NEET 2020 Physics Question 158 Solution

When a uranium-235 nucleus absorbs a slow thermal neutron, it undergoes nuclear fission according to the reaction: $$^{235}_{92}\text{U} + ^1_0\text{n} \to ^{89}_{36}\text{Kr} + ^A_Z\text{X} + 3 ^1_0\text{n}$$ where $$^A_Z\text{X}$$ is the unknown co-product nucleus.

Total charge on reactant side equals total charge on product side: $$\sum Z_{\text{reactants}} = \sum Z_{\text{products}}$$ $$92 + 0 = 36 + Z + 3(0)$$ $$92 = 36 + Z$$ $$Z = 92 - 36 = 56$$ The element with atomic number $$Z = 56$$ is Barium ($$\text{Ba}$$).

Total mass number on reactant side equals total mass number on product side: $$\sum A_{\text{reactants}} = \sum A_{\text{products}}$$ $$235 + 1 = 89 + A + 3(1)$$ $$236 = 89 + A + 3$$ $$236 = 92 + A$$ $$A = 236 - 92 = 144$$

The missing nuclide is barium-144, represented as $$^{144}_{56}\text{Ba}$$. This matches Option (1).