High Quality Content by WIKIPEDIA articles! High Quality Content by WIKIPEDIA articles! Stable isotopes are chemical isotopes that are not radioactive (they have not been observed to decay, though a few of them may be theoretically unstable with exceedingly long half-lives). By this definition, there are 256 known stable isotopes of the 80 elements, which have one or more stable nuclides. A list of these is given at the end of this article. About two thirds of the elements have more than one stable isotope. One element (tin) has ten stable isotopes.