observationally stable

English

Bismuth-209 was observationally stable; but it was found to decay, albeit so slowly that it is for all practical purposes stable. No element beyond bismuth has a known stable isotope.

Adjective

observationally stable (not comparable)

1. Of a nuclide (isotope): theoretically capable of radioactive decay (on the grounds that it would be energy-profitable), but not having been observed to do so.