Ar and 38
Ar content may be as high as 2.07% and 4.3% respectively in natural samples. 40
Ar is the remainder in such cases, whose content may be as low as 93.6%.
|Standard atomic weight Ar, standard(Ar)|
Argon (18Ar) has 26 known isotopes, from 29Ar 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 29Ar with a half-life of approximately 4×10−20 seconds.
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
|Z||N||Isotopic mass (u)
[n 2][n 3]
[n 6][n 7]
|Natural abundance (mole fraction)|
|Excitation energy||Normal proportion||Range of variation|
|31Ar||18||13||31.01216(22)#||15.1(3) ms||β+, p (68.3%)||30S||5/2+|
|β+, 2p (9.0%)||29P|
|β+, 3p (0.07%)||28Si|
|32Ar||18||14||31.9976378(19)||98(2) ms||β+ (64.42%)||32Cl||0+|
|β+, p (35.58%)||31S|
|33Ar||18||15||32.9899255(4)||173.0(20) ms||β+ (61.3%)||33Cl||1/2+|
|β+, p (38.7%)||32S|
|36Ar||18||18||35.967545105(29)||Observationally Stable[n 8]||0+||0.003336(4)|
|39Ar[n 9]||18||21||38.964313(5)||269(3) y||β−||39K||7/2−||Trace[n 10]|
|40Ar[n 11]||18||22||39.9623831238(24)||Stable||0+||0.996035(4)[n 12]|
|47Ar||18||29||46.9727681(12)||1.23(3) s||β− (99.8%)||47K||(3/2−)|
|β−, n (0.2%)||46K|
|51Ar||18||33||50.99280(64)#||60# ms [>200 ns]||β−||51K||3/2−#|
- mAr – Excited nuclear isomer.
- ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
- # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
Modes of decay:
n: Neutron emission p: Proton emission
- Bold symbol as daughter – Daughter product is stable.
- ( ) spin value – Indicates spin with weak assignment arguments.
- # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
- 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.
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- Half-life, decay mode, nuclear spin, and isotopic composition is sourced in:
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