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 ⁴⁰K 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.

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% ⁴⁰Ar and mostly (85%) ³⁶Ar.

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. 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.

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.

Standard argon atomic mass: 39.948(1) u.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)	daughter isotope(s)	nuclear spin	representative isotopic composition (mole fraction)	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			0+	0.003336(4)
37Ar	18	19	36.96677632(22)	35.04(4) d	EC	37Cl	3/2+
38Ar	18	20	37.9627324(4)	Stable			0+	6.29(1)×10−4
39Ar	18	21	38.964313(5)	269(3) a	β−	39K	7/2-	Trace
40Ar	18	22	39.9623831225(29)	Stable			0+	0.996035(4)
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-#
					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

Notes

• 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.

References

• 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 978-0849304859. {{cite book}}: Unknown parameter |nopp= ignored (|no-pp= suggested) (help)

External links

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