Promethium: the essentials
Great care is required while handling promethium as a consequence of its radioactivity. Promethium salts luminesce in the dark with a pale blue or greenish glow, due to their high radioactivity. Ion-exchange methods led to the preparation of about 10 g of promethium from atomic reactor fuel processing wastes in early 1963.
Little is yet generally known about the properties of metallic promethium. More than 30 promethium compounds have been prepared. Promethium is a rare earth metal. It appears that there is no known Pm existing in the earth's crust.
Promethium: historical information
Earlt claims to the discovery of promethium date back to 1924 but these appear have been substantiated. A group at Ohio State University (USA) claimed element 61 in experiments involving its synthesis in a cyclotron, but again the evidence did not satisfy everyone. In 1947, Marinsky, Glendenin, and Coryell at Oak Ridge, Tennessee, USA, made the first chemical identification of promethium by use of ion-exchange chromatography on residues in a nuclear reactor.
Promethium: physical properties
Promethium: orbital properties
Isolation: promethium metal is available commercially so it is not normally necessary to make it in the laboratory, which is just as well as it is difficult to isolate as the pure metal. This is largely because of the way it is found in nature. The lanthanoids are found in nature in a number of minerals. The most important are xenotime, monazite, and bastnaesite. The first two are orthophosphate minerals LnPO4 (Ln deonotes a mixture of all the lanthanoids except promethium which is vanishingly rare) and the third is a fluoride carbonate LnCO3F. Lanthanoids with even atomic numbers are more common. The most comon lanthanoids in these minerals are, in order, cerium, lanthanum, neodymium, and praseodymium. Monazite also contains thorium and ytrrium which makes handling difficult since thorium and its decomposition products are radioactive.
For many purposes it is not particularly necessary to separate the metals, but if separation into individual metals is required, the process is complex. Initially, the metals are extracted as salts from the ores by extraction with sulphuric acid (H2SO4), hydrochloric acid (HCl), and sodium hydroxide (NaOH). Modern purification techniques for these lanthanoid salt mixtures are ingenious and involve selective complexation techniques, solvent extractions, and ion exchange chromatography.
Pure promethium is available through the reduction of PmF3 with calcium metal.
2PmF3 + 3Ca → 2Pm + 3CaF2
This would work for the other calcium halides as well but the product CaF2 is easier to handle under the reaction conditions (heat to 50°C above the melting point of the element in an argon atmosphere). Excess calcium is removed from the reaction mixture under vacuum.
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