Isotopes of argon

Short description: Nuclides with atomic number of 18 but with different mass numbers

Main isotopes of Chemistry:argon (18Ar) | Iso­tope | Decay

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| abun­dance | half-life (t1/2) | mode | pro­duct
36Ar  | 0.334%  | stable
37Ar  | syn | 35 d  | ε | 37Cl
38Ar  | 0.063%  | stable
39Ar  | trace | 269 y  | β− | 39K
40Ar  | 99.604%  | stable
41Ar  | syn  | 109.34 min  | β− | 41K
42Ar  | syn  | 32.9 y  | β− | 42K

36Ar and 38Ar content may be as high as 2.07% and 4.3% respectively in natural samples. 40Ar is the remainder in such cases, whose content may be as low as 93.6%.

Standard atomic weight Ar, standard(Ar)|

* [39.792, 39.963][1]
* Conventional: 39.948

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Argon (18Ar) has 26 known isotopes, from 29Ar to 54Ar and 1 isomer (32mAr), of which three are stable (36Ar, 38Ar, and 40Ar). On the Earth, 40Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are 39Ar with a half-life of 268 years, 42Ar with a half-life of 32.9 years, and 37Ar with a half-life of 35.04 days. All other isotopes have half-lives of less than two hours, and most less than one minute. The least stable is 29Ar with a half-life of approximately 4×10−20 seconds.[2]

The naturally occurring 40K, with a half-life of 1.248×109 years, decays to stable 40Ar by electron capture (10.72%) and by positron emission (0.001%), and also transforms to stable 40Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.[3]

Despite the trapping of 40Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all of the argon in the Earth's atmosphere is the product of 40K decay, since 99.6% of Earth atmospheric argon is 40Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of < 15% 38Ar and mostly (85%) 36Ar. Similarly, the ratio of the three isotopes 36Ar:38Ar:40Ar in the atmospheres of the outer planets is measured to be 8400:1600:1.[4]

In the Earth's atmosphere, radioactive 39Ar (half-life 268(8) years) is made by cosmic ray activity, primarily from 40Ar. In the subsurface environment, it is also produced through neutron capture by 39K or alpha emission by calcium. The content of 39Ar in natural argon is measured to be of (8.0±0.6)×10−16 g/g, or (1.01±0.08) Bq/kg of 36, 38, 40Ar.[5] The content of 42Ar (half-life 33 years) in the Earth's atmosphere is lower than 6×10−21 parts per part of 36, 38, 40Ar.[6] Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon.[7] Lighter radioactive isotopes can decay to different elements (usually chlorine) while heavier ones decay to potassium.

36Ar, in the form of argon hydride, was detected in the Crab Nebula supernova remnant during 2013.[8][9] This was the first time a noble molecule was detected in outer space.[8][9]

Radioactive 37Ar is a synthetic radionuclide that is created from the neutron capture by 40Ca followed by an alpha particle emission as a result of subsurface nuclear explosions. It has a half-life of 35 days.[3]

## Contents

* 1 List of isotopes
* 2 See also
* 3 References
* 4 External links

## List of isotopes

Nuclide[10]
[n 1] | Z | N | Isotopic mass (u)[11]
[n 2][n 3] | Half-life
| Decay
mode
[n 4] | Daughter
isotope
[n 5] | Spin and
parity
[n 6][n 7] | Physics:Natural abundance (mole fraction)

