What is it?
The one rare earth with no stable form at all — every atom of it is radioactive and decays away, so essentially none exists naturally on Earth.
Why does it matter?
It completes the row, and it explains why the rare-earth series has a gap: promethium has to be made in a reactor, not mined.
Deeper written sections for this material have not been published yet. Everything below is drawn straight from the source datasets and is complete.
Read the numbers correctly.
Not mined; produced in nuclear reactors in gram quantities.
Reactor-produced isotope for nuclear batteries and thickness gauges.