Isotopes of argon

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

|

| 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, of which three are stable (36Ar, 38Ar, and 40Ar). On Earth, 40Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are 39Ar with a half-life of 302 years, 42Ar with a half-life of 32.9 years, and 37Ar with a half-life of 35.01 days. All other isotopes have half-lives of less than two hours, and most less than one minute. Isotopes lighter than 38Ar decay to chlorine or lighter elements, while heavier ones beta decay to potassium.

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 to stable 40Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.[2]

Despite the trapping of 40Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all argon in the Earth's atmosphere is the product of 40K decay, since 99.6% of Earth's atmospheric argon is 40Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of ~85% 36Ar, ~15% 38Ar and only trace 40Ar. Similarly, the ratio of the isotopes 36Ar:38Ar:40Ar in the atmospheres of the outer planets is measured to be 8400:1600:1.[3]

In the Earth's atmosphere, radioactive 39Ar (and to a lesser extent 37Ar) is made by cosmic ray activity, primarily from 40Ar. In the subsurface environment, 39Ar is also produced through neutron capture by 39K or 42Ca, with proton or alpha emission respectively; 37Ar was created in subsurface nuclear explosions similarly from 40Ca.[2] The content of 39Ar in natural argon is measured to be of (8.6±0.4)×10−16 g/g, or (0.964±0.024) Bq/kg weight.[4]

The content of 42Ar (half-life 33 years) in the Earth's atmosphere, though it had previously been reported as a cosmogenic isotope,[5] is lower than 6×10−21 of the element.[6] Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon[7] and this may be obtained from underground sources that have been isolated from the atmosphere long enough for these isotopes to decay.

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]

## Contents

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

## List of isotopes

Nuclide
[n 1] | Z | N | Isotopic mass (u)
[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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|
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|
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Excitation energy  | Normal proportion  | Range of variation
29Ar[10] | 18  | 11  | 29.04076(47)#  |  | 2p | 27S  | 5/2+#  |  |
30Ar  | 18  | 12  | 30.02369(19)#  | <10 ps  | 2p  | 28S  | 0+  |  |
31Ar  | 18  | 13  | 31.012125(14)[11] | 15.0(3) ms  | β+, p (68.3%)  | 30S  | 5/2+  |  |
β+ (22.63%)  | 31Cl
β+, 2p (9.0%)  | 29P
β+, 3p (0.07%)  | 28Si
β+, p, α? (<0.38%)  | 26Si
β+, α? (<0.03%)  | 27P
2p? (<0.03%)  | 29S
32Ar  | 18  | 14  | 31.9976378(19)  | 98(2) ms  | β+ (64.42%)  | 32Cl  | 0+  |  |
β+, p (35.58%)  | 31S
33Ar  | 18  | 15  | 32.98992555(43)  | 173.0(20) ms  | β+ (61.3%)  | 33Cl  | 1/2+  |  |
β+, p (38.7%)  | 32S
34Ar  | 18  | 16  | 33.980270092(83)  | 846.46(35) ms  | β+ | 34Cl  | 0+  |  |
35Ar  | 18  | 17  | 34.97525772(73)  | 1.7756(10) s  | β+ | 35Cl | 3/2+  |  |
36Ar  | 18  | 18  | 35.967545106(28)  | Observationally Stable[n 8] | 0+  | 0.003336(210)  |
37Ar  | 18  | 19  | 36.96677630(22)  | 35.011(19) d  | EC | 37Cl | 3/2+  | Trace[n 9] |
38Ar  | 18  | 20  | 37.96273210(21)  | Stable | 0+  | 0.000629(70)  |
39Ar[n 10] | 18  | 21  | 38.9643130(54)  | 302(10) y[12][13] | β− | 39K | 7/2−  | 8×10−16[14][n 9] |
40Ar[n 11] | 18  | 22  | 39.9623831220(23)  | Stable | 0+  | 0.996035(250)[n 12] |
41Ar  | 18  | 23  | 40.96450057(37)  | 109.61(4) min  | β− | 41K | 7/2−  | Trace[n 9] |
42Ar  | 18  | 24  | 41.9630457(62)  | 32.9(11) y  | β− | 42K  | 0+  |  |
43Ar  | 18  | 25  | 42.9656361(57)  | 5.37(6) min  | β− | 43K  | 5/2(−)  |  |
44Ar  | 18  | 26  | 43.9649238(17)  | 11.87(5) min  | β− | 44K  | 0+  |  |
45Ar  | 18  | 27  | 44.96803973(55)  | 21.48(15) s  | β− | 45K  | (5/2−,7/2−)  |  |
46Ar  | 18  | 28  | 45.9680392(25)  | 8.4(6) s  | β− | 46K  | 0+  |  |
47Ar  | 18  | 29  | 46.9727671(13)  | 1.23(3) s  | β− (>99.8%)  | 47K  | (3/2)−  |  |
β−, n? (<0.2%)  | 46K
48Ar  | 18  | 30  | 47.976001(18)  | 415(15) ms  | β− (62%)  | 48K  | 0+  |  |
β−, n (38%)  | 47K
49Ar  | 18  | 31  | 48.98169(43)#  | 236(8) ms  | β− | 49K  | 3/2−#  |  |
β−, n (29%)  | 48K
β−, 2n?  | 47K
50Ar  | 18  | 32  | 49.98580(54)#  | 106(6) ms  | β− (63%)  | 50K  | 0+  |  |
β−, n (37%)  | 49K
β−, 2n?  | 48K
51Ar  | 18  | 33  | 50.99303(43)#  | 30# ms
[>200 ns]  | β−?  | 51K  | 1/2−#  |  |
β−, n?  | 50K
β−, 2n?  | 49K
52Ar  | 18  | 34  | 51.99852(64)#  | 40# ms
[>620 ns]  | β−?  | 52K  | 0+  |  |
β−, n?  | 51K
β−, 2n?  | 50K
53Ar  | 18  | 35  | 53.00729(75)#  | 20# ms
[>620 ns]  | β−?  | 53K  | 5/2−#  |  |
β−, n?  | 52K
β−, 2n?  | 51K
54Ar  | 18  | 36  | 54.01348(86)#  | 5# ms
[>400 ns]  | β−?  | 54K  | 0+  |  |
β−, n?  | 53K
β−, 2n?  | 52K

