View source for Iron-56

{{Short description|Isotope of iron}}
{{Infobox isotope
 |alternate_names  =
 |mass_number      = 56
 |symbol           = Fe
 |num_neutrons     = 30
 |num_protons      = 26
 |abundance        = 91.754%
 |mass             = {{val|55.9349375|(7)}}
 |spin             = 0+
 |excess_energy    = {{val|−60601.003 |1.354}}
 |binding_energy   = {{val|492253.892 |1.356}}
}}
[[File:Binding energy curve - common isotopes.svg|thumb|290px|[[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. ]]

'''Iron-56''' (<sup>56</sup>Fe) is the most common [[isotope]] of [[iron]]. About 91.754% of all iron is iron-56.

Of all [[nuclide]]s, iron-56 has the lowest mass per [[nucleon]]. With 8.8&nbsp;[[electronvolt|MeV]] [[binding energy]] per nucleon, iron-56 is one of the most tightly bound nuclei.<ref>{{Cite web |title=Nuclear Binding Energy |url=http://hyperphysics.phy-astr.gsu.edu/hbase/NucEne/nucbin.html |access-date=2025-05-12 |website=hyperphysics.phy-astr.gsu.edu}}</ref>

The high nuclear binding energy for <sup>56</sup>Fe 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 [[Isotopes of nickel|<sup>56</sup>Ni]], which subsequently decays to [[Isotopes of cobalt|<sup>56</sup>Co]] and then <sup>56</sup>Fe.

== Relationship to nickel-62 ==
[[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 [[neutron]]s, which are slightly more massive than [[proton]]s. (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 <sup>62</sup>Ni might be expected to be common. However, during [[stellar nucleosynthesis]] the competition between [[photodisintegration]] and [[Alpha process|alpha capturing]] causes more [[Nickel-56|<sup>56</sup>Ni]] to be produced than <sup>62</sup>Ni (<sup>56</sup>Fe is produced later in the star's ejection shell as <sup>56</sup>Ni 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 {{val|0.011|ul=Da}} of energy.  As the [[universe]] ages, matter will slowly convert to ever more tightly bound nuclei, approaching <sup>56</sup>Fe, ultimately leading to the formation of [[iron star]]s over ≈&nbsp;10<sup>1500</sup> years, assuming an [[future of an expanding universe#Future without proton decay|expanding universe without proton decay]].<ref name=twoe>{{cite journal |title=Time without end: Physics and biology in an open universe |first=Freeman J. |last=Dyson |journal=[[Reviews of Modern Physics]] |volume=51 |issue=3 |year=1979 |pages=447–460 |doi=10.1103/RevModPhys.51.447 |bibcode = 1979RvMP...51..447D }}</ref>

== See also ==
* [[Isotopes of iron]]
* [[Iron star]]

== References ==
{{reflist}}
* {{CIAAW2003}}
{{Isotope sequence
|element=iron
|lighter=[[iron-55]]
|heavier=[[iron-57]]
|before=[[manganese-56]]<br />[[cobalt-56]]
|after=Stable
}}

[[Category:Isotopes of iron]]