Atom
The energy needed to break a nucleus into protons and neutrons is called its nuclear binding energy. This energy can be converted to mass, as stated by Einstein's famous formula E = mc2. Medium-sized nuclei, such as iron-56 and nickel-62, have the highest binding energy per proton or neutron. They will probably not go through fission or fusion, because they cannot release energy in this way. Very small and very large atoms have low binding energy, so they are most willing to go through fission or fusion.