There are 37 known isotopes of iodine from ¹⁰⁸I to ¹⁴⁴I; all undergo radioactive decay except ¹²⁷I, which is stable. Iodine is thus a monoisotopic element. Its longest-lived radioactive isotope, ¹²⁹I, has a half-life of 15.7 million years, which... Wikipedia