Iron-56

Iron-56 (⁵⁶Fe) is the most common isotope of iron. About 91.754% of all iron is iron-56.

Nuclear binding energy per nucleon of common isotopes; iron-56 labelled at the curve's crest. The rarer isotopes nickel-62 and iron-58, which both have higher binding energies, are not shown.

Of all nuclides, iron-56 has the lowest mass per nucleon. With 8.8 MeV binding energy per nucleon, iron-56 is one of the most tightly bound nuclei.[1]

Nickel-62, a relatively rare isotope of nickel, has a higher nuclear binding energy per nucleon; this is consistent with having a higher mass-per-nucleon because nickel-62 has a greater proportion of neutrons, which are slightly more massive than protons. (See the nickel-62 article for more). Light elements undergoing nuclear fusion and heavy elements undergoing nuclear fission release energy as their nucleons bind more tightly, so ⁶²Ni might be expected to be common. However, during nucleosynthesis in stars the competition between photodisintegration and alpha capturing causes more ⁵⁶Ni to be produced than ⁶²Ni (⁵⁶Fe is produced later in the star's ejection shell as ⁵⁶Ni decays).

Production of these elements has decreased considerably from what it was at the beginning of the stelliferous era.

Nonetheless, 28 atoms of nickel-62 fusing into 31 atoms of iron-56 releases 0.011 u of energy. As the Universe ages, matter will slowly convert to ever more tightly bound nuclei, approaching ⁵⁶Fe, ultimately leading to the formation of iron stars over ≈10¹⁵⁰⁰ years in an expanding universe without proton decay.[2]