91 thoriumprotactiniumuranium


Name, Symbol, Number protactinium, Pa, 91
Chemical series actinides
Group, Period, Block n/a, 7, f
Appearance bright, silvery metallic luster
Atomic mass 231.03588(2) g/mol
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/cm³
Melting point 1841 K
(1568 °C, 2854 °F)
Boiling point  ? 4300 K
(? 4027 °C, ? °F)
Heat of fusion 12.34 kJ/mol
Heat of vaporization 481 kJ/mol
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·K)
CAS registry number 7440-13-3
Notable isotopes
Main article: Isotopes of protactinium
iso NA half-life DM DE (MeV) DP
230Pa syn 17.4 d ε 1.310 230Th
β- 0.563 230U
231Pa syn 32760 y α 5.149 227Ac
233Pa syn 26.967 d β- 0.571 233U

Protactinium (chemical symbol Pa, atomic number 91) is a member of the actinide series of chemical elements. It is a toxic, highly radioactive material, of interest mainly for scientific research. Currently it has no practical applications, but some researchers have noted that it could be used in making nuclear weapons.



The isotope protactinium-231 occurs in pitchblende to the extent of about one part per ten million parts of ore. Some ores from the Democratic Republic of the Congo have been found to contain the element at a concentration of about three parts per million (ppm).


The existence of an element between thorium and uranium was predicted by Dmitri Mendeleev in 1871. In 1900, William Crookes isolated protactinium as a radioactive material from uranium, but he could not identify the material.[1]

Protactinium was first identified in 1913, when Kasimir Fajans and O. H. Göhring encountered short-lived isotope protactinium-234m, with a half-life of about 1.17 minutes, during their studies of the decay chain of uranium-238. They gave the new element the name brevium (Latin brevis, meaning brief or 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 Pa-231. The name was shortened to protactinium in 1949.

Aristid V. Grosse prepared two milligrams (mg) of the oxide Pa2O5 in 1927. About seven years later, he managed to isolate protactinium for the first time from 0.1 milligrams (mg) of Pa2O5, by 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.

In 1961, the U.K. Atomic Energy Authority produced 125 grams (g) of 99.9 percent pure protactinium, after processing 60 tons of waste material in a 12-stage process and spending 500,000 USD. This was the world's only supply of the element for many years to come, and it has been reported that the metal was sold to laboratories at a price of 2,800 USD per gram.

Notable Characteristics

Protactinium is an inner transition metal of the actinide series, situated in period seven of the periodic table, between thorium and uranium. It retains a bright silvery luster for some time in the air. It is superconductive at temperatures below 1.4 K.


Many radioisotopes of protactinium have been characterized. The most stable of these are 231-Pa, with a half-life (t½) of 32,760 years; 233-Pa, with a half-life of 26.967 days; and 230-Pa, with a half-life of 17.4 days. All the remaining radioactive isotopes have half-lives that are less than 1.6 days, and the majority of these have half-lives that are less than 1.8 seconds. This element also has two meta states, 217m-Pa (t½ 1.15 milliseconds) and 234m-Pa (t½ 1.17 minutes).

The primary decay mode before the most stable isotope, 231-Pa, is alpha decay and the primary mode after is beta minus decay. The primary decay products before 231-Pa are actinium isotopes, and the primary products after are uranium isotopes.


Known compounds of protactinium include the following:

  • Fluorides:
    • protactinium(IV) fluoride (PaF4)
    • protactinium(V) fluoride (PaF5)
  • Chlorides:
    • protactinium(IV) chloride (PaCl4)
    • protactinium(V) chloride (PaCl5)
  • Bromides:
    • protactinium(IV) bromide (PaBr4)
    • protactinium(V) bromide (PaBr5)
  • Iodides:
    • protactinium(III) iodide (PaI3)
    • protactinium(IV) iodide (PaI4)
    • protactinium(V) iodide (PaI5)
  • Oxides:
    • protactinium(II) oxide (PaO)
    • protactinium(IV) oxide (PaO2)
    • protactinium(V) oxide (Pa2O5)


Given its scarcity, high radioactivity, and toxicity, there are currently no practical uses for protactinium, and it is mainly used for basic scientific research.

Some researchers have observed that protactinium-231, which is formed by the alpha decay of uranium-235, 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 kilograms (kg). Other authors conclude that no chain reactions are possible with protactinium-231.


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

See also


  1. John Emsley, Nature's Building Blocks (Oxford: Oxford Univ. Press, 2001), 347. ISBN 0198503407


  • Emsley, John. 2001. Nature's Building Blocks: An A–Z Guide to the Elements. Oxford: Oxford Univ. Press. ISBN 0198503407
  • Greenwood, N.N., and A. Earnshaw. 1998. Chemistry of the Elements, 2nd ed. Oxford, UK; Burlington, MA: Butterworth-Heinemann. ISBN 0750633654
  • Hampel, Clifford A. 1968. The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corp. ISBN 0442155980
  • Morss, Lester R., Norman M. Edelstein, and Jean Fuger, eds. 2006. The Chemistry of the Actinide and Transactinide Elements. 3rd ed. 5 vols. Joseph J. Katz, adapter. Dordrecht: Springer. ISBN 1402035551
  • Stwertka, Albert. 1998. Guide to the Elements. Rev. ed. Oxford: Oxford University Press. ISBN 0-19-508083-1

External links

All links retrieved June 11, 2015.


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