In nuclear reactions, which of the following is conserved?

The correct answer is Option (4) → Total mass-energy, momentum and the number of nucleons

Option (1) Mass only — Incorrect. Mass alone is not conserved. In a nuclear reaction, part of the rest mass is converted into other forms of energy, such as kinetic energy, or the reverse.

Option (2) Energy only — Incorrect. Energy, counted together with the energy associated with mass, is conserved. But momentum and the number of nucleons are conserved as well.

Option (3) Momentum only — Incorrect. Momentum is conserved in a nuclear reaction, as in any isolated system. But mass-energy and the number of nucleons are also conserved.

Option (4) Total mass-energy, momentum and the number of nucleons — Correct. All three are conserved in a nuclear reaction.

• The initial and final energies are equal once the energy associated with mass is included.
• The total momentum of the isolated system does not change.
• The number of protons and the number of neutrons are each conserved, so the number of nucleons is the same on both sides.

NCERT (2026-27):
Mass – Energy – “… In a reaction the conservation law of energy states that the initial energy and the final energy are equal provided the energy associated with mass is also included.”

Example 13.4 – “… However, the number of protons and the number of neutrons are both separately conserved in a nuclear reaction.”