|Atomic Number:||91||Atomic Radius:||243 pm (Van der Waals)|
|Atomic Symbol:||Pa||Melting Point:||1572 °C|
|Atomic Mass:||231||Boiling Point:||~4000 °C|
[Rn] 7s2 5f2 6d1
|Oxidation States:||5, 4, 3, 2, 0|
(Gr. protos: first) The first isotope of element 91 to be discovered was 234Pa, also known as UX2, a short-lived member of the naturally occurring 238U decay series. It was identified by K. Fajans and O.H. Gohring in 1913 and the named the new element brevium. When the longer-lived isotope 231Pa was identified by Hahn and Meitner in 1918, the name protoactinium was adopted as being more consistent with the characteristics of the most abundant isotope. Sody, Cranson, and Fleck were also active in this work. The name protoactinium was shortened to protactinium in 1949. In 1927, Grosse prepared 2 mg of a white powder, which was shown to be Pa2O5. Later, in 1934, from 0.1 g of pure Pa2O5 he isolated the element by two methods, one of which was by converting the oxide to an iodide and “cracking” it in a high vacuum by an electrically heated filament by the reaction: 2PaI5 –> 2Pa + 5I2.
Protactinium is one of the rarest and most expensive naturally occurring elements. In 1959 and 1961, it was announced that the Great Britain Atomic Energy Authority extracted by a 12-stage process 125 g of 99.9% protactinium, the world’s only stock of the metal for many years to come. The extraction was made from 60 tons of waste material at a cost of about $500,000.
Protactinium has a bright metallic luster which it retains for some time in air. The element occurs in pitchblende to the extent of about 1 part 231Pa to 10 million parts of ore. Ores from Zaire have about 3 ppm. Protactinium has 20 isotopes, the most common of which is 231Pa with a half-life of 32,700 years. A number of protactinium compounds are known, some of which are colored. The element is superconductive below 1.4K.
The element is a dangerous material and requires precautions similar to those used when handling plutonium.
Los Alamos National Laboratory, Periodic Table of Elements, http://periodic.lanl.gov/list.shtml.