f.5 nuclear fission

The mass of a neutron in a nuclide is a little bit lighter than that when the neutron exists as a free neutron. The same happens to a proton in a nuclide and a proton in free state. The difference in mass goes to the binding energy of the nucleons in order to counter balacne against the strong electrostatic repulsive force arising from the the positively charged protons.

Therefore, although the number of protons and neutrons before and after fission are equal, the total mass of neutrons and protons that are present in a heavy nucleus are larger than the total mass of the neutrons and protons in lighter nucleus. When fission takes place, the excess binding energy (which comes from the difference in mass of protons and neutrons) will be released.

The deficit is due to the conversion of neutrons to protons and electrons during nuclear fission.

As the mass of neutron is a little bit larger than proton(the mass of electron can be neglected), which the mass of the former is ~1.6749 X 10^-27 kg and that of the latter is 1.6726 X 10^-27 kg.

However, there are billions of neutrons converting to protons duing the fission, so the total decrease in mass becomes significant.