Question

If both the number of protons and the neutrons are conserved in each nuclear reaction, in what way is mass converted into energy (or vice versa) in a nuclear reaction? Explain.

Answer 1

In nuclear reactions total number of nucleons (protons + neutrons) is conserved. But total mass is not conserved exactly.

This is explained using Einstein’s mass–energy relation:

E = mc²

The mass of a nucleus is less than the sum of the masses of its individual nucleons. This difference is called a mass defect.

∆m = (sum of individual masses) − (actual nuclear mass)

The missing mass appears as binding energy:

E_b = ∆mc²

This energy holds nucleons together inside the nucleus.

During nuclear fission or nuclear fusion reactions, products have different binding energies compared to reactants. If final nuclei have higher binding energy per nucleon, the total mass decreases, and excess mass is released as energy.

In the energy-mass conservation, if the mass decreases, the energy is released, and if the energy is supplied, the mass can be increased.

Thus, even though nucleon number is conserved, a small amount of mass is converted into energy (or vice versa).