Isotopes of argon

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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 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.[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] 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[edit]

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)
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
  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[edit]

See also[edit]


References[edit]

  1. ^ a b "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. 
  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. ^ a b Quenqua, Douglas (13 December 2013). "Noble Molecules Found in Space". New York Times. Retrieved 13 December 2013. 
  6. ^ a b 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. 
  7. ^ http://www.nucleonica.net/unc.aspx

External links[edit]


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