141 and krypton-92, with combined mass number 141 + 92 = 233.
- Nucleon number must be conserved in the reaction.
- Difference is 236 - 233 = 3 free neutrons emitted.
Why D is correct:
- Conservation of nucleon number (total protons + neutrons) requires exactly 3 neutrons to balance the initial 236 nucleons against the 233 in the fission fragments.
Why the others are wrong:
- A. 0 neutrons leaves 233 nucleons, violating conservation by 3.
- B. 1 neutron leaves 234 nucleons, violating conservation by 2.
- C. 2 neutrons leaves 235 nucleons, violating conservation by 1.
Final answer: D
Topic: Mass defect and nuclear binding energy
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