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Thorium - Protactinium - Uranium

Name, Symbol, NumberProtactinium, Pa, 91
Chemical series Transition metals
Group, Period, Block_ [?], 7, f
Density, Hardness 15370 kg/m3, no data
Appearance bright, silvery metallic luster
Atomic Properties
Atomic weight 231.03588(2) amu
Atomic radius (calc.) 180 (no data) pm
Covalent radius no data
van der Waals radius no data
Electron configuration [Rn]7s²5f²6d¹
e- 's per energy level2, 8, 18, 32, 20, 9, 2
Oxidation states (Oxide) 5 (weak base)
Crystal structure Orthorhombic[?]
Physical Properties
State of matter solid
Melting point 2113 K (2912 °F)
Boiling point 4300 K (7281 °F)
Molar volume 15.18 ×10-3 m3/mol
Heat of vaporization 470 kJ/mol
Heat of fusion 15 kJ/mol
Vapor pressure 5.1E-5 Pa at 2200 K
Velocity of sound no data
Electronegativity 1.5 (Pauling scale)
Specific heat capacity 120 J/(kg*K)
Electrical conductivity 5.29 106/m ohm
Thermal conductivity 47 W/(m*K)
1st ionization potential 568 kJ/mol
2nd ionization potential no data
3rd ionization potential no data
4th ionization potential no data
Most Stable Isotopes
isoNAhalf-life DMDE MeVDP
230Pa{syn.}17.4 dε1.310230Th
231Pa{syn.}32760 a α5.149227Ac
233Pa{syn.}26.967 dβ-0.571233U
SI units & STP are used except where noted.

Protactinium is a chemical element in the periodic table that has the symbol Pa and atomic number 91.

Table of contents

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 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 (lat. 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 Great Britain) independently discovered 231-Pa, and 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 a iodide and then cracking it in a high vacuum by an electrically heated filament by the reaction 2PaI5 → 2Pa + 5I2.

In 1961, the Great Britain 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; this was the world's only supply of the element for many years to come, and it is reported that the metal was sold to laboratories for a cost of 2,800 USD / g in the following years.

Biological Role

Protactinium does not play any biological role.


Proactinium occurs in pitchblende to the extent of about 1 part 231-Pa to 10 million of ore; ores from Zaire have about 3 ppm.


Known Protactinium compounds include:


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 1.7 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 element Ac (Actinium) isotopes and the primary products after are element U (Uranium) isotopes.


Protactinium is both toxical and highly radioactive; it requires precautions similar to those used when handling plutonium.

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