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

Explanation

Of course in a nuclear reaction both the number of protons and neutrons (i.e., the mass number as well as atomic number) remains conserved, yet total mass of products is not same as the total mass of reactants. There is some loss /gain in mass, which is converted into energy. Alternately we can say that total binding energy of products is not same as the total binding energy of reactants and the difference in these binding energies appears as energy released or absorbed in a nuclear reaction.
As an example consider fission of a heavy nucleus (X) of mass number A = 240 into two nuclides (Y) each of mass number A. = 120 each. Binding energy per nucleon of parent nucleus X is 7.6 MeV but that of each daughter nuclide Y is 8.5 MeV. This difference in binding energy is the cause of energy released.