(a) Define (i) mass defect and (ii) binding energy per nucleon for an atomic nucleus.
(b) The binding energy per nucleon of a deuterium nucleus is approximately 1.1 MeV, while that of a helium-4 nucleus is about 7.1 MeV. Using these data, explain qualitatively why energy is liberated when two deuterium nuclei undergo nuclear fusion to form helium-4.
(c) Give a brief reason why thermonuclear fusion in main-sequence stars is energetically favourable only up to the production of nuclei around the mass number 56.