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 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 less than two hours, and most less than one minute. The least stable is ³⁰Ar with a half-life shorter than 20 nanoseconds.

The naturally occurring ⁴⁰K, with a half-life of 1.248×10⁹ 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 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 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] Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon.^[5] In December 2013, ³⁶Argon, in the form of argon hydride, was found in cosmic dust associated with the Crab nebula supernova.^[6][7] This was the first time a noble molecule was detected in outer space.^[6][7]

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.^[1]

Relative atomic mass: 39.948(1)

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)[8]	daughter isotope(s)[n 1]	nuclear spin	representative isotopic composition (mole fraction)[n 2]	range of natural variation (mole fraction)
	excitation energy
^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+
					β^+, p (43.01%)	^31S
^32mAr	5600(100) keV			unknown			5−#
^33Ar	18	15	32.9899257(5)	173.0(20) ms	β^+ (61.35%)	^33Cl	1/2+
					β^+, p (38.65%)	^32S
^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+	0.000629(1)
^39Ar[n 4]	18	21	38.964313(5)	269(3) y	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) y	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−#
					β^−, n (1%)	^46K
^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−)#
					β^−, n	^52K

1. Bold for stable isotopes
2. 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.
3. Believed to undergo double electron capture to ³⁶S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)
4. Used in argon-argon dating
5. Cosmogenic nuclide
6. Used in argon-argon dating and potassium-argon dating
7. Generated from ⁴⁰K in rocks. These ratios are terrestrial. Cosmic abundance is far less than ³⁶Ar.

Notes

• 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.

See also

References

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 34, 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.
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.
5. H. O. Back; et al. (2012). "Depleted Argon from Underground Sources". Physics Procedia. 37: 1105–1112. doi:10.1016/j.phpro.2012.04.099.
6. Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". The New York Times. Retrieved 13 December 2013.
7. Barlow, M. J.; et al. (2013). "Detection of a Noble Gas Molecular Ion, ³⁶ArH+, in the Crab Nebula". Science. 342 (6164): 1343–1345. doi:10.1126/science.124358213.
8. "Universal Nuclide Chart". nucleonica. (registration required (help)).

• Isotope masses from:
  • G. Audi; A. H. Wapstra; C. Thibault; J. Blachot; 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.
• 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; 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.
  • 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. Lay summary.
• 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; 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.
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. Check date values in: |access-date= (help)
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0-8493-0485-9.

External links

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