protactinium

thorium ← protactinium → uranium Pr ↑ Pa ↓ (Uqu) 91Pa Periodic table
Appearance
bright, silvery metallic luster
General properties
Name, symbol, number	protactinium, Pa, 91
Element category	actinide
Group, period, block	n/a, 7, f
Standard atomic weight	231.03588 g·mol−1
Electron configuration	[Rn] 7s2 6d1 5f2
Electrons per shell	2, 8, 18, 32, 20, 9, 2 (Image)
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, ? 7280 °F
Heat of fusion	12.34 kJ·mol−1
Heat of vaporization	481 kJ·mol−1
Atomic properties
Oxidation states	2, 3, 4, 5 (weakly basic oxide)
Electronegativity	1.5 (Pauling scale)
Ionization energies	1st: 568 kJ·mol−1
Atomic radius	163 pm
Covalent radius	200 pm
Miscellanea
Crystal structure	orthorhombic
Magnetic ordering	paramagnetic[1]
Electrical resistivity	(0 °C) 177 nΩ·m
Thermal conductivity	(300 K) 47 W·m−1·K−1
CAS registry number	7440-13-3
Most stable 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 trace 1.17 min β− 2.29 234U IT 0.0694 234Pa 234Pa trace 6.75 h β− 0.23 234U
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Protactinium (pronounced /ˌproʊtækˈtɪniəm/, PROH-tak-TIN-ee-əm) is a chemical element with the symbol Pa and atomic number 91. Its longest-lived and most abundant naturally-occurring isotope by far, Pa-231, is a decay product of uranium-235 (U-235), and it has a half-life of 32,760 years. Much smaller trace amounts of shorter lived Pa-234 occur as decay products of uranium-238 (U-238).

Characteristics

Protactinium is a metallic element that belongs to the actinoid group, with a bright metallic luster that it retains for some time in contact with air.^[2] Protactinium is superconductive at temperatures below 1.4 K.^[3]

Applications

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

Protactinium-231 is formed by the alpha decay of U-235 followed by beta decay of thorium-231. The physicist Walter Seifritz once estimated that protactinium might possibly be used to build a nuclear weapon with a critical mass of 750±180 kg. This possibility (of a chain reaction) has been ruled out by other nuclear physicists since then.

The ratio of protactinium-231 to thorium-230 in ocean sediments has also been used in paleoceanography to reconstruct the movements of North Atlantic water bodies during the last melting of Ice Age glaciers.^[4]

History

In 1890, Mendeleev predicted the existence of an element between thorium and uranium. Due to the fact that the actinide element group was unknown uranium was positioned below tungsten and thorium below zirconium leaving the space below tantalum empty. Until the 1950s periodic tables were published with this structure.^[5] For a long time chemists searched for eka-tantalum as an element with similar chemical properties as tantalum, making a discovery of protactinium nearly impossible.

In 1900, William Crookes isolated protactinium as a radioactive material from uranium; however, he did not identify it as a new element.^[6]

Protactinium was first identified in 1913, when Kasimir Fajans and O. H. Göring encountered the short-lived isotope Pa-234m (half-life of about 1.17 minutes), during their studies of the decay chains of uranium-238 (U-238). They gave the new element the name brevium (from the Latin word, brevis, meaning brief or short);^[7][8] the name was changed to protoactinium in 1918 when two groups of scientists (lead by Otto Hahn and Lise Meitner of Germany; and Frederick Soddy and John Cranston of Great Britain) independently discovered Pa-231. The name was shortened to Protactinium in 1949.

Aristid von Grosse produced 2 mg of Pa₂O₅ in 1927,^[9] and in 1934 performed the first isolation of elemental protactinium from 0.1 mg of Pa₂O₅ using the iodide process: converting the oxide to an iodide and then reducing it in a vacuum with an electrically-heated metal filament:

2 PaI₅ → 2 Pa + 5 I₂

In 1961, the British Atomic Energy Authority (UKAEA) was able to produce 125 grams of 99.9% pure protactinium by processing 60 tons of waste material in a 12-stage process. For many years, this was the world's only significant supply of protactinium.

Occurrence

Protactinium occurs in pitchblende to the extent of about 1.0 part ²³¹Pa per 10 million parts of ore (i.e., 0.1 ppm). Some ores from the Democratic Republic of the Congo have about 3.0 ppm. Protactinium is one of the rarest and most expensive naturally occurring elements.^[2]

Compounds

Examples of protactinium compounds:

• 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

See also Protactinium compounds.

Isotopes

Twenty-nine radioisotopes of protactinium have been discovered, with the most stable being Pa-231 with a half life of 32760 years, Pa-233 with a half-life of 27.0 days, and Pa-230 with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives that are less than 1.60 days, and the majority of these have half-lives that are less than 1.8 seconds. Protactinium also has two meta states, Pa-217m (half-life 1.2 milliseconds) and Pa-234m (half-life 1.17 minutes).

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

Precautions

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

References

External links

Wikimedia Commons has media related to: Protactinium

Look up protactinium in Wiktionary, the free dictionary.

• ANL factsheet
• WebElements.com - Protactinium
• It's Elemental - Protactinium
• PROTACTINIUM