Naturally occurring xenon is made of eight stable isotopes , the most of any element with the exception of tin , which have ten . Xenon and tin are the only elements to have more than seven stable isotopes . The isotopes 124Xe and 134Xe are predicted to undergo double beta decay , but this have never been observed so they are considered to be stable . Besides these stable forms , there are over 40 unstable isotopes that have been studied . The longest lived of these isotopes is 136Xe , which have been observed to undergo double beta decay with a half @-@ life of 2 @.@ 11 × 1021 yr . 129Xe is produced by beta decay of 129I , which have a half @-@ life of 16 million years , while 131mXe , 133Xe , 133mXe , and 135Xe are some of the fission products of both 235U and 239Pu , and therefore used as indicators of nuclear explosions .