Iron-56

With 8.8 MeV binding energy per nucleon, iron-56 is one of the most tightly bound nuclei.

Because of this, it is among the heaviest elements formed in stellar nucleosynthesis reactions in massive stars.

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.

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.

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, assuming an expanding universe without proton decay.

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.