Promethium is a rare earth element that is classified as a lanthanide. It was discovered in 1945 by scientists at the Oak Ridge National Laboratory in Tennessee, United States. Promethium is highly radioactive and has no stable isotopes. Its most stable isotope, promethium-145, has a half-life of only 17.7 years.

Although promethium has no significant commercial uses due to its rarity and high cost, it has niche applications in nuclear batteries and portable X-ray machines. Its radioisotopes can be used in pacemakers, thickness gauges, and beta radiation sources. Promethium also has potential uses in cancer treatment, such as in brachytherapy, where small pellets containing radioactive isotopes are implanted into cancerous tissues to destroy them.

Promethium is typically produced artificially by neutron activation of a naturally occurring isotope in nuclear reactors. However, this process remains expensive and its environmental impact is a concern. As a result, researchers are seeking more efficient and sustainable ways to produce promethium and other rare earth elements.