One of protactinium’s particularities is that it was independently identified (and named) twice, just a few years apart. In 1913, Kasimir Fajans and Oswald Helmuth Göhring discovered a short-lived element 91 (234mPa) — whose half-life was only one minute — which they accordingly named brevium. A few years later, in 1918, when Lise Meitner and Otto Hahn were examining a silica residue extracted from pitchblende, they successfully identified the element that decays into actinium. It turned out to be a different isotope of brevium (231Pa) — but one that had a much longer half-life. This gave their suggested name, protoactinium (which later evolved in the simpler protactinium), the edge over Fajans’ brevium.
Protactinium isn’t really so well-suited to extensive, thorough, detailed characterization — it is rare, difficult to isolate, highly radioactive and toxic. It further confused chemists by its seemingly contradictory properties. Was it a transition metal, as its pentavalent oxidation state seemed to suggest? An actinide, owing to its tetravalent oxidation state available on reduction? The situation became clearer once the place of actinides in the periodic table was established — yet protactinium still differentiates itself from most actinides, find out how in the article.
Despite a period of activity related to thorium-based nuclear power, protactinium hasn’t elicited much interest in terms of practical applications. Its electronic structure however, especially owing to its 5f electron, makes it a very valuable element in computational studies to understand the reactivity of 5f elements. As Richard Wilson observes, “[protactinium’s] future contributions to chemistry may well come from where Meitner and Hahn first found it, in silico”.
Anne Pichon (Associate Editor, Nature Chemistry)