Promethium is a silver-white metal that tarnishes slowly in air to a pink tint and burns readily in air at 150 °C. No stable isotopes of the element exist and 38 radioisotopes have been identified. The original isotope discovered was Promethium-147 which has a half-life of 2.62 years, and is a uranium fission product from nuclear reactors via the beta decay of neodymium-147. Promethium-145 has the longest half-life at 17.7 years. The metal has a double hexagonal close-packed structure, a density of 7.22 g/cm3, a melting point of 1168 °C, and an estimated boiling point of 2460 °C (Wheelwright, 1969). Promethium oxide, or promethia, is a pale pink color and occurs as a sesquioxide with the formula Pm2O3. Dependant on temperature, the oxide occurs in three crystal systems. At ambient temperatures, promethium oxide occurs in cubic form with a specific gravity of 6.85 g/cm3, above 750-800 °C it occurs in monoclinic form with a specific gravity of 7.48 g/cm3, and above 1740 °C it occurs in hexagonal form with a specific gravity of 7.62 g/cm3.
Promethium does not occur in sufficient quantities in the Earth‘s crust to be economically mined.
