Promethium (Pm), atomic number 61, is an extremely rare metallic element and a member of the lanthanide series, often called rare earth metals. Unlike its neighbors, Promethium is radioactive, as all its isotopes are unstable. It is also one of only two elements below lead (atomic number 82) that is essentially synthetic, meaning it is not found in substantial, lasting quantities in nature.
Defining Promethium’s Identity
Chemically, Promethium fits into the lanthanide series, sitting between neodymium and samarium. Its presence was predicted long before its actual discovery when scientists noticed the missing element that should occupy the 61st spot. Physically, Promethium is expected to be a silvery-white, relatively soft metal, though its highly radioactive nature makes studying its pure metallic form challenging.
Like most lanthanides, Promethium predominantly forms a +3 oxidation state in its chemical compounds. In this common form, its salts and solutions often exhibit a characteristic pink or rose-tinged color. The chemical similarity to its neighbors, particularly neodymium and samarium, makes separating Promethium from other elements a difficult task.
The first conclusive identification of Promethium occurred in 1945 at the Oak Ridge National Laboratory, isolated from the products of uranium fission. This confirmed the existence of the final undiscovered element in the rare earth series. Its name is derived from the Greek Titan Prometheus, who stole fire from the gods.
The Element’s Radioactive Instability
Promethium is one of the only elements below atomic number 83 that has no stable isotopes. This lack of stability means any Promethium existing at the Earth’s formation has long since decayed, making it extremely rare in the crust. Trace amounts are found fleetingly in nature, primarily resulting from the spontaneous fission of uranium-238 and the alpha decay of europium-151.
For practical use, Promethium must be produced synthetically; the primary source is the waste stream from nuclear power reactors. It is separated from other materials after being created as a fission product in nuclear fuel, typically from the decay chain of uranium. The most commonly utilized isotope is Promethium-147 (\(\text{Pm}^{147}\)), which has a half-life of 2.62 years.
Promethium-147 decays through beta emission, where a neutron converts into a proton, emitting a high-energy electron (a beta particle). This decay transforms Promethium-147 into stable Samarium-147 (\(\text{Sm}^{147}\)). The beta particles emitted by \(\text{Pm}^{147}\) are considered “soft” because they are low-energy and do not produce gamma radiation. While the beta particles can be easily shielded, their interaction with high atomic number materials can generate secondary X-rays, necessitating careful handling.
Practical Applications and Utilization
The radioactive properties of Promethium-147 make it valuable for several high-tech applications. Its soft beta emission is leveraged in radioluminescent paint, a self-sustaining light source. The beta particles strike a phosphor material, exciting it to glow without needing external power. This paint was historically used to replace the more hazardous radium in watch dials and instrument panels, though its short half-life limits long-term suitability.
The heat and energy generated by the radioactive decay are harnessed in miniature power sources known as betavoltaic or nuclear batteries. These devices convert the energy from the emitted beta particles directly into electrical current. Such compact, long-lasting power sources have been used in pacemakers, remote sensing equipment, and components for guided missiles.
Beyond light and power, Promethium-147 finds use in industrial gauges for measuring the thickness of materials. The beta-ray backscatter technique relies on the consistent emission of beta particles to accurately determine the thickness of a substance. Research continues into potential medical applications, including its use in radioactive imaging and as a component in cancer treatments.