Read the following passage and answer the questions.
Nuclear power stations and the sun both draw their energy from atomic nuclei. In a reactor, a neutron strikes a nucleus of uranium-235. The nucleus splits into two nuclei and a few neutrons. About 200 MeV of energy is released each time a uranium nucleus splits in this way. A reactor uses this energy to produce electricity. The sun works in a different way. In its core, very light nuclei, such as those of hydrogen, join to form heavier nuclei. These nuclei are positively charged and repel each other. So the joining takes place only at a very high temperature, where the nuclei move fast enough to come very close. Scientists are also trying to make light nuclei join in a controlled way on the earth.
When a uranium nucleus splits, the number of protons and the number of neutrons are the same before and after the reaction. The energy released comes from
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → the difference between the total binding energies of the nuclei on the two sides
The binding energy of a nucleus makes its mass smaller than the mass of its free nucleons. In a nuclear reaction the protons and neutrons only regroup into new nuclei.
Option (1) the kinetic energy of the incoming neutron — Incorrect. The neutron only starts the reaction. The 200 MeV comes from the change inside the nuclei.
Option (2) a decrease in the rest mass of each proton and neutron — Incorrect. A proton or a neutron keeps its own rest mass. Their numbers are also the same on both sides, so their total rest mass does not change.
Option (3) the difference between the total binding energies of the nuclei on the two sides — Correct. The new nuclei are more tightly bound than the uranium nucleus. Their total binding energy is larger, and this extra binding energy is set free.
Option (4) the chemical energy of the uranium atoms — Incorrect. Chemical energy comes from the binding of atoms and molecules and is only a few electron volts. The energy here comes from nuclear binding.
NCERT (2026-27):
Example 13.4 – “… since proton number and neutron number are conserved in a nuclear reaction, the total rest mass of neutrons and protons is the same on either side of a reaction. But the total binding energy of nuclei on the left side need not be the same as that on the right hand side. The difference in these binding energies appears as energy released or absorbed in a nuclear reaction. …”