Talk:Bismuth/@comment-29013044-20160828203706/@comment-24468230-20160904175535

Power Ed, a "half-life" is how long it takes half of a radioactive substance to decay. So if something has a half-life of ten years, it takes ten years for half of it to decay, twenty years for three-fourths to decay, thirty years for seven-eighths to decay, etc.

The estimates of bismuth-209's half-life must have been based on noticing a couple molecules of the stuff were missing after several years of observation.