iron56

Iron-56 (56Fe) is a stable isotope of iron with 26 protons and 30 neutrons, giving it a mass number of 56. Its nucleus is among the most tightly bound of all nuclides, with a binding energy per nucleon of about 8.8 MeV and a total binding energy around 492 MeV. This high binding energy contributes to the stability and abundance of 56Fe compared with other iron isotopes.

Naturally, iron is dominated by the 56Fe isotope, which accounts for roughly 92% of elemental iron on Earth. Smaller fractions of 54Fe, 57Fe, and 58Fe are also present. 56Fe is stable and does not undergo radioactive decay, making it a long-lived constituent of planetary cores and meteorites.

In astrophysics, 56Fe is significant because it represents the endpoint of exothermic fusion in stellar cores. Fusion of lighter elements releases energy up to the production of iron-group nuclei, but fusion of elements heavier than iron consumes energy. This energy balance halts further energy production by fusion in massive stars, contributing to core collapse and supernova events that synthesize and eject iron-group elements, including 56Fe, into the interstellar medium.

There is a related nuance: some nuclear models indicate nickel-62 (62Ni) has a slightly higher binding energy per nucleon, but 56Fe is commonly cited as the most tightly bound nucleus in many discussions and is especially noted for its prominence in stellar nucleosynthesis and planetary composition.