| Protactinium The name �protactinium� was derived from the Greek �protos aktino�, which mean first ray. A short-lived isotope of Pa234, also known as UX2, with a half-life of a little more than one minute was discovered in the decay products of U238. The discovery was made in 1913 in Karlsruhe, Germany, by Kasimir Fajans and O.H. Gohring. Gohring named the isotope �brevium,� after uranium X2, because it was a short-lived member of the uranium radioactive decay series. The long-lived isotope, Pa231 was discovered in 1918 in Berlin, Germany, by Otto Hahn and Lise Meitner. It was discovered in processing products of pitchblende. The long-lived isotope was also independently discovered in 1917 in Glasgow, United Kingdom, by Frederick Soddy, John Cranston, and Sir Alexander Fleck. The isotope was named �protoactinium� but in 1949 the name was shortened to �protactinium.� (1) Protactinium is a radioactive element of the actinide group. Its atomic number is 91, which means it has 91 protons and 91 electrons in pure form. Its atomic mass number is about 231 meaning that Pa231, the most stable isotope, has 140 neutrons. In the Periodic System protactinium is located between thorium and uranium. It is in solid state at 293K and has a bright light gray metallic luster, which it retains for some time in air. Its melting point is about 1873.2K; its boiling point is 4500K. Its calculated density at 293K is about 15.37g/cm3. The element becomes super conductive at 2K. The electronic configuration of protactinium is (Rn)5f26d17s2 or 2-8-18-32-20-9-2. It has seven shells and two electrons in its outermost shell. Protactinium has two valency states, +4 and +5. It is a dangerous toxic material and requires precautions similar to those used when handling plutonium. The element is an -emitter and has a radiological hazard similar to polonium. If it gets into the human body, it starts building up inside the kidneys and bones. The maximum safe dose of protactinium for a human being is 0.05 micrograms. Pa231, which is present in the air in aerosol form, is 2.5 x 108 times more toxic than hydrocyanic acid at equal concentrations. Protactinium is found naturally in small amounts in uranium ores. It is also extracted from spent fuel rods from nuclear reactors. Protactinium is one of the rarest and most expensive naturally occurring elements. It is mostly found in pitchblende, and uranium ores in Zaire. (2) Approximately 2mg of protactinium were first isolated in 1927 by Grosse, who studied its principal chemical properties and determined the atomic weight of the element. The research on the element continued in the Curie Laboratory in France, became part of the Manhattan Project in the United States, and later in the Soviet Union and England. Scientists Hassinsky and Bouissieres found that protactinium probably exists in the tetravalent state while studying the properties of the element. Maddock studied the distribution of protactinium in the processing products of the Springfield uranium plant in England, where he identified many potential sources from which protactinium could be isolated. An extensive study of the properties of protactinium was made possible in 1960 when more than 130g of Pa231 were isolated in England and at that time there were also large amounts of protactinium isolated in the Soviet Union and other countries but in smaller amounts. (1) Protactinium is often described as an �intractable� element because it has a very strong tendency to become hydrolyzed in solution and often loses its individual features during isolation from natural products where it occurs in very small amounts. Because of its properties and chemical behavior protactinium is one of the most interesting heavy elements. Protactinium has become more interesting recently because of development of the homogenous thorium reactor. (1) It has a very low natural abundance, about 0.000034% of the uranium content in uranium ores. There are twenty known isotopes of protactinium. However, only two of them, Pa231 and Pa234, occur in nature; the rest are artificially made in laboratories. In practical use, Pa231 and Pa233 are the most important. Most isotopes of protactinium are very short lived. The half-life of the longest-lived isotope Pa231 is 32,480 years. This isotope decays to actinium227 with a half-life of 32,500 years. It is constantly being produced by decaying U235, however, the content of protactinium is also constantly decreasing because the uranium in the Earth�s crust is decaying. For example, 4.5 billion years ago there was 16 times as much naturally occurring protactinium as there is today. (1) References: 1.�Analytical Chemistry of Protactinium,� E.S. Palshin, B.F. Myasoedov, A.V. Davydov, Ann Arbor-Humphrey Publishers, 1970 2.�Protactinium,� Cornelius Keller, Springer-Verlag, 1977 |
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