Protactinium
Symbol: Pa
Atomic Number: 91
Atomic Mass: 231.03587 amu (atomic mass units)
Melting Point: 1600.0 C (1873.15 K, 2912.0 F)
Boiling Point: Unknown
Number of Protons: 91
Number of Electrons: 91
Number of Neutrons: 140
Classification: Rare Earth Metal
Crystal Structure: Orthorhombic
Density @ 293 K: 15.4 grams per cubic centimeter
Color: Silver
Protactinium is one of the few naturally occurring members of the actinide or actinoid group of rare earth elements. It is the most expensive and one of the rarest of all the naturally occurring elements found on earth.
It was originally called brevium by Kasimir Fajans and O.H. Gohring who identified the isotope protactinium-134 in 1913. This isotope which has a half-life of 1.17 minutes was identified as part of their study on the decay of uranium. The name they gave to the element meant brief and related to this isotopes short half-life.
In 1918 two separate groups of scientists, Otto Hahn and Lise Meitner in Germany and Frederick Soddy and John Cranston in the United Kingdom independently discovered the isotope protactinium-231. This isotope has a half-life of 32760 years so the name brevium was no longer suitable for the element. They called the element protoactinium meaning “parent of actinium”. This relates to the fact that the protactinium-231 decays by alpha decay to form actinium-227. In 1949 the name was shortened from protoactinium to protactinium.
The pure metal was not isolated until 1934 by Alfred Grosse. He developed two methods for this. One method was the reduction of protactinium pentoxide using a stream of electrons in a vacuum. The other method was heating protactinium iodide in a vacuum.
Protactinium is highly radioactive and toxic. Handling the element requires using precautions similar to those taken when handling the element polonium. Below a temperature of 1.14 K protactinium is super-conductive.
There are no stable isotopes of protactinium. Numerous unstable isotopes of this element have been discovered with mass numbers ranging from 212 to 240. Protactinium-219 has the shortest half-life at 53 nanoseconds. The most commonly found isotope is protactinium-231.
It is found in very small quantities in the mineral ore pitchblende. In 1961 the Atomic Energy Authority of Great Britain produced 125 grams of protactinium which was 99.99% pure. To do this they had to process 55000 kilograms of ore and it cost them 500000 dollars.
The radioactive nature of protactinium combined with its toxicity and rarity means that the element has no uses outside of scientific research within specialized laboratories.
