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Isotopes of argon

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Isotopes of argon (18Ar)
Main isotopes Decay
abun­dance half-life (t1/2) mode pro­duct
Ar 0.334% stable
Ar trace 35 d ε Cl
Ar 0.0630% stable
Ar trace 268 y β K
Ar 99.6% stable
Ar trace 109.34 min β K
Ar synth 32.9 y β K
Standard atomic weight Ar°(Ar)

Argon (18Ar) has 26 known isotopes, from Ar to Ar, of which three are stable (Ar, Ar, and Ar). On the Earth, Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are Ar with a half-life of 268 years, Ar with a half-life of 32.9 years, and Ar 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 naturally occurring K, with a half-life of 1.248×10 years, decays to stable Ar by electron capture (10.72%) and by positron emission (0.001%), and also transforms to stable Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.

Despite the trapping of Ar 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 K decay, since 99.6% of Earth atmospheric argon is Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of < 15% Ar and mostly (85%) Ar. Similarly, the ratio of the three isotopes Ar:Ar:Ar in the atmospheres of the outer planets is measured to be 8400:1600:1.

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

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

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

List of isotopes


Nuclide
 Z N Isotopic mass (Da)
 Half-life
 Decay
mode
 Daughter
isotope
 Spin and
parity
 Natural abundance (mole fraction)
Excitation energy Normal proportion Range of variation
Ar 18 11 29.04076(47)# 2p S 5/2+#
Ar 18 12 30.02369(19)# <10 ps 2p S 0+
Ar 18 13 31.01216(22)# 15.0(3) ms β, p (68.3%) S 5/2+
β (22.63%) Cl
β, 2p (9.0%) P
β, 3p (0.07%) Si
β, p, α? (<0.38%) Si
β, α? (<0.03%) P
2p? (<0.03%) S
Ar 18 14 31.9976378(19) 98(2) ms β (64.42%) Cl 0+
β, p (35.58%) S
Ar 18 15 32.98992555(43) 173.0(20) ms β (61.3%) Cl 1/2+
β, p (38.7%) S
Ar 18 16 33.980270092(83) 846.46(35) ms β Cl 0+
Ar 18 17 34.97525772(73) 1.7756(10) s β Cl 3/2+
Ar 18 18 35.967545106(28) Observationally Stable 0+ 0.003336(210)
Ar 18 19 36.96677630(22) 35.011(19) d EC Cl 3/2+ Trace
Ar 18 20 37.96273210(21) Stable 0+ 0.000629(70)
Ar 18 21 38.9643130(54) 268.2+3.1
−2.9 y 		β K 7/2− 8×10
Ar 18 22 39.9623831220(23) Stable 0+ 0.996035(250)
Ar 18 23 40.96450057(37) 109.61(4) min β K 7/2− Trace
Ar 18 24 41.9630457(62) 32.9(11) y β K 0+
Ar 18 25 42.9656361(57) 5.37(6) min β K 5/2(−)
Ar 18 26 43.9649238(17) 11.87(5) min β K 0+
Ar 18 27 44.96803973(55) 21.48(15) s β K (5/2−,7/2−)
Ar 18 28 45.9680392(25) 8.4(6) s β K 0+
Ar 18 29 46.9727671(13) 1.23(3) s β (>99.8%) K (3/2)−
β, n? (<0.2%) K
Ar 18 30 47.976001(18) 415(15) ms β (62%) K 0+
β, n (38%) K
Ar 18 31 48.98169(43)# 236(8) ms β K 3/2−#
β, n (29%) K
β, 2n? K
Ar 18 32 49.98580(54)# 106(6) ms β (63%) K 0+
β, n (37%) K
β, 2n? K
Ar 18 33 50.99303(43)# 30# ms
 β? K 1/2−#
β, n? K
β, 2n? K
Ar 18 34 51.99852(64)# 40# ms
 β? K 0+
β, n? K
β, 2n? K
Ar 18 35 53.00729(75)# 20# ms
 β? K 5/2−#
β, n? K
β, 2n? K
Ar 18 36 54.01348(86)# 5# ms
 β? K 0+
β, n? K
β, 2n? K
This table header & footer:
  1. Ar – 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 S (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 K in rocks. These ratios are terrestrial. Cosmic abundance is far less than Ar.

References

  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.
  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. ^ "Ar/Ar dating and errors". Archived from the original on 9 May 2007. Retrieved 7 March 2007.
  5. 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.
  6. P. Benetti; et al. (2007). "Measurement of the specific activity of Ar 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.
  7. V. D. Ashitkov; et al. (1998). "New experimental limit on the Ar 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.
  8. 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.
  9. ^ Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times. Retrieved 13 December 2013.
  10. ^ Barlow, M. J.; et al. (2013). "Detection of a Noble Gas Molecular Ion, ArH+, 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.
  11. Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
  12. 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.
  13. Golovko, Victor V. (15 October 2023). "Application of the most frequent value method for Ar half-life determination". The European Physical Journal C. 83 (10): 930. arXiv:2310.06867. Bibcode:2023EPJC...83..930G. doi:10.1140/epjc/s10052-023-12113-6. ISSN 1434-6052.
  14. Lu, Zheng-Tian (1 March 2013). "What trapped atoms reveal about global groundwater". Physics Today. 66 (3): 74–75. Bibcode:2013PhT....66c..74L. doi:10.1063/PT.3.1926.

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Isotopes of the chemical elements
Group 1 2   3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
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