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

Of allnuclides, iron-56 has the lowest mass pernucleon. With 8.8 MeVbinding energy per nucleon, iron-56 is one of the most tightly bound nuclei.^[1]

The high nuclear binding energy for⁵⁶Fe represents the point where further nuclear reactions become energetically unfavorable. Because of this, it is among the heaviest elements formed instellar nucleosynthesis reactions in massive stars. These reactions fuse lighter elements like magnesium, silicon, and sulfur to form heavier elements. Among the heavier elements formed is⁵⁶Ni, which subsequently decays to⁵⁶Co and then⁵⁶Fe.

Nickel-62, a relatively rare isotope of nickel, has a highernuclear binding energy per nucleon; this is consistent with having a higher mass-per-nucleon because nickel-62 has a greater proportion ofneutrons, which are slightly more massive thanprotons. (See thenickel-62 article for more). Light elements undergoingnuclear fusion and heavy elements undergoingnuclear fission release energy as their nucleons bind more tightly, so⁶²Ni might be expected to be common. However, duringstellar nucleosynthesis the competition betweenphotodisintegration andalpha capturing causes more⁵⁶Ni to be produced than⁶²Ni (⁵⁶Fe is produced later in the star's ejection shell as⁵⁶Ni 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 releases0.011 Da of energy. As theuniverse ages, matter will slowly convert to ever more tightly bound nuclei, approaching⁵⁶Fe, ultimately leading to the formation ofiron stars over ≈ 10¹⁵⁰⁰ years, assuming anexpanding 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.