Protactinium

Protactinium (symbol Pa, atomic number 91) is a dense, radioactive, silvery-gray metal belonging to the actinide series. It reacts readily with oxygen, water vapor, and inorganic acids. In its compounds, protactinium most often appears in the +5 oxidation state, though it can also take the +4, +3, or +2 states. On Earth, it is extremely scarce—typically just a few parts per trillion in the crust, though concentrations can climb to a few parts per million in certain uraninite deposits. Because it is so rare, highly radioactive, and toxic, protactinium has no practical applications beyond scientific research, and for that purpose it is usually obtained from spent nuclear fuel. It is also the only highly radioactive element that is mononuclidic (occurring almost entirely as a single nuclide) without being monoisotopic; nearly all natural protactinium comes from the decay of uranium-235.

The element was first detected in 1913 by Kazimierz Fajans and Oswald Helmuth Göhring, who named it “brevium” after the short half-life of the isotope they studied, 234mPa. A more stable isotope, 231Pa, was discovered in 1917–18 by Lise Meitner working with Otto Hahn; they called it protactinium. In 1949, the International Union of Pure and Applied Chemistry (IUPAC) officially adopted the name “protactinium,” meaning “precursor of actinium,” and credited Hahn and Meitner as discoverers. John Arnold Cranston, collaborating with Frederick Soddy and Ada Hitchins, also identified the same stable isotope in 1915 but delayed publication due to his service in World War I.

The longest-lived and overwhelmingly dominant natural isotope is 231Pa, with a half-life of 32,760 years; it arises from the decay chain of uranium-235. Tiny traces of short-lived 234Pa and its nuclear isomer 234mPa come from the uranium-238 decay chain. Another isotope, 233Pa, forms when thorium-233 decays—a step in producing uranium-233 by neutron irradiation of thorium-232. In thorium-based nuclear reactors, 233Pa is an unwanted intermediate and must be removed from the reactor core during breeding. Oceanographers use protactinium to study ancient ocean geography: by measuring the relative amounts of uranium, thorium, and protactinium isotopes in water and sediments, they can date deposits up to 175,000 years old and model geological processes.

Protactinium is unusual among non-primordial elements because its natural isotopic composition is extremely uniform across typical terrestrial samples. This is because essentially all naturally occurring protactinium is 231Pa, produced solely from the decay of primordial uranium-235.

**History**

In 1871, Dmitri Mendeleev predicted an element between thorium and uranium. Since the actinide series was not yet recognized, he placed uranium under tungsten in group VI and thorium under zirconium in group IV, leaving a gap under tantalum in group V. Until the actinide concept was accepted in the late 1940s, periodic tables followed this arrangement. Chemists long searched for “eka-tantalum,” an element chemically similar to tantalum, which made discovering protactinium nearly impossible. Tantalum’s heavier analogue later turned out to be the transuranic element dubnium—though dubnium is actually more similar to protactinium than to tantalum.

In 1900, William Crookes isolated a highly radioactive material from uranium but could not identify it as a new element; he called it uranium X (UX). By dissolving uranium nitrate in ether, he left most of the 234Th and 234Pa in the aqueous phase. This method was used into the 1950s to separate those isotopes from uranium compounds. Protactinium was first identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring studied the uranium-238 decay chain: 238U → 234Th → 234mPa → 234U. They named the new element “brevium” (from Latin *brevis*, meaning short) because of the 1.16-minute half-life of 234mPa (uranium X2). In 1917–18, two independent groups—Lise Meitner with Otto Hahn in Germany, and Frederick Soddy with John Cranston in Britain—discovered the longer-lived isotope 231Pa (half-life 32,760 years). Meitner changed the name from “brevium” to “protactinium,” since the element preceded actinium in the uranium-235 decay chain (from Greek *prôtos*, meaning “first”). IUPAC confirmed this name in 1949. The discovery filled one of the last gaps in early periodic tables and brought fame to those involved.

Aristid von Grosse produced 2 milligrams of Pa₂O₅ in 1927, and in 1934 he first isolated elemental protactinium from 0.1 milligrams of Pa₂O₅. He used two methods: one irradiated protactinium oxide with 35 keV electrons in a vacuum; the other, the van Arkel–de Boer process, converted the oxide to a halide (chloride, bromide, or iodide) and then reduced it in a vacuum with an electrically heated metal filament: 2 PaI₅ → 2 Pa + 5 I₂. In 1961, the United Kingdom Atomic Energy Authority produced 127 grams of 99.9% pure protactinium-231 by processing 60 tonnes of waste material in a 12-stage process, costing about US$500,000. For many years, this was the world’s only significant supply, distributed to laboratories for research. Oak Ridge National Laboratory in the US also provided protactinium at a cost.