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% 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