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

* Banana equivalent dose

Daughter products other than argon

* Isotopes of potassium
* Isotopes of chlorine
* Isotopes of sulfur
* Isotopes of phosphorus

## 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 "40Ar/39Ar dating and errors". http://www.geoberg.de/text/geology/07011601.php.
3. ↑ 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.
4. ↑ P. Adhikari (2023). "Precision Measurement of the Specific Activity of 39Ar in Atmospheric Argon with the DEAP-3600 Detector". The European Physical Journal C 83 (7): 642. doi:10.1140/epjc/s10052-023-11678-6. https://doi.org/10.1140/epjc/s10052-023-11678-6.
5. ↑ For example in Barabash, A.S.; Saakyan, R.R.; Umatov, V.I. (2016). "On concentration of 42Ar in the Earth's atmosphere". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 839: 39–42. doi:10.1016/j.nima.2016.09.042.
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. ↑ 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.
11. ↑ Chen, Z. Y.; Yan, X. L.; Hou, S. Q.; Liu, J. B.; Shi, J. Y.; Zhou, X. H.; Zhang, Y. H.; Wang, M. et al. (1 December 2025). "Precision Mass Measurement of 26 P and 27 S and Their Impact on the 26 P( p , γ ) 27 S Reaction in Stellar X-Ray Bursts". The Astrophysical Journal 994 (2): 270. doi:10.3847/1538-4357/ae1470.
12. ↑ P. Adhikari (2025). "Direct Measurement of the Half-Life of 39Ar from 3.4 Years of Data with the DEAP-3600 Detector". The European Physical Journal C 85 (7): 728. doi:10.1140/epjc/s10052-025-14289-5.
13. ↑ This appears reliable but the NUBASE2020 value is 268(8) years and the discrepancy is yet to be explained.
14. ↑ Lu, Zheng-Tian (1 March 2013). "What trapped atoms reveal about global groundwater". Physics Today 66 (3): 74–75. doi:10.1063/PT.3.1926. Bibcode: 2013PhT....66c..74L.

## External links

* Argon isotopes data from The Berkeley Laboratory Isotopes Projects

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