- Promethium (Pm) is a rare, silvery metal with atomic number 61, belonging to the lanthanide series of the periodic table.
- It is unique among the lanthanides because it has no stable isotopes—all of its isotopes are radioactive. Its electron configuration is [Xe] 4f⁵ 6s², and it typically forms the Pm³⁺ ion as its most stable oxidation state. The atomic structure contains sixty-one protons, a variable number of neutrons depending on the isotope, and sixty-one electrons arranged in six shells.
- The most stable isotope, promethium-145 (¹⁴⁵Pm), has a half-life of about 17.7 years, decaying to stable samarium-145 via electron capture. Other isotopes, such as promethium-147 (¹⁴⁷Pm), are also notable—¹⁴⁷Pm has a half-life of 2.62 years and is widely used in practical applications.
- Promethium does not occur naturally in significant quantities on Earth because its isotopes are unstable and decay relatively quickly on a geological timescale. Trace amounts can be found in uranium ores as a result of spontaneous fission of uranium-238, and it can also form in nature from the decay of europium-151. However, almost all promethium used today is produced artificially in nuclear reactors, typically by neutron irradiation of neodymium-146 (¹⁴⁶Nd), which captures a neutron to form neodymium-147, then undergoes beta decay to promethium-147.
- The existence of an element between neodymium and samarium was first suggested by gaps in the periodic table, but early claims of discovery in the early 20th century proved incorrect. Promethium was conclusively identified in 1945 by American chemists Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell at the Oak Ridge National Laboratory, while they were analyzing fission products from uranium fuel used in the Manhattan Project. The element’s name comes from Prometheus, the Titan of Greek mythology who stole fire from the gods, symbolizing both the element’s radioactive energy and its man-made origin.
- Promethium has limited but valuable applications, mainly due to its radioactivity. Its beta radiation can be used in nuclear batteries (betavoltaic devices) to power spacecraft instruments, remote weather stations, and other devices where long-lasting, maintenance-free energy sources are needed. ¹⁴⁷Pm is also used in luminous paints for instrument dials, signs, and watches—though this use has declined in favor of safer alternatives. Additionally, it serves as a radiation source for thickness gauges and other industrial measuring instruments.
- Chemically, promethium behaves much like other trivalent lanthanides, readily forming ionic compounds in the +3 oxidation state. It tarnishes slowly in air, forming a thin oxide layer, and reacts with acids to form soluble salts.
- Biologically, promethium has no known essential role in living organisms, and due to its radioactivity, it is considered hazardous. Exposure can lead to beta radiation damage, particularly if inhaled or ingested, so strict safety protocols are required in its handling.
- From an environmental perspective, promethium is not naturally abundant enough to pose a large-scale ecological hazard. However, improper disposal of radioactive promethium from industrial or research uses could present localized contamination risks.
