Isotopes of iron

Naturally occurring iron (26Fe) consists of four stable isotopes: 5.845% of 54Fe (possibly radioactive with a half-life over 3.1×1022 years), 91.754% of 56Fe, 2.119% of 57Fe and 0.286% of 58Fe. There are 24 known radioactive isotopes and their half-lives are shown below. See Brookhaven National Laboratory Interactive Table of Nuclides for a more accurate reading.

Much of the past work on measuring the isotopic composition of Fe has centered on determining 60Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation. In the last decade however, advances in mass spectrometry technology have allowed the detection and quantification of minute, naturally occurring variations in the ratios of the stable isotopes of iron. Much of this work has been driven by the Earth and planetary science communities, although applications to biological and industrial systems are beginning to emerge.[2]

Main isotopes of iron (26Fe)
Iso­tope Decay
abun­dance half-life (t1/2) mode pro­duct
54Fe 5.85% stable
55Fe syn 2.73 y ε 55Mn
56Fe 91.75% stable
57Fe 2.12% stable
58Fe 0.28% stable
59Fe syn 44.6 d β 59Co
60Fe trace 2.6×106 y β 60Co
Standard atomic weight Ar, standard(Fe)


54Fe is observationally stable, but theoretically can decay to 54Cr, with a half-life of more than 3.1x1022 years via double electron capture (εε).


The isotope 56Fe is the isotope with the lowest mass per nucleon, 930.412 MeV/c2, though not the isotope with the highest nuclear binding energy per nucleon, which is nickel-62.[3] However, because of the details of how nucleosynthesis works, 56Fe is a more common endpoint of fusion chains inside extremely massive stars and is therefore more common in the universe, relative to other metals, including 62Ni, 58Fe and 60Ni, all of which have a very high binding energy.


The isotope 57Fe is widely used in Mössbauer spectroscopy and the related nuclear resonance vibrational spectroscopy due to the low natural variation in energy of the 14.4 keV nuclear transition.[4] The transition was famously used to make the first definitive measurement of gravitational redshift, in the 1960 Pound-Rebka experiment.[5]


Iron-60 is an iron isotope with a half-life of 2.6 million years,[6][7] but was thought until 2009 to have a half-life of 1.5 million years. It undergoes beta decay to cobalt-60, which then decays with a half-life of about 5 years to stable nickel-60. Traces of iron-60 have been found in lunar samples.

In phases of the meteorites Semarkona and Chervony Kut a correlation between the concentration of 60Ni, the granddaughter isotope of 60Fe, and the abundance of the stable iron isotopes could be found, which is evidence for the existence of 60Fe at the time of formation of the solar system. Possibly the energy released by the decay of 60Fe contributed, together with the energy released by decay of the radionuclide 26Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of 60Ni present in extraterrestrial material may also provide further insight into the origin of the solar system and its early history.

Iron-60 found in fossilised bacteria in sea floor sediments suggest there was a supernova in the vicinity of the solar system approximately 2 million years ago.[8][9] Iron-60 is also found in sediments from 8 million years ago.[10]

