Atomic number 91

Protactinium

Pa
+ phase
− phase

Shells Orbitals 5f 6d 7s

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91 electrons in 7 shells: 2, 8, 18, 32, 20, 9, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Actinide
Period: 7
Block: f
Standard atomic weight: 231.04(1) Da
Electron configuration: [Rn] 5f² 6d¹ 7s²
Phase at 25 °C: solid
Melting point: 1568 °C
Boiling point: 4027 °C
Density: 15.37 g/cm³
Electronegativity (Pauling): 1.5
Discovered: 1913; 1917/18, Kasimir Fajans and O. H. Göhring; Otto Hahn and Lise Meitner, and Frederick Soddy and John Cranston

Neighbours in the table

Th U

Element 91 carried three names before it kept one. In 1900 William Crookes separated an intensely radioactive substance from uranium but could not prove it a new element, so he called it uranium-X, Los Alamos records. In 1913 Fajans and Göhring found a short-lived isotope of it. They proposed brevium, meaning brief, because of that short half-life, according to CIAAW, the atomic-weights commission of IUPAC, the International Union of Pure and Applied Chemistry.

The longer-lived protactinium-231 turned up in 1917/18, so proto-actinium became the better fit. The Greek protos, first, marks the element that comes before actinium, because protactinium-231 loses an alpha particle and becomes actinium-227. IUPAC shortened the name to protactinium in 1949.

Protactinium holds 91 electrons as [Rn] 5f² 6d¹ 7s², spread 2, 8, 18, 32, 20, 9 and 2 over its shells. The dense, silvery-grey metal keeps a bright lustre briefly in air, yet it reacts readily with oxygen, water vapour and inorganic acids. In solid compounds its most stable oxidation state, a bookkeeping charge that treats every bond as ionic, is +5. Dissolved, the +5 state quickly binds hydroxide ions into hydroxy-oxide solids that tend to cling to vessel walls. Below 1.4 K, about −272 °C, the metal superconducts, carrying an electric current with no resistance, Los Alamos adds.

Where it is found

Protactinium ranks among the rarest and most expensive of the naturally occurring elements, in Los Alamos’s judgement. Crustal rock usually holds a few parts per trillion, and some uraninite deposits reach a few parts per million. Pitchblende carries about one part protactinium-231 in ten million parts of ore, and ores from Zaire about 3 parts per million. Jefferson Lab’s estimates give 1.4 × 10⁻⁶ milligrams per kilogram of crust and 5 × 10⁻¹¹ milligrams per litre of seawater.

Nearly all natural protactinium is protactinium-231, formed along the uranium-235 decay chain. Uranium-235 emits an alpha particle and becomes thorium-231, which a beta emission turns into protactinium-231, according to IUPAC’s isotope notes. Protactinium-234 belongs to the uranium-238 chain instead. Nearly all uranium-238 decays pass through protactinium-234m, a metastable isomer (the same nucleus held briefly in a higher-energy state), and then protactinium-234, in Los Alamos’s account. In the Atomic Mass Data Center (AMDC) list, the isomer itself beta-decays almost every time, and only 0.16 per cent of its decays reach protactinium-234. Research material today generally comes out of spent nuclear fuel.

Why it matters

In 1927 Aristid V. Grosse made 2 mg of white powder, later shown to be Pa₂O₅. In 1934 he freed the metal by two methods. One turned the oxide into an iodide and split that apart in a high vacuum on an electrically heated filament.

A 12-stage process at Britain’s Atomic Energy Authority gave 125 g of 99.9 per cent pure protactinium, for years the only stock of the metal anywhere. It came from 60 tons of waste material at about $500,000, announced in 1959 and 1961, Los Alamos says. Jefferson Lab gives about 55,000 kg of ore and $500,000, in 1961.

Scarcity, strong radioactivity and toxicity leave protactinium with no practical use beyond basic research, according to Los Alamos and Jefferson Lab. In that research it traces the oceans. Uranium-235 and uranium-234 in seawater keep producing protactinium-231 and thorium-230. Sinking particles strip thorium-230 from the water more efficiently, while currents carry protactinium-231 farther. So in settling particles the ratio of the two usually stays under its production value of 0.093 unless the water stands still, IUPAC’s isotope notes explain. Seabed records of that ratio can therefore show past changes in the strength of major ocean currents.

Properties

Properties of protactinium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	91
Standard atomic weight	231.04(1) Da	Da	IUPAC/CIAAW 2024
Melting point	1568 °C	°C	Los Alamos: 1568 °C
Boiling point	4027 °C	°C	Los Alamos: 4027 °C
Density	15.37 g/cm³	g/cm³
Electronegativity (Pauling)	1.5
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f² 6d¹
Phase at 25 °C	solid

Safety

Protactinium-231 is an alpha emitter and a radiological hazard similar to polonium, Los Alamos warns. Its internal toxicity is very high on the rating of Haz-Map, an occupational health database quoted in PubChem’s safety record. More than 80 per cent of the inhalation hazard from protactinium-231 comes from actinium-227 and the nuclides it decays into, on Argonne National Laboratory’s figures quoted by Haz-Map. Guidance is in the PubChem Laboratory Chemical Safety Summary.

Questions

Who discovered protactinium?

Credit is shared. Mendeleev predicted an element between thorium and uranium in 1871. Kasimir Fajans and O. H. Göhring found its first isotope in 1913: protactinium-234 for Los Alamos, lasting 6.74 hours, and 6.7 hours for CIAAW, or the short-lived isomer protactinium-234m, about 1.17 minutes, for Jefferson Lab. Hahn and Meitner in Germany and, independently, Soddy and Cranston in Britain found protactinium-231 in 1917/18, and in 1949 IUPAC confirmed Hahn and Meitner as co-discoverers.

What is the half-life of protactinium-231?

Half a sample decays in 32,570 years for the International Atomic Energy Agency (IAEA), 32,650 for the Atomic Mass Data Center, about 32,760 for Jefferson Lab, 3.25 × 10⁴ for CIAAW and 33,000 for Haz-Map. Nearly every decay emits an alpha particle, leaving actinium-227. Rarer branches: spontaneous fission, the nucleus splitting by itself, at 3 × 10⁻¹⁰ per cent in IAEA data, and emission of whole neon-24 and fluorine-23 nuclei in the Data Center's list.

What is protactinium used for?

Outside basic research, nothing practical. In geology, protactinium-231 and thorium-230 date events over 10³ to 10⁵ years, IUPAC's isotope notes record, such as rates of magma melting beneath volcanoes and the ages of corals studied in relation to climate change. Protactinium-233 also appears in the thorium route to uranium-233, lasting about 27 days for Jefferson Lab and 26.975 days for the Atomic Mass Data Center before its beta decay.

Is protactinium dangerous?

Yes. Protactinium-231 is dangerous and highly radioactive, giving off alpha particles of 5.0 MeV (million electronvolts), Los Alamos says. Haz-Map rates its internal toxicity very high. Once inhaled, oxides and hydroxides take years to be half cleared from the lungs and other compounds weeks, in Haz-Map's summary. Handling guidance is in the PubChem Laboratory Chemical Safety Summary for protactinium.