stylistic:style_transfer
History

In 1871, Dmitri Mendeleev predicted the existence of an element between thorium and uranium. The actinide series was unknown at the time, so Mendeleev positioned uranium below tungsten in group VI, and thorium below zirconium in group IV, leaving the space below tantalum in group V empty. Until the general acceptance of the actinide concept in the late 1940s, 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 – although dubnium is more chemically similar to protactinium, not tantalum.

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 contained most of the Th and Pa. His method was used into the 1950s to isolate Th and Pa from uranium compounds. Protactinium was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring encountered the isotope Pa during their studies of the decay chains of uranium-238: U → Th → Pa → U. They named the new element " brevium " (from the Latin word brevis, meaning brief or short) because of the short half-life of 1.16 minutes for Pa (uranium X2). In 1917–18, two groups of scientists, Lise Meitner in collaboration with Otto Hahn of Germany and Frederick Soddy and John Cranston of Great Britain, independently discovered another isotope, Pa, having a much longer half-life of 32,760 years. Meitner changed the name "brevium" to protactinium as the new element was part of the decay chain of uranium-235 as the parent of actinium (from the Greek: πρῶτος prôtos, meaning "first, before"). The IUPAC confirmed this naming in 1949. The discovery of protactinium completed one of the last gaps in early versions of the periodic table, and brought fame to the involved scientists.

Aristid von Grosse produced 2 milligrams of Pa O in 1927, and in 1934 first isolated elemental protactinium from 0.1 milligrams of Pa O. He used two different procedures: in the first, protactinium oxide was irradiated by 35 keV electrons in vacuum. In the other, 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 US$500,000. For many years, this was the world's only significant supply of protactinium, which was provided to various laboratories for scientific studies. The Oak Ridge National Laboratory in the US provided protactinium at a cost of about US$280/gram.