List of isotopes

Z(p) N(n)  
Atomic mass (u)
half-life decay
mode(s)[11][n 1]
isotope(s)[n 2]
spin and
(mole fraction)
range of natural
(mole fraction)
excitation energy
45Fe 26 19 45.01458(24)# 1.89(49) ms β+ (30%) 45Mn 3/2+#
2p (70%) 43Cr
46Fe 26 20 46.00081(38)# 9(4) ms
[12(+4-3) ms]
β+ (>99.9%) 46Mn 0+
β+, p (<.1%) 45Cr
47Fe 26 21 46.99289(28)# 21.8(7) ms β+ (>99.9%) 47Mn 7/2−#
β+, p (<.1%) 46Cr
48Fe 26 22 47.98050(8)# 44(7) ms β+ (96.41%) 48Mn 0+
β+, p (3.59%) 47Cr
49Fe 26 23 48.97361(16)# 70(3) ms β+, p (52%) 48Cr (7/2−)
β+ (48%) 49Mn
50Fe 26 24 49.96299(6) 155(11) ms β+ (>99.9%) 50Mn 0+
β+, p (<.1%) 49Cr
51Fe 26 25 50.956820(16) 305(5) ms β+ 51Mn 5/2−
52Fe 26 26 51.948114(7) 8.275(8) h β+ 52mMn 0+
52mFe 6.81(13) MeV 45.9(6) s β+ 52Mn (12+)#
53Fe 26 27 52.9453079(19) 8.51(2) min β+ 53Mn 7/2−
53mFe 3040.4(3) keV 2.526(24) min IT 53Fe 19/2−
54Fe 26 28 53.9396090(5) Observationally Stable[n 3] 0+ 0.05845(35) 0.05837–0.05861
54mFe 6526.9(6) keV 364(7) ns 10+
55Fe 26 29 54.9382934(7) 2.737(11) y EC 55Mn 3/2−
56Fe[n 4] 26 30 55.9349363(5) Stable 0+ 0.91754(36) 0.91742–0.91760
57Fe 26 31 56.9353928(5) Stable 1/2− 0.02119(10) 0.02116–0.02121
58Fe 26 32 57.9332744(5) Stable 0+ 0.00282(4) 0.00281–0.00282
59Fe 26 33 58.9348755(8) 44.495(9) d β 59Co 3/2−
60Fe 26 34 59.934072(4) 2.6×106 y β 60Co 0+ trace
61Fe 26 35 60.936745(21) 5.98(6) min β 61Co 3/2−,5/2−
61mFe 861(3) keV 250(10) ns 9/2+#
62Fe 26 36 61.936767(16) 68(2) s β 62Co 0+
63Fe 26 37 62.94037(18) 6.1(6) s β 63Co (5/2)−
64Fe 26 38 63.9412(3) 2.0(2) s β 64Co 0+
65Fe 26 39 64.94538(26) 1.3(3) s β 65Co 1/2−#
65mFe 364(3) keV 430(130) ns (5/2−)
66Fe 26 40 65.94678(32) 440(40) ms β (>99.9%) 66Co 0+
β, n (<.1%) 65Co
67Fe 26 41 66.95095(45) 394(9) ms β (>99.9%) 67Co 1/2−#
β, n (<.1%) 66Co
67mFe 367(3) keV 64(17) µs (5/2−)
68Fe 26 42 67.95370(75) 187(6) ms β (>99.9%) 68Co 0+
β, n 67Co
69Fe 26 43 68.95878(54)# 109(9) ms β (>99.9%) 69Co 1/2−#
β, n (<.1%) 68Co
70Fe 26 44 69.96146(64)# 94(17) ms 0+
71Fe 26 45 70.96672(86)# 30# ms
[>300 ns]
72Fe 26 46 71.96962(86)# 10# ms
[>300 ns]
  1. ^ Abbreviations:
    EC: Electron capture
    IT: Isomeric transition
  2. ^ Bold for stable isotopes
  3. ^ Believed to decay by β+β+ to 54Cr with a half-life of over 3.1×1022 a
  4. ^ Lowest mass per nucleon of all nuclides; End product of stellar nucleosynthesis


  • Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
  • Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.
  • Nuclide masses are given by IUPAP Commission on Symbols, Units, Nomenclature, Atomic Masses and Fundamental Constants (SUNAMCO)
  • Atomic masses of the stable nuclides (54Fe, 56Fe, 57Fe, and 58Fe) are given by the AME2012 atomic mass evaluation. The one standard deviation errors are given in parentheses after the corresponding last digits.[12]
  • Isotope abundances are given by IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW)

See also


  1. ^ Meija, Juris; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91. doi:10.1515/pac-2015-0305.
  2. ^ N. Dauphas; O. Rouxel (2006). "Mass spectrometry and natural variations of iron isotopes". Mass Spectrometry Reviews. 25 (4): 515–550. Bibcode:2006MSRv...25..515D. doi:10.1002/mas.20078. PMID 16463281.
  3. ^ Fewell, M. P. "The atomic nuclide with the highest mean binding energy". American Journal of Physics 63 (7): 653-58. Accessed: 2011-03-22. (Archived by WebCite® at
  4. ^ R. Nave. "Mossbauer Effect in Iron-57". HyperPhysics. Georgia State University. Retrieved 2009-10-13.
  5. ^ Pound, R. V.; Rebka Jr. G. A. (April 1, 1960). "Apparent weight of photons". Physical Review Letters. 4 (7): 337–341. Bibcode:1960PhRvL...4..337P. doi:10.1103/PhysRevLett.4.337.
  6. ^ Rugel, G.; Faestermann, T.; Knie, K.; Korschinek, G.; Poutivtsev, M.; Schumann, D.; Kivel, N.; Günther-Leopold, I.; Weinreich, R.; Wohlmuther, M. (2009). "New Measurement of the 60Fe Half-Life". Physical Review Letters. 103 (7): 72502. Bibcode:2009PhRvL.103g2502R. doi:10.1103/PhysRevLett.103.072502. PMID 19792637.
  7. ^ "Eisen mit langem Atem".
  8. ^ Belinda Smith (Aug 9, 2016). "Ancient bacteria store signs of supernova smattering". Cosmos.
  9. ^ Peter Ludwig; et al. (Aug 16, 2016). "Time-resolved 2-million-year-old supernova activity discovered in Earth's microfossil record". PNAS. 113 (33): 9232–9237. arXiv:1710.09573. Bibcode:2016PNAS..113.9232L. doi:10.1073/pnas.1601040113. PMC 4995991. PMID 27503888.
  10. ^ Colin Barras (Oct 14, 2017). "Fires may have given our evolution a kick-start". New Scientist.
  11. ^ "Universal Nuclide Chart". nucleonica.
  12. ^ M. Wang, G. Audi, A. H. Wapstra, F. G. Kondev, M. MacCormick, X. Xu, and B. Pfeiffer (2012), "The AME2012 atomic mass evaluation (II). Tables, graphs and references", Chinese Physics C, Vol. 36, 1603-2014.

