Potassium has 26 known isotopes from ³¹ K to ⁵⁷ K, with the exception of still-unknown ³² K, as well as an unconfirmed report of ⁵⁹ K. Three of those isotopes occur naturally: the two stable forms ³⁹ K and ⁴¹ K, and a very long-lived radioisotope... Wikipedia