The amount of energy needed to remove a particle from a system of particles, for example, from an atom’s nucleus. The energy required to separate all nucleons from an atom’s nucleus is called nuclear binding energy and is very important in nuclear physics. The average binding energy per nucleon in the hydrogen isotope deuterium is 1,1 MeV and in uranium, 7,6 MeV.
During the fission reaction, the nucleus of a heavy element, for example, uranium, is transformed into two nuclei with a lower binding energy than the previous atom. Excess binding energy is released and can be used for energy purposes. This is the principle of a fission power plant.
In fusion, binding energy is released when two nuclei of light atoms, for example, two isotopes of hydrogen, deuterium, and tritium, are merged into one nucleus. This process will be the source of energy in the fusion power plant.
In the middle of the periodic table of chemical elements, there is only a small difference in binding energy per nucleon between atoms, so nothing is gained by their assembling or disassembling. Those atoms have no energy use, neither for fission nor for fusion power plants.