Isotopes of argon

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 269 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.[2]

Main isotopes of 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 ^36 Ar and ^38 Ar content may be as high as 2.07% and 4.3% respectively in natural samples. ^40 Ar 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

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.[3]

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.[4]

In the Earth's atmosphere, radioactive ³⁹Ar (half-life 269 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.[5] 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.[6] Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon.[7]

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]

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.[3]

List of isotopes

Nuclide[10] [n 1]	Z	N	Isotopic mass (Da)[11] [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[2]	18	11		~4×10^−20 s	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	ε	^37Cl	3/2+
^38Ar	18	20	37.96273210(21)	Stable			0+	0.000629(1)
^39Ar[n 9]	18	21	38.964313(5)	269(3) y	β^−	^39K	7/2−	Trace[n 10]
^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−
^42Ar	18	24	41.963046(6)	32.9(11) y	β^−	^42K	0+	Trace
^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[12]	18	36			β^−	^54K	0+

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:

   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. Used in argon–argon dating
10. Cosmogenic nuclide
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

References

1. Meija, Juris; 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. 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.
3. "⁴⁰Ar/³⁹Ar dating and errors". Archived from the original on 9 May 2007. Retrieved 7 March 2007.
4. 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.
5. 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.
6. 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.
7. 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.
8. Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times. Retrieved 13 December 2013.
9. 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.
10. 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.
11. 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.
12. 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.

External links

• Argon isotopes data from The Berkeley Laboratory Isotopes Project's