Tin-126 is a radioisotope of tin and one of only seven long-lived fission products. For example, its yield from U-235 is 0.0007% per thermal fission and 0.002% per fast fission. Fast fission or fission of some heavier actinides will produce 121mSn at higher yields. In a normal thermal reactor, it has a very low fission product yield thus, this isotope is not a significant contributor to nuclear waste. Tin-121m is a radioisotope and nuclear isomer of tin with a half-life of 43.9 years. One of its uses is in a particulate suspension to treat canine synovitis (radiosynoviorthesis). ^ Believed to undergo β −β − decay to 124Te with a half-life over 100×10 15 years.^ Believed to undergo β −β − decay to 122Te.^ a b c d e f g Theoretically capable of spontaneous fission.^ Heaviest known nuclide with more protons than neutrons.^ ( ) spin value – Indicates spin with weak assignment arguments.^ Bold symbol as daughter – Daughter product is stable.^ a b c # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).^ ( ) – Uncertainty (1 σ) is given in concise form in parentheses after the corresponding last digits.

The other 28 radioisotopes have half-lives less than a year. The longest-lived radioisotope is 126Sn, with a half-life of 230,000 years. Twenty-nine additional unstable isotopes are known, including the ' doubly magic' tin-100 ( 100Sn) (discovered in 1994) and tin-132 ( 132Sn). Tin ( 50Sn) 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.

Element sn protons neutrons electrons