I know this is nitpicking, but the definition of a “stable” vs. a “radioactive” isotope seems a little silly to me.
The usual definition of a stable nuclide (or isotope) is one that humans have not observed to decay. A radioactive nuclide is one that humans have observed to decay. A stable element is one that has at least one stable isotope.
We pretty much know from theory exactly which nuclides eventually decay, and how. For example, we know that argon-36 decays by double electron capture, and I bet we can even make a rough estimate of its half life (like, “ten to the thirty-something years”). But it is stable, because we have never observed it to decay.
The last remaining stable isotope of bismuth, bismuth-209, was observed to decay in 2003, with a half life estimated at 1.8×1019 years. Since then, the element bismuth has been radioactive.
I stress that I don’t mean it was radioactive all along, and was only discovered to be radioactive in 2003. I mean that bismuth was truly, literally, factually not radioactive until 2003.
I’m sure I’ve seen it written that bismuth was “discovered to be radioactive” in 2003, with the writer implying that it was in fact radioactive all along. But this is inconsistent with the standard terminology. Either the terminology is bad, or the writer is making a subtle error.