Symbol Name Atomic Number Atomic Weight Group Number
Pa Protactinium 91 231.0388


Standard State: Solid at 298 K
Color: Silvery metallic

The first isotope of element 91 to be discovered was a short lived member of the naturally occurring uranium decay series.  It was identified by K. Fajans and O.H. Gohring in 1913 and they named the new element brevium. When the longer-live isotope Pa was identified by Hahn and Meitner in 1918, the name Protoactinium was adopted as being more consistent with the characteristics of the most abundant isotope. In 1959 and 1961, it was announced that the Great Britain Atomic Energy Authority extracted by  12-stage process 125g of 99.9% protactinium, the world's only stock of the metal for many years to come.  Protactinium is one the rarest and most expensive naturally occurring elements.