Isotopes of argon

Argon (18Ar) has 26 known isotopes, from 29Ar to 54Ar, and one 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.[4]

Isotopes of argon (18Ar)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
36Ar 0.334% stable
37Ar trace 35 d ε 37Cl
38Ar 0.0630% stable
39Ar trace 268 y β− 39K
40Ar 99.6% stable
41Ar trace 109.34 min β− 41K
42Ar synth 32.9 y β− 42K
Standard atomic weight Ar°(Ar)

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.[5]

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.[6]

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.[7] 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.[8] Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon.[9] 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.[10][11] This was the first time a noble molecule was detected in outer space.[10][11]

37Ar is a synthetic radionuclide that is created via neutron capture of 40Ca followed by alpha particle emission, as a result of subsurface nuclear explosions. It has a half-life of 35 days.[5]

List of isotopes
Nuclide[12]
[n 1]		 Z N Isotopic mass (Da)[13]
[n 2][n 3]		 Half-life
 Decay
mode
[n 4]		 Daughter
isotope
[n 5]		 Spin and
parity
[n 6][n 7]		 Natural abundance (mole fraction)
Excitation energy Normal proportion Range of variation
29Ar[4] 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[14]		 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[15] 18 36 β− 54K 0+
This table header & footer:
  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. 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.

References
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  3. Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (4 May 2022). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
  4. Mukha, I.; et al. (2018). "Deep excursion beyond the proton dripline. I. Argon and chlorine isotope chains". Physical Review C. 98 (6): 064308–1–064308–13. arXiv:1803.10951. Bibcode:2018PhRvC..98f4308M. doi:10.1103/PhysRevC.98.064308. S2CID 119384311.
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  6. 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. Bibcode:1973SSRv...14..392C. doi:10.1007/BF00214750. S2CID 119861943.
  7. P. Benetti; et al. (2007). "Measurement of the specific activity of 39Ar in natural argon". Nuclear Instruments and Methods A. 574 (1): 83–88. arXiv:astro-ph/0603131. Bibcode:2007NIMPA.574...83B. doi:10.1016/j.nima.2007.01.106. S2CID 17073444.
  8. V. D. Ashitkov; et al. (1998). "New experimental limit on the 42Ar content in the Earth's atmosphere". Nuclear Instruments and Methods A. 416 (1): 179–181. Bibcode:1998NIMPA.416..179A. doi:10.1016/S0168-9002(98)00740-2.
  9. H. O. Back; et al. (2012). "Depleted Argon from Underground Sources". Physics Procedia. 37: 1105–1112. Bibcode:2012PhPro..37.1105B. doi:10.1016/j.phpro.2012.04.099.
  10. Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times. Retrieved 13 December 2013.
  11. Barlow, M. J.; et al. (2013). "Detection of a Noble Gas Molecular Ion, 36ArH+, in the Crab Nebula". Science. 342 (6164): 1343–1345. arXiv:1312.4843. Bibcode:2013Sci...342.1343B. doi:10.1126/science.1243582. PMID 24337290. S2CID 37578581.
  12. 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" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
  13. 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" (PDF). Chinese Physics C. 41 (3): 030003-1–030003-442. doi:10.1088/1674-1137/41/3/030003.
  14. Golovko, Victor V. (15 October 2023). "Application of the most frequent value method for 39Ar half-life determination". The European Physical Journal C. 83 (10). arXiv:2310.06867. doi:10.1140/epjc/s10052-023-12113-6. ISSN 1434-6052.
  15. 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. arXiv:1901.07632. Bibcode:2019PhRvL.122f2502N. doi:10.1103/PhysRevLett.122.062502. PMID 30822058. S2CID 73508148.
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