Transcript: The mass of any atomic nucleus is less than the separate masses of its protons and neutrons. The difference is the binding energy. In other words sticking protons and neutrons together somehow causes some of their mass to vanish. The answer is connected with Einstein’s equation E = mc2. Mass and energy are really two different forms of the same thing so the vanishing mass of the protons and neutrons is simply converted into energy, and that’s the idea behind fusion. The binding energy of a particular isotope is the amount of energy released when it’s created. You can calculate its amount by using Einstein’s equation. The binding energy is also the amount of energy you need to add to a nucleus to break it up again into protons and neutrons. The binding energy per nucleon, that is proton or neutron, differs for different elements. It peaks at iron; that’s the most stable element. So for elements heavier than iron a decay by fission into smaller pieces releases energy. The binding energy is less of the residual pieces and the excess is released as energy.