Argon (18Ar) has 26 known isotopes, from Ar to Ar, of which three are stable (Ar, Ar, and Ar). On Earth, Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are Ar with a half-life of 302 years, Ar with a half-life of 32.9 years, and Ar with a half-life of 35.01 days. All other isotopes have half-lives of less than two hours, and most less than one minute. Isotopes lighter than Ar decay to chlorine or lighter elements, while heavier ones beta decay to potassium.
The naturally occurring K, with a half-life of 1.248×10 years, decays to stable Ar by electron capture (10.72%) and by positron emission (0.001%), and also to stable Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.
