Protactinium Element Facts

The chemical element protactinium is classed as an actinide metal. It was discovered in 1913 by Kasimir Fajans and Oswald Göhring.

Lise Meitner and Otto Hahn in the laboratory.
Lise Meitner and Otto Hahn

Data Zone

Classification: Protactinium is an actinide metal
Color: silvery
Atomic weight: 231.0359, no stable isotopes
State: solid
Melting point: 1570 oC, 1843 K
Boiling point: 4000 oC, 4273 K
Electrons: 91
Protons: 91
Neutrons in most abundant isotope: 140
Electron shells: 2,8,18,32,20,9,2
Electron configuration: [Rn] 5f2 6d1 7s2
Density @ 20oC: 15.4 g/cm3
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities
Atomic volume: 15.0 cm3/mol
Structure: fcc: face-centered cubic
Specific heat capacity 0.12 J g-1 K-1
Heat of fusion 12.34 kJ mol-1
Heat of atomization
Heat of vaporization 481 kJ mol-1
1st ionization energy 568 kJ mol-1
2nd ionization energy
3rd ionization energy
Electron affinity
Minimum oxidation number 0
Min. common oxidation no. 0
Maximum oxidation number 5
Max. common oxidation no. 5
Electronegativity (Pauling Scale) 1.5
Polarizability volume 25.4 Å3
Reaction with air
Reaction with 15 M HNO3
Reaction with 6 M HCl
Reaction with 6 M NaOH none
Oxide(s) PaO PaO2 Pa2O5
Hydride(s) PaH3
Chloride(s) PaCl4 PaCl5
Atomic radius 163 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion)
Ionic radius (3+ ion) 118 pm
Ionic radius (1- ion)
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity 47 W m-1 K-1
Electrical conductivity 5.6 x 106 S m-1
Freezing/Melting point: 1570 oC, 1843 K

An investigation into the decay of protactinium.

Discovery of Protactinium

Dr. Doug Stewart

The discovery story of protactinium spans several years. Polish chemist Kasimir Fajans and German chemist Oswald Göhring discovered protactinium in 1913 at Karlsruhe, Germany. They called the element ‘brevium’ because the isotope they had found (protactinium-234) has a very short half-life (1.17 minutes). (1),(2)

Otto Hahn and Lise Meitner at the Kaiser Wilhelm Institute in Berlin, discovered a much longer lived isotope in 1917: protactinium-231 (half-life of 32 670 years). Hahn and Meitner were searching for a ‘mother substance’ that decayed to actinium.

Otto Hahn wrote that their goal was “to find that substance which… forms the starting point for the actinium series, and to determine whether and through which intermediates actinium is derived.” (1)

Their work was interrupted by the First World War. In 1917 they examined silica residue they had extracted from pitchblende (uranium oxide) over two years earlier.

They identified protactinium-231 by establishing that no known substance could have emitted the alpha particles that were observed and by the actinium produced.

Frederic Soddy and John Cranston also discovered protactinium independently in 1917, at the University of Glasgow, Scotland. (1),(3)

In 1927 Aristid von Grosse isolated protactinium oxide (Pa2O5). In 1934 he isolated the element from the oxide by converting it to the iodide (PaI5) and then decomposed it using a heated filament in a high vacuum. (4)

The element name comes from a combination of the Greek word ‘protos’ meaning first and ‘actinium.’

Appearance and Characteristics

Harmful effects:

Protactinium is harmful due to its radioactivity and is also toxic.


Protactinium is a very rare shiny, silvery, highly radioactive metal that tarnishes slowly in air to the oxide.

Almost all naturally occurring protactinium is the 231 isotope. It emits alpha radiation and is produced through the decay of uranium-235.

Protactinium is one of the rarest and most expensive naturally occurring elements.

The largest amount of protactinium obtained so far has been 125 grams in 1961 from the Great Britain Atomic Energy Authority. The extraction was made from 60 tons of nuclear waste material. (5)

Uses of Protactinium

Protactinium is used mainly for research purposes.

Protactinium-231 combined with the thorium-230 can be used to date marine sediments. (6)

Abundance and Isotopes

Abundance earth’s crust: 0.1 part per trillion , 0.001 by moles

Abundance solar system: negligible

Cost, pure: $280 per g

Cost, bulk: per 100g

Source: Protactinium occurs naturally in minute amounts (up to 3 parts per million) in uranium ores. Protactinium can also obtained as a by-product of uranium processing.

Isotopes: Protactinium has 27 isotopes whose half-lives are known, with mass numbers 212 to 238. Protactinium has no stable isotopes. Its longest lived isotopes are 231Pa, with a half-life of 32,760 years, 233Pa with a half-life of 26.967 days and 230Pa with a half-life of 17.4 days.


1. Ruth Lewin Sime, The Discovery of Protactinium., Journal of Chemical Education., 63.8 August 1986 pages 653 – 657.
2. Jeremy Bernstein, Plutonium: A History of the World’s Most Dangerous Element., National Academies Press, 2007 page 40.
3. Discovery of Protactinium, University of Glasgow
4. John Emsley, Nature’s building blocks: an A-Z guide to the elements., Oxford University Press, 2003 page 348.
5. Andrew Ede, The chemical element: a historical perspective., Greenwood Publishing Group, 2006 page 146.
6. John P Rafferty, Geochronology, Dating, and Precambrian Time: The Beginning of the World as We Know It., The Rosen Publishing Group, 2010 page 152.

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