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Title: Argon-36  
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Subject: Stable nuclide, Beta-decay stable isobars
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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, 40Ar.[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, 40Ar.[4]

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.


Z(p) N(n)  
isotopic mass (u)
half-life decay
isotope(s)[n 1]
(mole fraction)[n 2]
range of natural
(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+ 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-#
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
  • 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

Template:World Heritage Encyclopedia-Books


  • Isotope masses from:
  • Isotopic compositions and standard atomic masses from:
  • Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.

External links

Isotopes of chlorine Isotopes of argon Isotopes of potassium
Table of nuclides
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