(a) Define mass defect of a nucleus.
(b) Explain, with the help of the principle of mass–energy equivalence, why the mass of a bound nucleus is smaller than the combined mass of its constituent nucleons. Hence state how the concept of binding energy arises.
(c) The binding energy per nucleon first rises sharply with mass number, reaches a broad maximum around the iron region, and then falls slowly for heavier nuclei. Describe qualitatively how this trend accounts for the large energy release in both nuclear fission of very heavy nuclei and nuclear fusion of very light nuclei.