Which of the following is/are fission reaction(s)?
(I) \( {}^{1}_{0}n+{}^{235}_{92}\mathrm{U}\rightarrow{}^{236}_{92}\mathrm{U}\rightarrow{}^{133}_{51}\mathrm{Sb}+{}^{99}_{41}\mathrm{Nb}+4\,{}^{1}_{0}n \)
(II) \( {}^{1}_{0}n+{}^{235}_{92}\mathrm{U}\rightarrow{}^{140}_{54}\mathrm{Xe}+{}^{94}_{38}\mathrm{Sr}+2\,{}^{1}_{0}n \)
(III) \( {}^{2}_{1}\mathrm{H}+{}^{2}_{1}\mathrm{H}\rightarrow{}^{3}_{2}\mathrm{He}+{}^{1}_{0}n \)
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Both I and II
In fission a heavy nucleus, here \( {}^{235}_{92}\mathrm{U} \) after absorbing a neutron, breaks into two intermediate-mass fragments and a few neutrons. In fusion two light nuclei combine to form a heavier one. In every nuclear reaction the mass numbers and the charge numbers balance on both sides.
(I) \( {}^{1}_{0}n+{}^{235}_{92}\mathrm{U}\rightarrow{}^{236}_{92}\mathrm{U}\rightarrow{}^{133}_{51}\mathrm{Sb}+{}^{99}_{41}\mathrm{Nb}+4\,{}^{1}_{0}n \) — Fission. \( {}^{236}_{92}\mathrm{U} \) splits into antimony and niobium; mass numbers 133 + 99 + 4 = 236 and charge numbers 51 + 41 = 92 balance.
(II) \( {}^{1}_{0}n+{}^{235}_{92}\mathrm{U}\rightarrow{}^{140}_{54}\mathrm{Xe}+{}^{94}_{38}\mathrm{Sr}+2\,{}^{1}_{0}n \) — Fission. The uranium nucleus splits into xenon and strontium; mass numbers 140 + 94 + 2 = 236 and charge numbers 54 + 38 = 92 balance.
(III) \( {}^{2}_{1}\mathrm{H}+{}^{2}_{1}\mathrm{H}\rightarrow{}^{3}_{2}\mathrm{He}+{}^{1}_{0}n \) — Not fission. Two light deuterons join to form the heavier helium-3 nucleus, so this is fusion.
NCERT (2026-27):
Fission – “A most important neutron-induced nuclear reaction is fission. An example of fission is when a uranium isotope \( {}^{235}_{92}\mathrm{U} \) bombarded with a neutron breaks into two intermediate mass nuclear fragments … The same reaction can produce other pairs of intermediate mass fragments”
\( {}^{1}_{0}n+{}^{235}_{92}\mathrm{U}\rightarrow{}^{236}_{92}\mathrm{U}\rightarrow{}^{133}_{51}\mathrm{Sb}+{}^{99}_{41}\mathrm{Nb}+4\,{}^{1}_{0}n \qquad (13.11) \)
“Or, as another example,”
\( {}^{1}_{0}n+{}^{235}_{92}\mathrm{U}\rightarrow{}^{140}_{54}\mathrm{Xe}+{}^{94}_{38}\mathrm{Sr}+2\,{}^{1}_{0}n \qquad (13.12) \)
Nuclear Fusion – Energy Generation in Stars – “When two light nuclei fuse to form a larger nucleus, energy is released, since the larger nucleus is more tightly bound… Some examples of such energy liberating nuclear fusion reactions are :”
\( {}^{2}_{1}\mathrm{H}+{}^{2}_{1}\mathrm{H}\rightarrow{}^{3}_{2}\mathrm{He}+n+3.27\,\mathrm{MeV} \qquad [13.13(b)] \)