Fission and fusion
Physics Narrative for 14-16
Two possibilities for nuclei rearrangements
There are two cataclysmic possibilities for rearrangements of nuclei that result in a movement towards a lower-energy, more stable state: a large nucleus splits, or two small nuclei combine. As there is movement towards a lower-energy state, the binding energy per nucleon increases: the resultant nuclei that are the outputs are further down the energy hill than the nuclei you started with.
The most stable nucleus of the lot is iron: so nuclei with fewer protons than iron can fuse, or join, and end up in a more stable state. Nuclei with more protons than iron can fission, or split, to attain a more stable state.
In each of these cases there is a significant drop in the binding energy. Energy from the nuclear store is shifted to the kinetic stores of the outputs from the fission or fusion. The energy in the nuclear store is now lower, that's why we suggest a drop. As the binding energy is now lower, so the nucleus is more tightly bound. You'd have to provide more energy to pull the nucleus apart than before the energy was shifted to the kinetic stores.
There is a large quantity of energy shifted from the nuclear store per fission or fusion, compared with much smaller changes in the chemical store per reaction. So these nuclear changes enable a small quantity of matter to be a significant energy resource: for good or ill; for power stations or bombs.