Further reading

Future of an expanding universe

Observations suggest that the expansion of the universe will continue forever. If so, then a popular theory is that the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario once popularly called "Heat Death" is now known as the Big Chill or Big Freeze.If dark energy—represented by the cosmological constant, a constant energy density filling space homogeneously, or scalar fields, such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space—accelerates the expansion of the universe, then the space between clusters of galaxies will grow at an increasing rate. Redshift will stretch ancient, incoming photons (even gamma rays) to undetectably long wavelengths and low energies. Stars are expected to form normally for 1012 to 1014 (1–100 trillion) years, but eventually the supply of gas needed for star formation will be exhausted. As existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker. According to theories that predict proton decay, the stellar remnants left behind will disappear, leaving behind only black holes, which themselves eventually disappear as they emit Hawking radiation. Ultimately, if the universe reaches a state in which the temperature approaches a uniform value, no further work will be possible, resulting in a final heat death of the universe.


Iron () is a chemical element with symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal, that belongs to the first transition series and group 8 of the periodic table. It is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust.

Pure iron is very rare on the Earth's crust, basically being limited to meteorites. Iron ores are quite abundant, but extracting usable metal from them requires kilns or furnaces capable of reaching 1500 °C or higher, about 500 °C higher than what is enough to smelt copper. Humans started to dominate that process in Eurasia only about 2000 BCE, and iron began to displace copper alloys for tools and weapons, in some regions, only around 1200 BCE. That event is considered the transition from the Bronze Age to the Iron Age. Iron alloys, such as steel, inox, and special steels are now by far the most common industrial metals, because of their mechanical properties and their low cost.

Pristine and smooth pure iron surfaces are mirror-like silvery-gray. However, iron reacts readily with oxygen and water to give brown to black hydrated iron oxides, commonly known as rust. Unlike the oxides of some other metals, that form passivating layers, rust occupies more volume than the metal and thus flakes off, exposing fresh surfaces for corrosion.

The body of an adult human contains about 4 grams (0.005% body weight) of iron, mostly in hemoglobin and myoglobin. These two proteins play essential roles in vertebrate metabolism, respectively oxygen transport by blood and oxygen storage in muscles. To maintain the necessary levels, human iron metabolism requires a minimum of iron in the diet. Iron is also the metal at the active site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants and animals.Chemically, the most common oxidation states of iron are iron(II) and iron(III). Iron shares many properties of other transition metals, including the other group 8 elements, ruthenium and osmium. Iron forms compounds in a wide range of oxidation states, −2 to +7. Iron also forms many coordination compounds; some of them, such as ferrocene, ferrioxalate, and Prussian blue, have substantial industrial, medical, or research applications.


Iron-55 (55Fe) is a radioactive isotope of iron with a nucleus containing 26 protons and 29 neutrons. It decays by electron capture to manganese-55 and this process has a half-life of 2.737 years. The emitted X-rays can be used as an X-ray source for various scientific analysis methods, such as X-ray diffraction. Iron-55 is also a source for Auger electrons, which are produced during the decay.


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, ultimately leading to the formation of iron stars in around 101500 years.Production of these elements has decreased considerably from what it was at the beginning of the stelliferous era.

Mantle (geology)

A mantle is a layer inside a planetary body bounded below by a core and above by a crust. Mantles are made of rock or ices, and are generally the largest and most massive layer of the planetary body. Mantles are characteristic of planetary bodies that have undergone differentiation by density. All terrestrial planets (including Earth), a number of asteroids, and some planetary moons have mantles.

Monoisotopic mass

Monoisotopic mass (Mmi) is one of several types of molecular masses used in mass spectrometry. The theoretical monoisotopic mass of a molecule is computed by taking the sum of the accurate masses of the primary isotope of each atom in the molecule. For small molecules made up of low atomic number elements the monoisotopic mass is observable as an isotopically pure peak in a mass spectrum. This differs from the nominal molecular mass, which is the sum of the mass number of the primary isotope of each atom in the molecule and is an integer. It also is different from the molar mass, which is a type of average mass. For some atoms like carbon, oxygen, hydrogen, nitrogen, and sulfur the Mmi of these elements is exactly the same as the mass of its natural isotope, which is the lightest one. However, this does not hold true for all atoms. Iron's most common isotope has a mass number of 56, while the stable isotopes of iron vary in mass number from 54 to 58. Monoisotopic mass is typically expressed in unified atomic mass units (u), also called daltons (Da).

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