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Excitation energy  | Normal proportion  | Range of variation
29Ar[2] | 18  | 11  |  | ~40 zs  | 2p | 27S  |  |  |
30Ar  | 18  | 12  | 30.02247(22)  | <10 ps  | 2p  | 28S  | 0+  |  |
31Ar  | 18  | 13  | 31.01216(22)#  | 15.1(3) ms  | β+, p (68.3%)  | 30S  | 5/2+  |  |
β+ (22.63%)  | 31Cl
β+, 2p (9.0%)  | 29P
β+, 3p (0.07%)  | 28Si
32Ar  | 18  | 14  | 31.9976378(19)  | 98(2) ms  | β+ (64.42%)  | 32Cl  | 0+  |  |
β+, p (35.58%)  | 31S
32mAr  | 5600(100) keV  | unknown  |  |  | 5−#  |  |
33Ar  | 18  | 15  | 32.9899255(4)  | 173.0(20) ms  | β+ (61.3%)  | 33Cl  | 1/2+  |  |
β+, p (38.7%)  | 32S
34Ar  | 18  | 16  | 33.98027009(8)  | 843.8(4) ms  | β+ | 34Cl  | 0+  |  |
35Ar  | 18  | 17  | 34.9752577(7)  | 1.7756(10) s  | β+ | 35Cl | 3/2+  |  |
36Ar  | 18  | 18  | 35.967545105(29)  | Observationally Stable[n 8] | 0+  | 0.003336(4)  |
37Ar  | 18  | 19  | 36.96677631(22)  | 35.011(19) d  | EC | 37Cl | 3/2+  | Trace[n 9] |
38Ar  | 18  | 20  | 37.96273210(21)  | Stable | 0+  | 0.000629(1)  |
39Ar[n 10] | 18  | 21  | 38.964313(5)  | 268.2+3.1
−2.9 y[12] | β− | 39K | 7/2−  | Trace[n 9] |
40Ar[n 11] | 18  | 22  | 39.9623831238(24)  | Stable | 0+  | 0.996035(4)[n 12] |
41Ar  | 18  | 23  | 40.9645006(4)  | 109.61(4) min  | β− | 41K | 7/2−  | Trace[n 9] |
42Ar  | 18  | 24  | 41.963046(6)  | 32.9(11) y  | β− | 42K  | 0+  |  |
43Ar  | 18  | 25  | 42.965636(6)  | 5.37(6) min  | β− | 43K  | 5/2(−)  |  |
44Ar  | 18  | 26  | 43.9649238(17)  | 11.87(5) min  | β− | 44K  | 0+  |  |
45Ar  | 18  | 27  | 44.9680397(6)  | 21.48(15) s  | β− | 45K  | (5/2,7/2)−  |  |
46Ar  | 18  | 28  | 45.9680374(12)  | 8.4(6) s  | β− | 46K  | 0+  |  |
47Ar  | 18  | 29  | 46.9727681(12)  | 1.23(3) s  | β− (99.8%)  | 47K  | (3/2−)  |  |
β−, n (0.2%)  | 46K
48Ar  | 18  | 30  | 47.97608(33)  | 415(15) ms  | β− | 48K  | 0+  |  |
49Ar  | 18  | 31  | 48.98155(43)#  | 236(8) ms  | β− | 49K  | 3/2−#  |  |
50Ar  | 18  | 32  | 49.98569(54)#  | 106(6) ms  | β− | 50K  | 0+  |  |
51Ar  | 18  | 33  | 50.99280(64)#  | 60# ms [>200 ns]  | β− | 51K  | 3/2−#  |  |
52Ar  | 18  | 34  | 51.99863(64)#  | 10# ms  | β− | 52K  | 0+  |  |
53Ar  | 18  | 35  | 53.00729(75)#  | 3# ms  | β− | 53K  | (5/2−)#  |  |
β−, n  | 52K
54Ar[13] | 18  | 36  |  |  | β− | 54K  | 0+  |  |

1. ↑ mAr - Excited nuclear isomer.
2. ↑ ( ) - Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. ↑ # - Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. ↑ Modes of decay:  EC: | Electron capture

|

n: | Neutron emission
p: | Proton emission
5. ↑ Bold symbol as daughter - Daughter product is stable.
6. ↑ ( ) spin value - Indicates spin with weak assignment arguments.
7. ↑ # - Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
8. ↑ Believed to undergo double electron capture to 36S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)
9. ↑ 9.0 9.1 9.2 Cosmogenic nuclide
10. ↑ Used in argon–argon dating
11. ↑ Used in argon–argon dating and potassium–argon dating
12. ↑ Generated from 40K in rocks. These ratios are terrestrial. Cosmic abundance is far less than 36Ar.

