Isotopes of argon information

Isotopes of argon (₁₈Ar)
Standard atomic weight Ar°(Ar)
	[39.792, 39.963]; 39.95±0.16 (abridged);
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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.^[4]

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, 40}Ar.^[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, 40}Ar.^[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]

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.^[5]

^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
^ "Standard Atomic Weights: Argon". CIAAW. 2017.
^ 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.
^ Cite error: The named reference p-excursionArCl was invoked but never defined (see the help page).
^ ^a ^b "⁴⁰Ar/³⁹Ar dating and errors". Archived from the original on 9 May 2007. Retrieved 7 March 2007.
^ 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.
^ 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.
^ 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.
^ 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.
^ ^a ^b Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times. Retrieved 13 December 2013.
^ ^a ^b 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.

[NUBASE2020-1] Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.

[2] "Standard Atomic Weights: Argon". CIAAW. 2017.

[CIAAW2021-3] 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.

[p-excursionArCl-4] Cite error: The named reference p-excursionArCl was invoked but never defined (see the help page).

[iso-5] "⁴⁰Ar/³⁹Ar dating and errors". Archived from the original on 9 May 2007. Retrieved 7 March 2007.

[36Ar-Cameron-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.

[40Ar-Benetti-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.

[40Ar-Ashitkov-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.

[36Ar-Back-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.

[NYT-20131213-10] Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times. Retrieved 13 December 2013.

[SCI-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.

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