Final answer:
Nuclear fission releases energy when heavy nuclei split because the resulting products have a higher binding energy per nucleon, converting mass into energy. Fission of light nuclei requires energy input due to their already high binding energies. This process of splitting heavy nuclei to release energy is the basis for nuclear reactors and atomic bombs.
Step-by-step explanation:
The release of energy in nuclear fission of heavy nuclei is due to the nuclear physics principle that energy is released if the products of a reaction have a greater binding energy per nucleon (BE/A) than the parent nuclei. For heavy nuclei like those with atomic mass around 240 u, the BE/A is lower at about 7.6 MeV/nucleon compared to the BE/A for medium-mass nuclei, which is about 8.6 MeV/nucleon for nuclei with atomic mass around 120 u. Consequently, when a heavy nucleus splits, the mass of the products is less per nucleon, resulting in the conversion of the mass difference into energy according to Einstein's equation E=mc^2. Thus, fission of a heavy nucleus can release approximately 240 MeV of energy, signifying a substantial energy yield per reaction.
On the other hand, to fission light nuclei, energy input is required because their binding energy is already high; thus, adding energy is necessary to break those bonds and induce fission. This principle is central to why energy generation through fission is practical only with heavy nuclei.