Last time we discussed technetium, and now we shall discuss the only other element with Z < 82 with no stable isotope, promethium (Z = 61). But there is more business than just that, and it has to do with a suggestion that commenter Wreck Smurfy‘s suggestion that I use actual hyperlinks to key terms rather than just bolding them. There shall be more about that later.
Promethium is actually not as interesting as technetium, but still has its moments. It has a storied tale of claimed discoveries, and one of my personal interests is the history of chemistry, in particular infighting by contributors. I got into one of those contests myself back in the day, when I supported a particular geometry for the lowest triplet excited state for cyclohexen-2-one, but that is another story altogether.
Promethium, chemical symbol Pm, is a member of the lanthanide series, and those are often called the rare earth elements. They are not all that rare, at least several of them, but their chemistry is such that they were extremely difficult to separate and purify until modern ion exchange chromatographic methods were developed after World War II, many of those techniques outgrowths of classified work during the Manhattan Project.