What is the highest number of protons that a nucleus can have and still be considered stable?

Answer 1

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.

Answer 2

The largest atomic nucleus is for cesium-133 (aka caesium-133).

The heaviest stable element is lead-207 (207Pb).

Previously it was believed that bismuth-209 was stable, when in fact it is just meta-stable with a half life of 2 billion-billion years.

Answer 3

We don't know yet as we're still creating newer, heavier elements all the time. Currently, the most protons that we've seen in a nucleus is 118, although the element produced is highly unstable. The maximum amount of protons that can be in a stable nucleus used to be thought to be 83, corresponding to bismuth, but it's since been found to decay. Therefore, the honor now goes to lead, element 82.

Answer 4

The highest number of protons that a nucleus can have and still be considered stable is 83, which is the element bismuth. Nuclei with more than 83 protons tend to be unstable and undergo radioactive decay.