Promethium

Promethium is the only rare earth with no stable isotopes: every atom of it eventually decays. Chemists suspected a missing element here for decades, but it wasn't confirmed until 1945, when Jacob Marinsky, Lawrence Glendenin and Charles Coryell found it among the leftovers of uranium fission at Oak Ridge. It is named after Prometheus, who stole fire from the gods.

Traces form naturally in uranium ores, but nearly all promethium is made in nuclear reactors. Promethium-147 gives off weak radiation that was used in glow-in-the-dark watch dials and in small nuclear batteries for pacemakers and other devices that needed years of power. Its longest-lived form, promethium-145, has a half-life of about 18 years.