Promethium (Pm)
Promethium (Pm) is a metallic chemical element with the atomic number 61, classified among the rare earth elements in the lanthanide series of the periodic table. It was predicted to exist in 1902 and was successfully isolated in 1945 by scientists at Oak Ridge National Laboratory, marking a significant milestone in the study of rare earth elements. Promethium is characterized by its silvery metallic appearance and has no stable isotopes; its most stable isotopes, promethium-145 and promethium-147, have half-lives of 17.7 years and 2.62 years, respectively.
Due to its short half-lives, promethium does not occur naturally in significant amounts on Earth and is primarily produced from uranium ores. In addition to its role in research, promethium-147 has practical applications, notably in atomic batteries that power devices such as pacemakers and radios. The element's luminescent properties also allow it to be used in paints for traffic signals and road signs. However, due to its radioactive nature, handling promethium requires strict safety precautions to mitigate its volatile and toxic characteristics. With ongoing research, promethium holds potential for future applications in portable x-ray sources and thermal generators.
Subject Terms
Promethium (Pm)
- Element Symbol: Pm
- Atomic Number: 61
- Atomic Mass: 145
- Group # in Periodic Table: n/a
- Group Name: Lanthanides
- Period in Periodic Table: 6
- Block of Periodic Table: f-block
- Discovered by: Jacob A. Marinsky, Lawrence E. Glendenin, Charles D. Coryell (1945)
Promethium is a metallic chemical element in the periodic table. It is a rare earth element, belonging to the lanthanide group of elements, which includes fourteen other rare earth elements, such as samarium and neodymium. Rare earth elements, also known as rare earth metals, typically occur together in nature and are oftentimes difficult to separate.
In 1902 the Czech chemist Bohuslav Brauner predicted the existence of a then-unknown element between the elements neodymium and samarium in the periodic table. In 1914 Henry Moseley, an English physicist, supported Brauner’s claims, and the search for element 61 began. Over the years many experiments were performed in hopes of identifying the mysterious element; during this time there were also many claims of discovery. In 1926 groups of scientists in both Italy and the United States claimed to have discovered element 61, but upon further investigation, both claims were invalidated. A group of scientists at Ohio State University came close to discovering the elusive element in 1938 when the team members isolated small radioactive traces as the result of a nuclear experiment; however, this claim was not widely supported due to the lack of substantial evidence. Scientists Jacob A. Marinsky, Charles D. Coryell, and Lawrence E. Glendenin finally confirmed Brauner’s prediction of the existence of element 61 in 1945 at the Oak Ridge National Laboratory in Tennessee. Using ion exchange techniques, Marinsky, Coryell, and Glendenin separated the element from uranium fuel in a nuclear reactor. They named the element promethium. A metallic form of promethium was not produced until 1963.
Physical Properties
Promethium has a silvery metallic color. At 298 kelvins (K), promethium’s standard state is solid, and it has a density of 7.26 grams per cubic centimeter (g/cm3). The melting point of promethium is 1042 degrees Celsius (°C), and its boiling point is 3000 °C. The specific heat of promethium is unknown. Due to the element’s rarity, many of its physical properties are a mystery. These unknown properties are surmised based on those of similar elements that are close to it on the periodic table.
Promethium has an electrical conductivity of 1.3 × 106 siemens per meter (S/m) and a thermal conductivity of 15 watts per meter-kelvin (W/m·K). Promethium is paramagnetic, which means that it is magnetized when placed in a magnetic field but does not retain this property upon removal from this field.
Chemical Properties
The most common oxidation state of promethium is +3. Promethium salts usually have a pink or red hue that causes the surrounding oxygen to glow a blue or green color. Promethium has a double hexagonal, close-packed crystal structure, but it undergoes a crystal transformation at a temperature of 890 °C, at which point it has a body-centric cubic structure.
There are no naturally occurring isotopes of promethium, and all of its synthesized isotopes are unstable and radioactive. Promethium is one of only two elements with mass numbers less than 83 to have no stable isotopes, and it is the least stable of these first eighty-three elements. Of the seventeen known promethium isotopes, the most stable are promethium-145, with a half-life of 17.7 years, and promethium-147, with a half-life of 2.62 years. Unstable promethium isotopes with mass numbers less than 145 (promethium’s most stable isotope) typically undergo electron capture, a form of radioactivity in which one of the atom’s electrons combines with a proton to form a neutron and a neutrino; in promethium, this process usually results in neodymium isotopes. Heavier promethium isotopes, generally those with mass numbers greater than 147, undergo beta decay, another form of radioactivity, in which a proton is converted into a neutron (or vice versa) inside the nucleus of an atom. When the atom goes through this process, energy is released in the form of a beta particle (a fast-moving electron or positron). In promethium, this usually produces isotopes of samarium.
Applications
Due to the short half-lives of all of promethium’s isotopes, any naturally occurring promethium that may have existed on Earth would have disappeared long ago, and therefore the element is not found in the planet’s crust. Promethium is likewise not found freely in nature, but it is present in some uranium ores and is most commonly produced from uranium isotopes. By bombarding uranium-235 with thermal neutrons, significant amounts of promethium can be produced. Most of the world’s promethium is produced in Russia. Additionally, promethium has been discovered in the spectrums of various stars, including HR 465. Since all isotopes of the element have short half-lives, these traces of promethium must have been produced relatively recently, close to the surfaces of the stars, although scientists are thus far unsure of how this occurred.
Promethium is considered to be both highly volatile and highly toxic due to the radioactive nature of all of the element’s isotopes. Any handling of the element must be done with extreme care, so safety goggles, protective gloves, and other safety precautions are called for. Due to its highly radioactive nature, promethium is used primarily in research; however, because the most versatile and useful isotope of the element is promethium-147, it has other, more practical applications. Promethium-147 is primarily used in atomic batteries, which are found in devices such as pacemakers, radios, and missiles. Additionally, due to the luminescent properties of the element, which cause it to glow upon excitation, promethium is used in luminescent paint found in traffic signals and other road signs. In the future promethium also may be used as a portable source of x-rays, a thermal generator, or a power source.
Bibliography
Atwood, David A., ed. The Rare Earth Elements: Fundamentals and Applications. Hoboken: Wiley, 2012. Print.
"Facts about Promethium." LiveScience. Purch, 11 July 2013. Web. 10 Sept. 2015.
Haynes, William M., ed. CRC Handbook of Chemistry and Physics. 95th ed. Boca Raton: CRC, 2014. Print.
Lucas, Jacques, et al. Rare Earths: Science, Technology, Production and Use. Waltham: Elsevier, 2015. Print.
"Technical Data for Promethium." The Photographic Periodic Table of the Elements. Element Collection, n.d. Web. 10 Sept. 2015.