Given below are two statements: one is labelled Assertion (A) and the other is labelled Reason (R).
Assertion (A): Energy is released when a very heavy nucleus (A = 240) breaks into two nuclei of A = 120.
Reason (R): A nucleus with A = 240 has lower binding energy per nucleon than a nucleus with A = 120, so the nucleons get more tightly bound after the break-up.
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Assertion (A) Energy is released when a very heavy nucleus (A = 240) breaks into two nuclei of A = 120. — True. This break-up of a heavy nucleus into two intermediate mass nuclei is nuclear fission. It releases energy of the order of 200 MeV per nucleus.
Reason (R) A nucleus with A = 240 has lower binding energy per nucleon than a nucleus with A = 120, so the nucleons get more tightly bound after the break-up. — True. The binding energy per nucleon is about 7.6 MeV for A = 240 and about 8.5 MeV for A = 120. So each nucleon is held more tightly in the fragments.
Why R explains A?
The same 240 nucleons are present before and after the break-up. A greater binding energy means a smaller total mass. So the fragments together are lighter than the original nucleus. This gain in binding energy is given out as energy.
The gain is about 0.9 MeV for each nucleon, so the total energy released is about \( 240\times0.9\,\mathrm{MeV}\approx216\,\mathrm{MeV} \).
NCERT (2026-27):
Nuclear Binding Energy – “(iii) A very heavy nucleus, say A = 240, has lower binding energy per nucleon compared to that of a nucleus with A = 120. Thus if a nucleus A = 240 breaks into two A = 120 nuclei, nucleons get more tightly bound. This implies energy would be released in the process.”
Fission – “Let us take a nucleus with A = 240 breaking into two fragments each of A = 120. Then Ebn for A = 240 nucleus is about 7.6 MeV, Ebn for the two A = 120 fragment nuclei is about 8.5 MeV.”