In the nuclear reaction given below, a neutron hits a uranium nucleus. The values of \( a \) and \( b \) are respectively
\( \mathrm{n} + {}^{235}_{92}\mathrm{U} \rightarrow {}^{a}_{54}\mathrm{Xe} + {}^{94}_{b}\mathrm{Sr} + 2\,\mathrm{n} \)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 140 and 38
In a nuclear reaction the mass number (total number of nucleons) and the atomic number (total charge) are both conserved. A neutron has mass number 1 and atomic number 0.
Mass number:
\( \displaystyle 1 + 235 = a + 94 + 2\times1 \;\Rightarrow\; 236 = a + 96 \;\Rightarrow\; a = 140 \)
Atomic number:
\( \displaystyle 0 + 92 = 54 + b + 2\times0 \;\Rightarrow\; b = 38 \)
Note: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.
NCERT (2026-27):
Fission – “… A most important neutron-induced nuclear reaction is fission. … Or, as another example,
\( \displaystyle {}^{1}_{0}\mathrm{n} + {}^{235}_{92}\mathrm{U} \rightarrow {}^{140}_{54}\mathrm{Xe} + {}^{94}_{38}\mathrm{Sr} + 2\,{}^{1}_{0}\mathrm{n} \qquad (13.12) \)”