Protactinium

Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds in which protactinium is usually present in the oxidation state +5, but it can also assume +4 and even +3 or +2 states. Concentrations of protactinium in the Earth's crust are typically a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. Because of its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside scientific research, and for this purpose, protactinium is mostly extracted from spent nuclear fuel. Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds in which protactinium is usually present in the oxidation state +5, but it can also assume +4 and even +3 or +2 states. Concentrations of protactinium in the Earth's crust are typically a few parts per trillion, but may reach up to a few parts per million in some uraninite ore deposits. Because of its scarcity, high radioactivity and high toxicity, there are currently no uses for protactinium outside scientific research, and for this purpose, protactinium is mostly extracted from spent nuclear fuel. Protactinium was first identified in 1913 by Kasimir Fajans and Oswald Helmuth Göhring and named brevium because of the short half-life of the specific isotope studied, i.e. protactinium-234. A more stable isotope of protactinium, 231Pa, was discovered in 1917/18 by Otto Hahn and Lise Meitner, and they chose the name proto-actinium, but the IUPAC finally named it 'protactinium' in 1949 and confirmed Hahn and Meitner as discoverers. The new name meant '(nuclear) precursor of actinium' and reflected that actinium is a product of radioactive decay of protactinium. John Arnold Cranston (working with Frederick Soddy and Ada Hitchins) is also credited with discovering the most stable isotope in 1915, but delayed his announcement due to being called up for service in the First World War. The longest-lived and most abundant (nearly 100%) naturally occurring isotope of protactinium, protactinium-231, has a half-life of 32,760 years and is a decay product of uranium-235. Much smaller trace amounts of the short-lived protactinium-234 and its nuclear isomer protactinium-234m occur in the decay chain of uranium-238. Protactinium-233 results from the decay of thorium-233 as part of the chain of events used to produce uranium-233 by neutron irradiation of thorium-232. It is an undesired intermediate product in thorium-based nuclear reactors and is therefore removed from the active zone of the reactor during the breeding process. Analysis of the relative concentrations of various uranium, thorium and protactinium isotopes in water and minerals is used in radiometric dating of sediments which are up to 175,000 years old and in modeling of various geological processes. In 1871, Dmitri Mendeleev predicted the existence of an element between thorium and uranium. The actinide element group was unknown at the time. Therefore, uranium was positioned below tungsten in group VI, and thorium below zirconium in group IV, leaving the space below tantalum in group V empty and, until the 1950s, periodic tables were published with this structure. For a long time chemists searched for eka-tantalum as an element with similar chemical properties to tantalum, making a discovery of protactinium nearly impossible. Tantalum's heavier analogue was later found to be the transuranic element dubnium – which, however, does not react like tantalum, but like protactinium. In 1900, William Crookes isolated protactinium as an intensely radioactive material from uranium; however, he could not characterize it as a new chemical element and thus named it uranium-X (UX). Crookes dissolved uranium nitrate in ether, and the residual aqueous phase contains most of the 23490Th and 23491Pa. His method was still used in the 1950s to isolate 23490Th and 23491Pa from uranium compounds. Protactinium was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the isotope 234Pa during their studies of the decay chains of uranium-238: 23892U → 23490Th → 23491Pa → 23492U. They named the new element brevium (from the Latin word, brevis, meaning brief or short) because of its short half-life, 6.7 hours for 23491Pa. In 1917/18, two groups of scientists, Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of Great Britain, independently discovered another isotope of protactinium, 231Pa, having a much longer half-life of about 32,000 years. Thus the name brevium was changed to protoactinium as the new element was part of the decay chain of uranium-235 as the parent of actinium (from Greek: πρῶτος prôtos 'first, before'). For ease of pronunciation, the name was shortened to protactinium by the IUPAC in 1949. The discovery of protactinium completed one of the last gaps in the early versions of the periodic table, proposed by Mendeleev in 1869, and it brought to fame the involved scientists. Aristid von Grosse produced 2 milligrams of Pa2O5 in 1927, and in 1934 first isolated elemental protactinium from 0.1 milligrams of Pa2O5. He used two different procedures: in the first one, protactinium oxide was irradiated by 35 keV electrons in vacuum. In another method, called the van Arkel–de Boer process, the oxide was chemically converted to a halide (chloride, bromide or iodide) and then reduced in a vacuum with an electrically heated metallic filament: In 1961, the United Kingdom Atomic Energy Authority (UKAEA) produced 127 grams of 99.9% pure protactinium-231 by processing 60 tonnes of waste material in a 12-stage process, at a cost of about 500,000 USD. For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies. Oak Ridge National Laboratory in the US provided protactinium at a cost of about 280 USD/gram. Twenty-nine radioisotopes of protactinium have been discovered, the most stable being 231Pa with a half-life of 32,760 years, 233Pa with a half-life of 27 days, and 230Pa with a half-life of 17.4 days. All of the remaining isotopes have half-lives shorter than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. Protactinium also has two nuclear isomers, 217mPa (half-life 1.2 milliseconds) and 234mPa (half-life 1.17 minutes). The primary decay mode for isotopes of protactinium lighter than (and including) the most stable isotope 231Pa (i.e., 212Pa to 231Pa) is alpha decay and the primary mode for the heavier isotopes (i.e., 232Pa to 240Pa) is beta decay. The primary decay products of isotopes of protactinium lighter than (and including) 231Pa are actinium isotopes and the primary decay products for the heavier isotopes of protactinium are uranium isotopes.