Iron-56

Iron-56 is the most common isotope of iron. About 91.754% of all iron is iron-56. With 8.8 MeV binding energy per nucleon, iron-56 is one of the most tightly bound nuclei. Thus, light elements undergoing nuclear fusion and heavy elements undergoing nuclear fission release energy as their nucleons bind more tightly, and the resulting nuclei approach the maximum nucleon binding energy, which occurs at nickel-62. As the universe ages, more of the matter is converted into extremely tightly bound nuclei, such as iron-56. This progression of matter toward iron and nickel is one of the phenomena responsible for the heat death of the universe.