Isotopes of Argon

Isotopes Of Argon

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.

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% 40Ar and mostly (85%) 36Ar.

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.

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.

Standard argon atomic mass: 39.948(1) u.