Promethium (pro-mee- thee-əm) is a chemical element with the symbol "Pm" and atomic number 61. It is notable for being the only exclusively radioactive element besides technetium that is followed by chemical elements with stable isotopes.
The existence of promethium was first predicted by Bohuslav Brauner in 1902. During his research on the chemical properties of rare earth elements he found that the difference between neodymium and samarium is larger than between the other lanthanides. This prediction was supported in 1914 by Henry Moseley who, having discovered that atomic number was an experimentally measurable property of elements, found that no known element had atomic number 61.With the knowledge of a gap in the periodic table several groups started to search for the predicted element among other rare earths in the natural environment.
Promethium was first produced and characterized at Oak Ridge National Laboratory (ORNL) in 1945 by Jacob A. Marinsky, Lawrence E. Glendenin and Charles D. Coryell by separation and analysis of the fission products of uranium fuel irradiated in the Graphite Reactor; however, being too busy with military-related research during World War II, they did not announce their discovery until 1947.
Promethium can be formed in nature as a product of spontaneous fission of uranium- 238 and alpha decay of europium-151. Only trace amounts can be found in naturally occurring ores: a sample of pitchblende has been found to contain promethium at a concentration of four parts per quintillion (1018) by mass.It was calculated that the equilibrium mass of promethium in the earth's crust is about 560g due to uranium fission and about 12g due to the recently observed alpha decay of europium-151.
Promethium is a lanthanide metal whose chemical properties mostly resemble those of other lanthanides. The melting point of the element was claimed to have been calculated to be around 1042 °C (1908 °F, 1315 K), well suiting in the general trend of its increase with atomic numbers in lanthanides. Pure promethium exists in two allotropic forms, and its chemistry is similar to other lanthanides. Promethium salts luminesce in the dark with a pale blue or greenish glow, due to their high radioactivity.
Promethium metals tarnish slowly in air and burns readily at 150 °C to form promethium oxide:
4 Pm + 3 O2 → 2 Pm2O3
Promethium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form promethium hydroxide:
2 Pm (s) + 6 H2O (l) → 2 Pm(OH)3 (aq) + 3 H2 (g)
Uses for promethium include:
As a beta radiation source for thickness gauges.
As a light source for signals that require reliable, independent operation (using phosphor to absorb the beta radiation and produce light). In particular, Promethium(III) chloride (PmCl3) mixed with zinc sulfide (ZnS) was used for a time as a major luminous paint for watches after radium was discontinued. This mixture is still occasionally used for some luminous paint applications (though most such uses requiring radioactive materials have switched to tritium for safety reasons).
In an atomic battery in which cells convert the beta emissions into electric current, yielding a useful life of about five years, using Pm-147.
Promethium compounds include:
PmBr3 (coral - red)
PmF3 (purple - pink)
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