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91 thoriumprotactiniumuranium


Name, Symbol, Number protactinium, Pa, 91
Chemical series actinides
Group, Period, Block n/a, 7, f
Appearance bright, silvery metallic luster
Standard atomic weight 231.03588(2)  g·mol−1
Electron configuration [Rn] 5f2 6d1 7s2
Electrons per shell 2, 8, 18, 32, 20, 9, 2
Physical properties
Phase solid
Density (near r.t.) 15.37  g·cm−3
Melting point 1841 K
(1568 °C, 2854 °F)
Boiling point  ? 4300 K
(? 4027 °C, ? °F)
Heat of fusion 12.34  kJ·mol−1
Heat of vaporization 481  kJ·mol−1
Atomic properties
Crystal structure orthorhombic
Oxidation states 2, 3, 4, 5
(weakly basic oxide)
Electronegativity 1.5 (Pauling scale)
Ionization energies 1st: 568 kJ/mol
Atomic radius 180  pm
Magnetic ordering no data
Electrical resistivity (0 °C) 177 nΩ·m
Thermal conductivity (300 K) 47  W·m−1·K−1
CAS registry number 7440-13-3
Selected isotopes
Main article: Isotopes of protactinium
iso NA half-life DM DE (MeV) DP
229Pa syn 1.4 d α 5.58 225Ac
230Pa syn 17.4 d ε 1.310 230Th
β- 0.563 230U
231Pa ~100% 32760 y α 5.149 227Ac
232Pa syn 1.31 d β- 0.31 232U
233Pa syn 26.967 d β- 0.571 233U
234mPa syn 1.17 min β- 2.29 234U
IT 0.0694 234Pa
234Pa syn 6.75 h β- 0.23 234U

Protactinium (pronounced /ˌproʊtækˈtɪniəm/) is a chemical element with the symbol Pa and atomic number 91.


Notable Characteristics

Protactinium is a silver metallic element that belongs to the actinide group, with a bright metallic luster that it retains for some time in the air. It is superconductive at temperatures below 1.4 K.


Due to its scarcity, high radioactivity and toxicity, there are currently no uses for protactinium outside of basic scientific research.

Protactinium-231 (which is formed by the alpha decay of Uranium-235 followed by beta decay of Thorium-231) could possibly sustain a nuclear chain reaction and might, in principle, be used to build a nuclear weapon. The critical mass, according to Walter Seifritz, is 750±180 kg. Other authors conclude that no chain reactions are possible in Protactinium-231.


An element between thorium and uranium was predicted to exist by Mendeleev in 1871. In 1900 William Crookes isolated protactinium as a radioactive material from uranium which he could not identify[1].

Protactinium was first identified in 1913, when Kasimir Fajans and O. H. Göhring encountered short-lived isotope 234m-Pa, with a half-life of about 1.17 minutes, during their studies of the decay chain of 238-U. They gave the new element the name Brevium (Latin brevis, brief, short); the name was changed to Protoactinium in 1918 when two groups of scientists (Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of the UK) independently discovered 231-Pa. The name was shortened to Protactinium in 1949.

Aristid V. Grosse prepared 2 mg of Pa2O5 in 1927, and later on managed to isolate Protactinium for the first time in 1934 from 0.1 mg of Pa2O5, first converting the oxide to an iodide and then cracking it in a high vacuum by an electrically heated filament by the reaction 2PaI5 → 2Pa + 5I2 (iodide process).

In 1961, the United Kingdom Atomic Energy Authority was able to produce 125 g of 99.9% pure protactinium, processing 60 tons of waste material in a 12-stage process and spending 500,000 USD. For many years to come, this was the world's only supply of the element. It is reported that the metal was sold to laboratories for a cost of 2,800 USD / g in the following years.[citations needed]


Protactinium occurs in pitchblende to the extent of about 1 part 231Pa per 10 million parts of ore (i.e., 0.1 ppm). Some ores from the Democratic Republic of the Congo have about 3 ppm.


Known Protactinium compounds:

  • Fluorides: PaF4, PaF5
  • Chlorides: PaCl4, PaCl5
  • Bromides: PaBr4, PaBr5
  • Iodides: PaI3, PaI4, PaI5
  • Oxides: PaO, PaO2, Pa2O5
See also: Compounds of Protactinium


29 radioisotopes of protactinium have been characterized, with the most stable being 231-Pa with a half life of 32760 years, 233-Pa with a half-life of 26.967 days, and 230-Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lifes that are less than 1.6 days, and the majority of these have half lifes that are less than 1.8 seconds. This element also has 2 meta states, 217m-Pa (t½ 1.15 milliseconds) and 234m-Pa (t½ 1.17 minutes).

The primary decay mode for isotopes of Pa lighter than (and including) the most stable isotope 231-Pa (ie, 212Pa to 231Pa) is alpha decay and the primary mode for the heavier isotopes (ie, 232Pa to 240Pa) is beta minus (β) decay. The primary decay products of isotopes of Pa lighter than (and including) 231-Pa are element Ac (actinium) isotopes and the primary decay products for the heavier isotopes of Pa are element U (uranium) isotopes.


Protactinium is both toxic and highly radioactive. It requires precautions similar to those used when handling plutonium.


  1. ^ Emsley, John (2001). Nature's Building Blocks, (Hardcover, First Edition), Oxford University Press, page 347. ISBN 0198503407. 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Protactinium". A list of authors is available in Wikipedia.
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