What is the highest number of protons that a nucleus can have and still be considered stable?
There are 83 protons in the nucleus of bismuth-209 (Bi-209), and for the longest time, it was thought to be the heaviest element with at least one stable isotope. But it turns out that is it actually an alpha emitter with a half-life of about 19 x 10-18 years. As that is an extraordinarily long time, no wonder it wasn't discovered. But given such a long half-life, a lot of folks still consider it to be stable. But since it has been revealed that bismuth is unstable (though on a timescale longer than geologic), that leaves lead, with its 82 protons, as the heaviest element with a stable isotope.