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Element Fact
In 1961, the International Union of Pure and Applied Chemistry adopted the isotope carbon-12 as the basis of atomic weight, which has a mass of exactly 12.000 atomic mass unit.

Protactinium, Pa

protactinium sample

Atomic number: 91
Atomic mass: 231.03588(2)
Natural abundance: -
231Pa (231.03588) - *%

History and Production

From Greek protos aktino, meaning first ray. The first isotope, 234Pa (half-life=7.75 hours), was discovered by K. Fajans and O.H. Göhring in 1913, from the naturally occurring 238U decay series. However, the more stable isotope 231Pa (half-life=32800 years) was identified as a product of 235U decay by O. Hahn and L. Meitner and independently by F. Soddy and J.A. Cranston in 1918. It was initially named protoactinium but shortened to protactinium in 1949. Previously, it was produced only in milligram scale. In 1960 A.G. Maddock and co-workers at the UK Atomic Energy Authority had managed to extract about 130 g of the metal, at a cost of US$500,000 from 60 tons of waste material which had accumulated during the extraction of uranium from its ores.

Physical Data

Most of the properties of the metal can be properly investigated due to the production of the metal in 1960. It has a bright metallic luster. The element occurs in pitchblende to the extent about 1 part Pa to 10 million part of ore.

Interatomic distance: 312.2 pm
Melting point: 1572°C
Boiling point: 4000°C (est.)
Thermal conductivity/Wm-1K-1: 47 (27°C)
Density/kgm-3: 15370 (est.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 607 kJ/mol
Gibbs free energy of formation: 563 kJ/mol
Entropy: 198.1 J/mol K
Heat capacity: 22.9 J/mol K

Electronic data

Electronic configuration: [Rn] 5f2 6d1 7s2
Term symbol: 4K11/2
Electron affinity: -   Electronegativity (Pauline): 1.5
Ionization energy (first, second, third): 568.299, -, - kJ/mol

Chemical properties

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