Iron-56

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

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.

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 information regarding the ordering of binding energy per nucleon, and mass-per-nucleon, for various nuclides.

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 total energy per nucleon, which occurs at ⁶²Ni. 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). This means that as the Universe ages, more matter is converted into extremely tightly bound nuclei, such as ⁵⁶Fe. This progression of matter towards iron and nickel is one of the phenomena responsible for the heat death of the universe.

Production of these elements has decreased considerably from what it was at the beginning of the stelliferous era; in all likelihood, not all matter will be converted into such elements.

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