Protactinium is a member of the actinide series, which stretches from actinium (Ac) to lawrencium (Lr).
Kasimir Fajans and others predicted the existence of an element like tantalum in the early part of this century. The element itself was discovered independently by Otto Hahn and Lise Meitner, by Fajans, and by Soddy, J.A. Cranston, and A. Fleck in 1917. In each case the groups extracted the element from pitchblende, an ore known to contain some of the actinide elements and others such as polonium and radium.
In 1927 Aristid V. Grosse was successful in preparing 2 mg of white Pa2O5, protactinium(V) oxide. This can be converted to the iodide, PaI5, which is then reduced on a heated tungsten filament to give the metal.
2 PaI5 + heat 2 Pa + 5 I2
The name is taken from the Greek words proto and actinium which mean "parent of actinium".
Pa metal is shiny and silvery-white and oxidizes only slowly in air.
Although naturally occuring, it is one of the rarest of elements. As suggested in the lanthanide series (see praseodymium, for example), elements of odd atomic number are much less abundant than those of even atomic number, and Pa has an atomic number of 91.
The world's supply of pure Pa is only 125 g. Since there are no uses for the element, this supply is likely to last for some time. (To obtain this quantity of metal, 60 tons of ore were processed.)
Halides of Pa have been isolated where Pa has oxidation states of +5, +4, and +3. Among the known compounds are yellow PaCl5 and green-yellow PaCl4.
Twenty isotopes of Pa are known, with mass numbers ranging from 216 to 238. All are radioactive.