Fe56

Fe56 is the isotope of iron with 26 protons and 30 neutrons, giving it a mass number of 56. It is a stable nuclide and, along with a small number of neighboring isotopes, forms the iron group at the peak of nuclear binding energy. Its binding energy per nucleon is about 8.8 MeV, making it one of the most tightly bound nuclei in nature and contributing to the overall chemical and stellar role of iron.

Natural iron consists predominantly of Fe-56, which accounts for roughly 92% of iron found on Earth and in the solar system. The remaining iron isotopes, Fe-54, Fe-57, and Fe-58, occur in smaller amounts. Fe-56 is also a major product in certain stellar nucleosynthesis pathways, particularly explosive silicon burning in massive stars and supernovae. In such environments, nickel-56 is often produced and decays successively to cobalt-56 and then to iron-56, releasing gamma rays that power the observed light curves of supernovae.

Fe-56 has a central role in astrophysics because it represents the endpoint of exothermic fusion: fusion of lighter elements to iron releases energy, whereas fusion beyond iron would require energy input. This property helps determine the fate of massive stars, the timing of core collapse, and the distribution of iron-group elements in the universe. In nuclear physics and metrology, Fe-56 serves as a reference point in discussions of binding energy and nuclear stability.