High Quality Content by WIKIPEDIA articles! Tin (Sn) is the element with the greatest number of stable isotopes (ten) (three of them are potentially radioactive but have not been observed to decay), which is probably related to the fact that 50 is a "magic number" of protons. 29 additional unstable isotopes are known, including the "doubly magic" tin-100 (100Sn) (discovered in 1994) and tin-132 (132Sn). The longest-lived radioisotope is 126Sn with a half-life of 230,000 years. All other radioisotopes have half-lives less than a year.