Isotopes of argon

Argon (Ar) has 24 known isotopes, from ³⁰Ar to ⁵³Ar and 1 isomer (^32mAr), three of which 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 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 less than 2 hours, and most less than a minute. The least stable is ³⁰Ar with a half-life shorter than 20 nanoseconds.

Naturally occurring p⁴⁰K with a half-life of 1.248×10⁹ (3) years, decays to stable ⁴⁰Ar (10.72%) by electron capture and by positron emission, and also transforms to stable ⁴⁰Ca (89.28%) via beta decay. These properties and ratios are used to determine the age of rocks through potassium-argon dating.^[1]

Despite 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 potassium-40 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^[2]

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.^[3] 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.^[4] In December 2013, ³⁶Argon, in the form of argon hydride, was found in cosmic dust associated with the Crab nebula supernova.^[5]^[6] This was the first time a noble molecule was detected in outer space.^[5]^[6]

Radioactive ³⁷Ar is a synthetic radionuclide that is created from the neutron spallation of ⁴⁰Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days.^[1]

Standard argon atomic mass: 39.948(1) u.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)^[7]	daughter isotope(s)^{[n 1]}	nuclear spin	representative isotopic composition (mole fraction)^{[n 2]}	range of natural variation (mole fraction)
nuclide symbol	excitation energy			half-life	decay mode(s)^[7]	daughter isotope(s)^{[n 1]}	nuclear spin	representative isotopic composition (mole fraction)^{[n 2]}	range of natural variation (mole fraction)
³⁰Ar	18	12	30.02156(32)#	<20 ns	p	²⁹Cl	0+
³¹Ar	18	13	31.01212(22)#	14.4(6) ms	β⁺, p (55.0%)	³⁰S	5/2(+#)
					β⁺ (40.4%)	³¹Cl
					β⁺, 2p (2.48%)	²⁹P
					β⁺, 3p (2.1%)	²⁸Si
³²Ar	18	14	31.9976380(19)	98(2) ms	β⁺ (56.99%)	³²Cl	0+
³²Ar	18	14	31.9976380(19)	98(2) ms	β⁺, p (43.01%)	³¹S	0+
^32mAr	5600(100)# keV			unknown			5-#
³³Ar	18	15	32.9899257(5)	173.0(20) ms	β⁺ (61.35%)	³³Cl	1/2+
³³Ar	18	15	32.9899257(5)	173.0(20) ms	β⁺, p (38.65%)	³²S	1/2+
³⁴Ar	18	16	33.9802712(4)	844.5(34) ms	β⁺	³⁴Cl	0+
³⁵Ar	18	17	34.9752576(8)	1.775(4) s	β⁺	³⁵Cl	3/2+
³⁶Ar	18	18	35.967545106(29)	Observationally Stable^{[n 3]}			0+	0.003336(4)
³⁷Ar	18	19	36.96677632(22)	35.04(4) d	ε	³⁷Cl	3/2+
³⁸Ar	18	20	37.9627324(4)	Stable			0+	6.29(1)×10⁻⁴
³⁹Ar^{[n 4]}	18	21	38.964313(5)	269(3) a	β⁻	³⁹K	7/2-	Trace^{[n 5]}
⁴⁰Ar^{[n 6]}	18	22	39.9623831225(29)	Stable			0+	0.996035(4)^{[n 7]}
⁴¹Ar	18	23	40.9645006(4)	109.61(4) min	β⁻	⁴¹K	7/2-
⁴²Ar	18	24	41.963046(6)	32.9(11) a	β⁻	⁴²K	0+	Trace
⁴³Ar	18	25	42.965636(6)	5.37(6) min	β⁻	⁴³K	(5/2-)
⁴⁴Ar	18	26	43.9649240(17)	11.87(5) min	β⁻	⁴⁴K	0+
⁴⁵Ar	18	27	44.9680400(6)	21.48(15) s	β⁻	⁴⁵K	(1/2,3/2,5/2)-
⁴⁶Ar	18	28	45.96809(4)	8.4(6) s	β⁻	⁴⁶K	0+
⁴⁷Ar	18	29	46.97219(11)	1.23(3) s	β⁻ (99%)	⁴⁷K	3/2-#
⁴⁷Ar	18	29	46.97219(11)	1.23(3) s	β⁻, n (1%)	⁴⁶K	3/2-#
⁴⁸Ar	18	30	47.97454(32)#	0.48(40) s	β⁻	⁴⁸K	0+
⁴⁹Ar	18	31	48.98052(54)#	170(50) ms	β⁻	⁴⁹K	3/2-#
⁵⁰Ar	18	32	49.98443(75)#	85(30) ms	β⁻	⁵⁰K	0+
⁵¹Ar	18	33	50.99163(75)#	60# ms [>200 ns]	β⁻	⁵¹K	3/2-#
⁵²Ar	18	34	51.99678(97)#	10# ms	β⁻	⁵²K	0+
⁵³Ar	18	35	53.00494(107)#	3# ms	β⁻	⁵³K	(5/2-)#
⁵³Ar	18	35	53.00494(107)#	3# ms	β⁻, n	⁵²K	(5/2-)#

^ Bold for stable isotopes
^ Isotopic composition refers to that in air. ³⁶Ar is actually far more abundant than ⁴⁰Ar, universally. ⁴⁰Ar is most abundant in air because most of it is radiogenic. Such ⁴⁰Ar atoms are a decay product from ⁴⁰K via electron capture, whereas ⁴⁰K goes under mostly β^- decay to ⁴⁰Ca. ⁴⁰Ar escapes the ⁴⁰K-containing rocks into the atmosphere, thus making the argon in the air mostly ⁴⁰Ar, not ³⁶Ar.
^ Believed to undergo β⁺β⁺ decay to ³⁶S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)
^ Used in argon-argon dating
^ Cosmogenic nuclide
^ 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.

