Protactinium (Pa)

Isotopes of Protactinium

Isotope Atomic Mass Half-life Mode of Decay Nuclear Spin Nuclear Magnetic Moment
Pa-228 228.03100 22 hours EC to Th-228;
α to Ac-224
3 3.5
Pa-229 229.03209 1.50 days EC to Th-229;
α to Ac-225
5/2 No data available 
Pa-230 230.03453 17.04 days EC to Th-230;
α to Ac-226;
β- to U-230
2 2.0
Pa-231 231.035880 32,500 years α to Ac-226; SF 3/2 2.01
Pa-232 232.03858 1.31 days EC to Th-232;
β- to U-232
2 No data available 
Pa-233 233.04024 27.00 days β- to U-233 3/2 4.0
Pa-234 234.04330 6.69 hours β- to U-234 4 No data available 


Protactinium was discovered in 1913 by Otto Hahn, Lise Meitner, Frederick Soddy and John Cranston. Its name originates with the Greek word protos, meaning “first.”

A shiny white metal with bright metallic luster, protactinium is also hard and malleable, with a body-centered tetragonal structure. It has a vapor pressure of 3.88 x 10-2 torr at about 1930 ºC and is superconducting below -271.75 °C. In solution, Pa4+ is oxidized to Pa5+ by atmospheric oxygen. The chemistry of pentavalent protactinium is quite similar to that of niobium and tantalum. A number of salts and complexes are known. No commercial application of protactinium isotopes is known outside of scientific research. For this purpose, protactinium is generally extracted from spent nuclear fuel.

Protactinium is a very dangerous substance to work with. It is highly toxic and presents a radiation hazard (alpha emitter). The Proactinium-231 isotope is a long-lived alpha-emitter and is not excreted readily. Exposure can cause cancer.

Properties of Protactinium

Name Protactinium
Symbol Pa
Atomic number 91
Atomic weight 231.03588
Standard state Solid at 298 ºK
CAS Registry ID 7440-13-3
Group in periodic table N/A
Group name Actinoid
Period in periodic table 7 (Actinoid)
Block in periodic table f-block
Color Silvery metallic
Classification Metallic
Melting point 1568 °C
Boiling point 4000 °C
Thermal conductivity 47 (estimate) W/(m·K)
Electrical resistivity 18 x 10-8 Ω·m
Electronegativity 1.5
Heat of vaporization 470 kJ·mol-1
Heat of fusion 15 kJ·mol-1
Density of solid 15.370 g/cm3
Electron configuration [Rn]5f26d17s2
Atomic radius 1.63 Å (coordination number 12)
Oxidation states +4, +5

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