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

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.^[5]

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 captureby³⁹K or alpha emissionbycalcium. 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.^[6] 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.^[7] Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon.^[8] 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.^[9]^[10] This was the first time a noble molecule was detected in outer space.^[9]^[10]

³⁷Ar is a synthetic radionuclide that is created via neutron captureof⁴⁰Ca followed by alpha particle emission, as a result of subsurface nuclear explosions. It has a half-life of 35 days.^[4]

List of isotopes[edit]

Nuclide
^{[n 1]}

Isotopic mass (Da)^[11]
^{[n 2]}^{[n 3]}

Half-life^[1]

Decay
mode^[1]
^{[n 4]}

Daughter
isotope
^{[n 5]}

Spin and
parity^[1]
^{[n 6]}^{[n 7]}

Natural abundance (mole fraction)

Excitation energy

Normal proportion^[1]

Range of variation

²⁹Ar^[12]

29.04076(47)#

²⁷S

5/2+#

³⁰Ar

30.02369(19)#

<10 ps

²⁸S

³¹Ar

31.01216(22)#

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

31.9976378(19)

98(2) ms

β⁺ (64.42%)

³²Cl

β⁺, p (35.58%)

³¹S

³³Ar

32.98992555(43)

173.0(20) ms

β⁺ (61.3%)

³³Cl

1/2+

β⁺, p (38.7%)

³²S

³⁴Ar

33.980270092(83)

846.46(35) ms

β⁺

³⁴Cl

³⁵Ar

34.97525772(73)

1.7756(10) s

β⁺

³⁵Cl

3/2+

³⁶Ar

35.967545106(28)

Observationally Stable^{[n 8]}

0.003336(210)

³⁷Ar

36.96677630(22)

35.011(19) d

³⁷Cl

3/2+

Trace^{[n 9]}

³⁸Ar

37.96273210(21)

Stable

0.000629(70)

³⁹Ar^{[n 10]}

38.9643130(54)

268.2+3.1
−2.9 y^[13]

β⁻

³⁹K

7/2−

8×10⁻¹⁶^[14]^{[n 9]}

⁴⁰Ar^{[n 11]}

39.9623831220(23)

Stable

0.996035(250)^{[n 12]}

⁴¹Ar

40.96450057(37)

109.61(4) min

β⁻

⁴¹K

7/2−

Trace^{[n 9]}

⁴²Ar

41.9630457(62)

32.9(11) y

β⁻

⁴²K

⁴³Ar

42.9656361(57)

5.37(6) min

β⁻

⁴³K

5/2(−)

⁴⁴Ar

43.9649238(17)

11.87(5) min

β⁻

⁴⁴K

⁴⁵Ar

44.96803973(55)

21.48(15) s

β⁻

⁴⁵K

(5/2−,7/2−)

⁴⁶Ar

45.9680392(25)

8.4(6) s

β⁻

⁴⁶K

⁴⁷Ar

46.9727671(13)

1.23(3) s

β⁻ (>99.8%)

⁴⁷K

(3/2)−

β⁻, n? (<0.2%)

⁴⁶K

⁴⁸Ar

47.976001(18)

415(15) ms

β⁻ (62%)

⁴⁸K

β⁻, n (38%)

⁴⁷K

⁴⁹Ar

48.98169(43)#

236(8) ms

β⁻

⁴⁹K

3/2−#

β⁻, n (29%)

⁴⁸K

β⁻, 2n?

⁴⁷K

⁵⁰Ar

49.98580(54)#

106(6) ms

β⁻ (63%)

⁵⁰K

β⁻, n (37%)

⁴⁹K

β⁻, 2n?

⁴⁸K

⁵¹Ar

50.99303(43)#

30# ms
[>200 ns]

β⁻?

⁵¹K

1/2−#

β⁻, n?

⁵⁰K

β⁻, 2n?

⁴⁹K

⁵²Ar

51.99852(64)#

40# ms
[>620 ns]

β⁻?

⁵²K

β⁻, n?

⁵¹K

β⁻, 2n?

⁵⁰K

⁵³Ar

53.00729(75)#

20# ms
[>620 ns]

β⁻?

⁵³K

5/2−#

β⁻, n?

⁵²K

β⁻, 2n?

⁵¹K

⁵⁴Ar

54.01348(86)#

5# ms
[>400 ns]

β⁻?

⁵⁴K

β⁻, n?

⁵³K

β⁻, 2n?

⁵²K

This table header & footer:

^ ^mAr – Excited nuclear isomer.

^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.

^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).

^ Modes of decay:

EC:

Electron capture

Neutron emission

Proton emission

^ Bold symbol as daughter – Daughter product is stable.

^ ( ) spin value – Indicates spin with weak assignment arguments.

^ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).

^ Believed to undergo double electron capture to ³⁶S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)

^ ^a ^b ^c Cosmogenic nuclide

^ Used in argon–argon dating

^ Used in argon–argon dating and potassium–argon dating

^ Generated from ⁴⁰K in rocks. These ratios are terrestrial. Cosmic abundance is far less than ³⁶Ar.

References[edit]

^ ^a ^b ^c ^d ^e 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.

^ ^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.

^ 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.

^ 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.

^ 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): 930. arXiv:2310.06867. Bibcode:2023EPJC...83..930G. doi:10.1140/epjc/s10052-023-12113-6. ISSN 1434-6052.

^ 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.