Element Protactinium, Pa, Actinide or Actinoid

During increasing the uranium transformation research, it was becoming more and more clear that actinium is a product of such transformation. The constant ratio of the actinium and uranium concentrations in uranium ores is one of the confirmations of that. However the sequence of such transformation, the links of the transformation chain were still unclear. Soddy, Fajans and Russell independently predicted the existence of radioactive element, the member of uranium series which, as an analogue of tantalum, eka-tantalum, was supposed to occupy the place below vanadium. And, indeed, Otto Hahn and Lise Meitner in 1917 as well as Soddy, Cranston and Fleck one year later discovered the new element-91 which became the first in actinium series, a source of alpha-radiation with actinium as a decay product. The element was called protactinium, as the first element in the series before actinium. Protactinium is a member of actinide (or actinoid) group of chemical elements.

The primary protactinium generated in the beginning of the Earth forming, had been already completely decayed, died out. Nevertheless native protactinium still exists in very small quantities, estimated as 10^-10%; it is contained in uranium ores, approximately 3x10^-7 parts on a uranium part, as the ²³¹Pa isotope, the product of uranium-235 decay.