Protactinium

Isotope	Atomic mass (Da)	Isotopic abundance (amount fraction)
²³¹Pa	231.035 88(1)	1

The standard atomic weight of protactinium is determined solely by its isotope ²³¹Pa.

Primordial Pa has long since disappeared from the Earth. The only terrestrial source of Pa is by radioactive decay of heavier elements, almost entirely from uranium, the ores of which contain the two isotopes ²³¹Pa and ²³⁴Pa in equilibrium proportions. ²³⁴Pa is a decay product in the ²³⁸U series, whereas ²³¹Pa occurs in the ²³⁵U series. Owing to its much longer half-life, ²³¹Pa is by far the more abundant protactinium isotope even in uranium samples with more than 99 % ²³⁸U.

The atomic weight of protactinium of a given sample containing uranium depends not only on the geologically recent history of the specimen, but also on the isotopic composition of uranium. Consequently, in the past, the Commission did not recommend a standard atomic weight for protactinium. Subsequently, in 1969, the Commission recommended A_r(Pa) = 231.0359(1), which corresponded approximately to that of pure ²³¹Pa, but which included within its uncertainty also the average atomic weight of Pa in the presence of its radioactive U source. The atomic weight of Pa was dropped from the Table of Standard Atomic Weights in 1983 because it has no long-lived stable isotopes. Nevertheless, the atomic weight of Pa was restored in 1989 when the Commission recommended A_r(Pa) = 231.035 88(2), which corresponded to that of pure ²³¹Pa. The 1989 uncertainty reflects that of the atomic mass determination, expanded according to standard rules for monoisotopic elements. The atomic weight of Pa isotopes in radioactive equilibrium with normal uranium (including both ²³¹Pa and ²³⁴Pa) is not included within this uncertainty because ²³⁴Pa will decay rapidly to insignificance after chemical separation of the Pa from the U.

SOURCE Atomic weights of the elements: Review 2000 by John R de Laeter et al. Pure Appl. Chem. 2003 (75) 683-800
© IUPAC 2003

CIAAW

Protactinium
A_r(Pa) = 231.035 88(1) since 2017

The name derives from the Greek protos (first) for preceding the element actinium, because its most common isotope (²³¹Pa) decays to ²²⁷Ac by loss of an alpha particle.
In 1913 the German chemists K. Fajans and O. H. Gohring identified the first isotope of protactinium, ²³⁴Pa, and proposed the name brevium because of that isotope's short half-life of 6.7 h. ²³¹Pa, with a longer half-life of 3.25(1)×10⁴ a, was identified in 1918 by the German chemist O. Hahn and the Austrian physicist L. Meitner; and, independently in Britain, by F. Soddy and J. A. Cranston.