Atomic Number: 91
Atomic Weight: 231.03588
Discovery: Fajans & Gohring 1913; Fredrich Soddy, John Cranston, Otto Hahn, Lise Meitner 1917 (England/France)
Electron Configuration: [Rn] 7s2 5f2 6d1
Word Origin: Greek protos, meaning 'first'. Fajans and Gohring in 1913 named the element brevium, because the isotope they discovered, Pa-234, was short-lived. When Pa-231 was identified by Hahn and Meitner in 1918, the name protoactinium was adopted because this name was considered to be more consistent with the characteristics of the most abundant isotope (protactinium forms actinium when it radioactively decays). In 1949, the name protoactinium was shortened to protactinium.
Isotopes: Protactinium has 13 isotopes. The most common isotope is Pa-231, which has a half-life of 32,500 years. The first isotope to be discovered was Pa-234, which was also called UX2. Pa-234 is a short-lived member of the naturally occurring U-238 decay series. The longer-lived isotope, Pa-231, was identified by Hahn and Meitner in 1918.
Properties: The atomic weight of protactinium is 231.0359, its melting point is < 1600°C, specific gravity has been calculated to be 15.37, with a valence of 4 or 5. Protactinium has a bright metallic luster which is retained for a while in air. The element is superconductive below 1.4K. Several protactinium compounds are known, some of which are colored. Protactinium is an alpha emitter (5.0 MeV) and is a radiological hazard which requires special handling. Protactinium is one of the rarest and most expensive naturally occurring elements.
Sources: The element occurs in pitchblende to the extent of about 1 part Pa-231 to 10 million parts ore.
Element Classification: Radioactive Rare Earth (Actinide)
Density (g/cc): 15.37
Melting Point (K): 2113
Boiling Point (K): 4300
Appearance: silvery-white, radioactive metal
Atomic Radius (pm): 161
Atomic Volume (cc/mol): 15.0
Ionic Radius: 89 (+5e) 113 (+3e)
Specific Heat (@20°C J/g mol): 0.121
Fusion Heat (kJ/mol): 16.7
Evaporation Heat (kJ/mol): 481.2
Pauling Negativity Number: 1.5
Oxidation States: 5, 4
Lattice Structure: Tetragonal
Lattice Constant (Å): 3.920
References: Los Alamos National Laboratory (2001), Crescent Chemical Company (2001), Lange's Handbook of Chemistry (1952), CRC Handbook of Chemistry & Physics (18th Ed.)
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