Isotopes of argon

The main isotopes of argon (Ar) found on Earth are ⁴⁰Ar, ³⁶Ar, and ³⁸Ar. Naturally occurring ⁴⁰K with a half-life of 1.250×10⁹ years, decays to stable ⁴⁰Ar (11.2%) by electron capture and by positron emission, and also transforms to stable ⁴⁰Ca (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks through potassium-argon dating.

In the Earth's atmosphere, ³⁹Ar is made by cosmic ray activity, primarily with 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 ^natAr . ³⁷Ar is created from the decay of ⁴⁰Ca as a result of subsurface nuclear explosions. It has a half-life of 35 days. The content of ⁴²Ar in the Earth's atmosphere is lower than 6×10⁻²¹ parts per part of ^natAr .

Standard atomic mass: 39.948(1) u.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
	excitation energy
30Ar	18	12	30.02156(32)#	<20 ns	0+
31Ar	18	13	31.01212(22)#	14.4(6) ms	5/2(+#)
32Ar	18	14	31.9976380(19)	98(2) ms	0+
32mAr	5600(100)# keV			?	5-#
33Ar	18	15	32.9899257(5)	173.0(20) ms	1/2+
34Ar	18	16	33.9802712(4)	844.5(34) ms	0+
35Ar	18	17	34.9752576(8)	1.775(4) s	3/2+
36Ar	18	18	35.967545106(29)	STABLE	0+	0.003365(30)
37Ar	18	19	36.96677632(22)	35.04(4) d	3/2+
38Ar	18	20	37.9627324(4)	STABLE	0+	0.000632(5)
39Ar	18	21	38.964313(5)	269(3) a	7/2-
40Ar	18	22	39.9623831225(29)	STABLE	0+	0.996003(30)
41Ar	18	23	40.9645006(4)	109.61(4) min	7/2-
42Ar	18	24	41.963046(6)	32.9(11) a	0+
43Ar	18	25	42.965636(6)	5.37(6) min	(5/2-)
44Ar	18	26	43.9649240(17)	11.87(5) min	0+
45Ar	18	27	44.9680400(6)	21.48(15) s	(1/2,3/2,5/2)-
46Ar	18	28	45.96809(4)	8.4(6) s	0+
47Ar	18	29	46.97219(11)	1.23(3) s	3/2-#
48Ar	18	30	47.97454(32)#	0.48(40) s	0+
49Ar	18	31	48.98052(54)#	170(50) ms	3/2-#
50Ar	18	32	49.98443(75)#	85(30) ms	0+
51Ar	18	33	50.99163(75)#	60# ms [>200 ns]	3/2-#
52Ar	18	34	51.99678(97)#	10# ms	0+
53Ar	18	35	53.00494(107)#	3# ms	(5/2-)#

Notes

• The isotopic composition refers to that in air.
• Geologically exceptional samples are known in which the isotopic composition lies outside the reported range. The uncertainty in the atomic mass may exceed the stated value for such specimens.
• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.

References

• Isotope masses from Ame2003 Atomic Mass Evaluation by G. Audi, A.H. Wapstra, C. Thibault, J. Blachot and O. Bersillon in Nuclear Physics A729 (2003).
• Isotopic compositions and standard atomic masses from Atomic weights of the elements. Review 2000 (IUPAC Technical Report). Pure Appl. Chem. Vol. 75, No. 6, pp. 683-800, (2003) and Atomic Weights Revised (2005).
• Half-life, spin, and isomer data selected from these sources. Editing notes on this article's talk page.
  • Audi, Bersillon, Blachot, Wapstra. The Nubase2003 evaluation of nuclear and decay properties, Nuc. Phys. A 729, pp. 3-128 (2003).
  • National Nuclear Data Center, Brookhaven National Laboratory. Information extracted from the NuDat 2.1 database (retrieved Sept. 2005).
  • David R. Lide (ed.), Norman E. Holden in CRC Handbook of Chemistry and Physics, 85th Edition, online version. CRC Press. Boca Raton, Florida (2005). Section 11, Table of the Isotopes.

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External links

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