## See also

## References

1. ↑ Meija, Juris; Coplen, Tyler B.; Berglund, Michael; Brand, Willi A.; De Bièvre, Paul; Gröning, Manfred; Holden, Norman E.; Irrgeher, Johanna 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. ↑ 2.0 2.1 Mukha, I. (2018). "Deep excursion beyond the proton dripline. I. Argon and chlorine isotope chains". Physical Review C 98 (6): 064308–1–064308–13. doi:10.1103/PhysRevC.98.064308. Bibcode: 2018PhRvC..98f4308M.
3. ↑ 3.0 3.1 "40Ar/39Ar dating and errors". http://www.geoberg.de/text/geology/07011601.php.
4. ↑ Cameron, A.G.W. (1973). "Elemental and isotopic abundances of the volatile elements in the outer planets". Space Science Reviews 14 (3–4): 392–400. doi:10.1007/BF00214750. Bibcode: 1973SSRv...14..392C.
5. ↑ P. Benetti (2007). "Measurement of the specific activity of 39Ar in natural argon". Nuclear Instruments and Methods A 574 (1): 83–88. doi:10.1016/j.nima.2007.01.106. Bibcode: 2007NIMPA.574...83B.
6. ↑ V. D. Ashitkov (1998). "New experimental limit on the 42Ar content in the Earth's atmosphere". Nuclear Instruments and Methods A 416 (1): 179–181. doi:10.1016/S0168-9002(98)00740-2. Bibcode: 1998NIMPA.416..179A.
7. ↑ H. O. Back (2012). "Depleted Argon from Underground Sources". Physics Procedia 37: 1105–1112. doi:10.1016/j.phpro.2012.04.099. Bibcode: 2012PhPro..37.1105B.
8. ↑ 8.0 8.1 Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times. https://www.nytimes.com/2013/12/17/science/space/noble-molecules-found-in-space.html.
9. ↑ 9.0 9.1 Barlow, M. J. (2013). "Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula". Science 342 (6164): 1343–1345. doi:10.1126/science.1243582. PMID 24337290. Bibcode: 2013Sci...342.1343B.
10. ↑ Half-life, decay mode, nuclear spin, and isotopic composition is sourced in:
Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties". Chinese Physics C 41 (3): 030001. doi:10.1088/1674-1137/41/3/030001. Bibcode: 2017ChPhC..41c0001A. https://www-nds.iaea.org/amdc/ame2016/NUBASE2016.pdf.
11. ↑ Wang, M.; Audi, G.; Kondev, F. G.; Huang, W. J.; Naimi, S.; Xu, X. (2017). "The AME2016 atomic mass evaluation (II). Tables, graphs, and references". Chinese Physics C 41 (3): 030003-1—030003-442. doi:10.1088/1674-1137/41/3/030003. http://nuclearmasses.org/resources_folder/Wang_2017_Chinese_Phys_C_41_030003.pdf.
12. ↑ Golovko, Victor V. (2023-10-15). "Application of the most frequent value method for 39Ar half-life determination". The European Physical Journal C 83 (10). doi:10.1140/epjc/s10052-023-12113-6. ISSN 1434-6052.
13. ↑ Neufcourt, L.; Cao, Y.; Nazarewicz, W.; Olsen, E.; Viens, F. (2019). "Neutron drip line in the Ca region from Bayesian model averaging". Physical Review Letters 122 (6): 062502–1–062502–6. doi:10.1103/PhysRevLett.122.062502. PMID 30822058. Bibcode: 2019PhRvL.122f2502N.

## External links

* Argon isotopes data from The Berkeley Laboratory Isotopes Project's

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