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.^[1]

The high nuclear binding energy for ⁵⁶Fe represents the point where further nuclear reactions become energetically unfavorable. Because of this, it is among the heaviest elements formed in stellar nucleosynthesis reactions in massive stars. These reactions fuse lighter elements like magnesium, silicon, and sulfur to form heavier elements. Among the heavier elements formed is ⁵⁶Ni, which subsequently decays to ⁵⁶Co and then ⁵⁶Fe.

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 stellar nucleosynthesis 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).

Although nickel-62 has a higher binding energy per nucleon, the conversion of 28 atoms of nickel-62 into 31 atoms of iron-56 releases 0.011 Da 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, assuming an expanding universe without proton decay.^[2]

• Isotopes of iron
• Iron star

• de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.