Potassium has 26 known isotopes from 31K to 57K, with the exception of still-unknown 32K, as well as an unconfirmed report of 59K. Three of those isotopes occur naturally: the two stable forms 39K and 41K, and a very long-lived radioisotope 40K Naturally occurring radioactive 40K decays with a half-life of 1.248×109 years. Wikipedia