BOILING POINT: Unknown
DENSITY: 15.37 g/cm 3
MOST COMMON IONS: Pa 4+ , PaO(OH)2 +
An isotope of protactinium (having mass number 234 and a half-life of 1.1 minutes) was first identified by Kasimir Fajans and O. Gohring in 1913 as a short-lived member of the naturally occurring 238 U decay series and was given the name brevium , meaning brief. The existence of protactinium was confirmed in 1918 when another isotope of protactinium (of mass 231 and a half-life of 3.3 × 10 4 years) was studied independently by Otto Hahn and Lise Meitner in Germany, and by Frederick Soddy and John Cranston in Great Britain. The current name of the element is a shortened version of the original protoactinium , derived in part from the Greek protos, meaning parent; protoactinium thus meant parent of actinium (its decay product). There are twenty-four known isotopes of Pa, having mass numbers ranging from 214 to 238, the most stable isotope being 231 Pa. Protactinium metal is silvery and relatively nonreactive. It occurs at ppm levels in uranium ores and is extracted from these ores. There are about 125 grams (4.4 ounces) of protactinium in the world today. Its ground state electronic configuration is [Rn]5 f 2 6 d 1 7 s 2 , placing it in Group IIIB. Its principal oxidation state is +5, but there is no stable Pa 5+ ion because it is hydrolyzed so quickly to species such as PaO(OH) 2+ , or forms complexes with anions such as fluoride. Protactinium in its +4 state may exist in aqueous solution or in compounds. The most important solid compound of protactinium is Pa 2 O 5 .
Cotton, F. Albert, and Wilkinson, Geoffrey (1988). Advanced Inorganic Chemistry, 5th edition. New York: Wiley.