Isotopes of argon
|Standard atomic weight (Ar, standard)|
Argon (18Ar) has 25 known isotopes, from 30Ar to 54Ar and 1 isomer (32mAr), of which three 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 of 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 by electron capture (10.72%) and by positron emission (0.001%), and also transforms to stable 40Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.
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 40K 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.
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. 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. Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon. In December 2013, 36Ar, in the form of argon hydride, was found in cosmic dust associated with the Crab Nebula supernova. This was the first time a noble molecule was detected in outer space.
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.
List of isotopes
This article needs to be updated.(July 2018)
isotopic mass (u)
(mole fraction)[n 2]
|range of natural|
|31Ar||18||13||31.01212(22)#||14.4(6) ms||β+, p (55.0%)||30S||5/2(+#)|
|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|
|33Ar||18||15||32.9899257(5)||173.0(20) ms||β+ (61.35%)||33Cl||1/2+|
|β+, p (38.65%)||32S|
|36Ar||18||18||35.967545106(29)||Observationally Stable[n 3]||0+||0.003336(4)|
|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]|
|47Ar||18||29||46.97219(11)||1.23(3) s||β− (99%)||47K||3/2−#|
|β−, n (1%)||46K|
|51Ar||18||33||50.99163(75)#||60# ms [>200 ns]||β−||51K||3/2−#|
- Bold for stable isotopes
- 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.
- Believed to undergo double electron capture to 36S (lightest theoretically unstable nuclide for which no evidence of radioactivity has been observed)
- Used in argon–argon dating
- Cosmogenic nuclide
- Used in argon–argon dating and potassium–argon dating
- Generated from 40K in rocks. These ratios are terrestrial. Cosmic abundance is far less than 36Ar.
- 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 (CIAAW).
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- Isotope masses from:
- Isotopic compositions and standard atomic masses from:
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