Naturally occurring iron (Fe) consists of four 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 0.282% 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.[1]
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54Fe is observationally stable, with a branching theory that it decays to 54Cr, with a half-life of more than 3.1x1022 years via double electron capture (2β+). All other natural isotopes are known to be stable, which makes 54Fe seemingly strange.
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.[2] 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 due to the low natural variation in energy of the 14.4keV nuclear transition.[3]
Iron-60 is another isotope. It has a half-life of 2.6 million years,[4][5] but was thought until 2009 to have a half-life of 1.5 million years. It undergoes beta decay to cobalt-60.
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.
Standard atomic mass: 55.845(2) u
| nuclide symbol |
Z(p) | N(n) | isotopic mass (u) |
half-life | decay mode(s)[6][n 1] |
daughter isotope(s)[n 2] |
nuclear spin |
representative isotopic composition (mole fraction) |
range of natural variation (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+ | 52Mn | 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 | 53.9396105(7) | 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) a | EC | 55Mn | 3/2- | ||
| 56Fe[n 4] | 26 | 30 | 55.9349375(7) | Stable | 0+ | 0.91754(36) | 0.91742-0.91760 | ||
| 57Fe | 26 | 31 | 56.9353940(7) | Stable | 1/2- | 0.02119(10) | 0.02116-0.02121 | ||
| 58Fe | 26 | 32 | 57.9332756(8) | 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 a | 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] |
7/2+# | ||||
| 72Fe | 26 | 46 | 71.96962(86)# | 10# ms [>300 ns] |
0+ | ||||
| Isotopes of manganese | Isotopes of iron | Isotopes of cobalt |
| Index to isotope pages · Table of nuclides | ||