Nuclei such as uranium and plutonium are used to produce energy by nuclear fission.
(a) Explain why only the heaviest nuclei can be used to produce energy by nuclear fission.
(b) Stars produce energy by nuclear fusion of small-mass nuclei into larger-mass nuclei. Explain why nuclear fusion releases energy.
(c) Which two elements form the boundary between those that can be used for fusion and those that can be used for fission?