Nuclide masses are given by IUPAP Commission on Symbols, Units, Nomenclature, Atomic Masses and Fundamental Constants (SUNAMCO).
Isotope abundances are given by IUPAC Commission on Isotopic Abundances and Atomic Weights.

References

^ ^a ^b "⁴⁰Ar/³⁹Ar dating and errors". Archived from the original on 2007-05-09. Retrieved 2007-03-07.
^ Cameron, A. G. W., "Elemental and Isotopic Abundances of the Volatile Elements in the Outer Planets" (Article published in the Space Science Reviews special issue on 'Outer Solar System Exploration - An Overview', ed. by J. E. Long and D. G. Rea.) Journal: Space Science Reviews, Volume 14, Issue 3-4, pp. 392-400 (1973).
^ P. Benetti; et al. (2007). "Measurement of the specific activity of ³⁹Ar in natural argon". Nuclear Instruments and Methods A. 574: 83. arXiv:astro-ph/0603131. Bibcode:2007NIMPA.574...83B. doi:10.1016/j.nima.2007.01.106. {{cite journal}}: Explicit use of et al. in: |author= (help)
^ V. D. Ashitkov; et al. (1998). "New experimental limit on the ⁴²Ar content in the Earth's atmosphere". Nuclear Instruments and Methods A. 416: 179. doi:10.1016/S0168-9002(98)00740-2. {{cite journal}}: Explicit use of et al. in: |author= (help)
^ ^a ^b Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". New York Times. Retrieved 13 December 2013.
^ ^a ^b 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.124358213. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ http://www.nucleonica.net/unc.aspx

Isotope masses from:
- G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.{{cite journal}}: CS1 maint: multiple names: authors list (link)
Isotopic compositions and standard atomic masses from:
- J. R. de Laeter, J. K. Böhlke, P. De Bièvre, H. Hidaka, H. S. Peiser, K. J. R. Rosman and P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.{{cite journal}}: CS1 maint: multiple names: authors list (link)
- M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051. {{cite journal}}: Unknown parameter |laysummary= ignored (help)
Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
- G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. Bibcode:2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001.{{cite journal}}: CS1 maint: multiple names: authors list (link)
- National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. {{cite web}}: Check date values in: |accessdate= (help)
- N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide (ed.). CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9. {{cite book}}: Unknown parameter |nopp= ignored (|no-pp= suggested) (help)

External links

Argon isotopes data from The Berkeley Laboratory Isotopes Project's

[8] Bold for stable isotopes

[9] Isotopic composition refers to that in air. ³⁶Ar is actually far more abundant than ⁴⁰Ar, universally. ⁴⁰Ar is most abundant in air because most of it is radiogenic. Such ⁴⁰Ar atoms are a decay product from ⁴⁰K via electron capture, whereas ⁴⁰K goes under mostly β^- decay to ⁴⁰Ca. ⁴⁰Ar escapes the ⁴⁰K-containing rocks into the atmosphere, thus making the argon in the air mostly ⁴⁰Ar, not ³⁶Ar.

[10] Believed to undergo β⁺β⁺ decay to ³⁶S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)

[11] Used in argon-argon dating

[c-12] Cosmogenic nuclide

[13] Used in argon-argon dating and potassium-argon dating

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

[iso-1] "⁴⁰Ar/³⁹Ar dating and errors". Archived from the original on 2007-05-09. Retrieved 2007-03-07.

[2] Cameron, A. G. W., "Elemental and Isotopic Abundances of the Volatile Elements in the Outer Planets" (Article published in the Space Science Reviews special issue on 'Outer Solar System Exploration - An Overview', ed. by J. E. Long and D. G. Rea.) Journal: Space Science Reviews, Volume 14, Issue 3-4, pp. 392-400 (1973).

[3] P. Benetti; et al. (2007). "Measurement of the specific activity of ³⁹Ar in natural argon". Nuclear Instruments and Methods A. 574: 83. arXiv:astro-ph/0603131. Bibcode:2007NIMPA.574...83B. doi:10.1016/j.nima.2007.01.106. {{cite journal}}: Explicit use of et al. in: |author= (help)

[4] V. D. Ashitkov; et al. (1998). "New experimental limit on the ⁴²Ar content in the Earth's atmosphere". Nuclear Instruments and Methods A. 416: 179. doi:10.1016/S0168-9002(98)00740-2. {{cite journal}}: Explicit use of et al. in: |author= (help)

[NYT-20131213-5] Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". New York Times. Retrieved 13 December 2013.

[SCI-2013-6] 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.124358213. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[7] ttp://www.nucleonica.net/unc.aspx

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See also

References

External links