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Ok so promethium is actually my least favourite lanthanide because I
can’t actually work with it. It’s highly radioactive and at any one time on
the earth’s surface there is not an awful lot of it so you can’t physically
get enough of it to do any chemistry with.
So promethium is the only one of the rare earth elements that is
radioactive, but because it’s radioactive and the atoms decay away it’s
not found in nature. So it was only after the start of nuclear research
that people discovered promethium, even some of the products from the
decay from other radioactive elements. And you can see this quite well:
this is an old chemistry book that I’ve got, written by Aston, the famous
scientist who discovered isotopes – atoms of the same element that have
different weights - and in this book which he published in 1922…
Hold it a bit more vertical. Ok go ahead.
…he had a periodic table and if you look down here, here are the rare
earths and in this position here there is a space for element 61,
promethium. So element 61 wasn’t known in 1922 and it was found
much later. But you can see the rest of the elements were, except
there’s a gap down here under tungsten because, again, the element
rhenium was only discovered relatively recently compared to most of the
other elements.