Promethium Information
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Promethium Element
Promethium is a chemical
element with the symbol Pm and atomic number 61. All of its isotopes are
radioactive; it is extremely rare, with only about 500–600 grams naturally
occurring in Earth's crust at any given time. Promethium is one of only two radioactive
elements that are followed in the periodic table by elements with stable forms,
the other being technetium. Chemically, promethium is a lanthanide. Promethium
shows only one stable oxidation state of +3.
In 1902 Bohuslav Brauner
suggested that there was a then-unknown element with properties intermediate
between those of the known elements neodymium (60) and samarium (62); this was
confirmed in 1914 by Henry Moseley, who, having measured the atomic numbers of
all the elements then known, found that atomic number 61 was missing. In 1926,
two groups (one Italian and one American) claimed to have isolated a sample of
element 61; both "discoveries" were soon proven to be false. In 1938,
during a nuclear experiment conducted at Ohio State University, a few
radioactive nuclides were produced that certainly were not radioisotopes of
neodymium or samarium, but there was a lack of chemical proof that element 61
was produced, and the discovery was not generally recognized. Promethium was
first produced and characterized at Oak Ridge National Laboratory in 1945 by
the separation and analysis of the fission products of uranium fuel irradiated
in a graphite reactor. The discoverers proposed the name "prometheum"
(the spelling was subsequently changed), derived from Prometheus, the Titan in
Greek mythology who stole fire from Mount Olympus and brought it down to
humans, to symbolize "both the daring and the possible misuse of mankind's
intellect". However, a sample of the metal was made only in 1963.
Conclusive chemical proof of
the existence of promethium, the last of the rare-earth elements to be
discovered, was obtained in 1945 (but not announced until 1947) by American
chemists Jacob A. Marinsky, Lawrence E. Glendenin, and Charles D. Coryell, who
isolated the radioactive isotopes promethium-147 (2.62-year half-life) and
promethium-149 (53-hour half-life) from uranium fission products at Clinton
Laboratories (now Oak Ridge National Laboratory) in Tennessee. Identification
was firmly established by ion-exchange chromatography. (Earlier investigators
thought that they had found the element with atomic number 61 in naturally
occurring rare earths and had prematurely called it illinium and florentium.)
Promethium-147 is effectively
separated from the other rare-earth fission products by an ion-exchange method.
Promethium has also been prepared by slow neutron bombardment of the isotope
neodymium-146; the resulting isotope, neodymium-147, decays by electron emission
to promethium-147. The metal itself was first prepared in 1963 by reduction of
the fluoride, PmF3, with lithium. Two allotropic (structural) modifications of
promethium are known: the α-phase is double close-packed hexagonal with a =
3.65 Å and c = 11.65 Å at room temperature. The β-phase is body-centred cubic
with a = 4.10 Å (estimated) at 890 °C (1,634 °F).
There are two possible sources
for natural promethium: rare decays of natural europium-151 (producing
promethium-147) and uranium (various isotopes). Practical applications exist
only for chemical compounds of promethium-147, which are used in luminous
paint, atomic batteries and thickness-measurement devices, even though
promethium-145 is the most stable promethium isotope. Because natural
promethium is exceedingly scarce, it is typically synthesized by bombarding
uranium-235 (enriched uranium) with thermal neutrons to produce promethium-147
as a fission product.
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