Topic summary
Observationally stable
Extracted from the Wikipedia article Stable isotope.
Still-unobserved decay
Isotopes that are theoretically believed to be unstable but have not been observed to decay are termed observationally stable. Currently there are 105 "stable" isotopes which are theoretically unstable, 40 of which have been observed in detail with no sign of decay, the lightest in any case being Ar. Many "stable" nuclides are metastable but have not been observed to decay, and are expected to undergo very rare kinds of radioactive decay, including double beta decay.