Protactinium

Protactinium is a dense, silvery actinide metal with the configuration 5f2 6d1 7s2. It sits at a turning point in the actinide series: like thorium before it, it often behaves as a transition metal, most commonly forming the +5 oxidation state, similar to tantalum above it in group 5. Its 5f and 6d orbitals are close in energy, which gives it a mix of behaviours. In water, protactinium(V) compounds tend to hydrolyse and stick to the walls of glass containers, making it notoriously awkward to handle.

All its isotopes are radioactive. Protactinium-231, with a half-life of 32,760 years, belongs to the uranium-235 decay chain and decays to actinium-227, which is how the element got its name. Protactinium-233, with a 27-day half-life, is a key intermediate in thorium reactors, where thorium-233 decays into it before it becomes uranium-233. Because the ratio of protactinium-231 to thorium-230 in seafloor sediments changes predictably with time, oceanographers use it to reconstruct past ocean circulation.

Discovery

Kasimir Fajans and Oswald Göhring identified the short-lived isotope protactinium-234m in 1913 and called it brevium for its brief life. In 1917–18 Otto Hahn and Lise Meitner in Berlin, and independently Frederick Soddy and John Cranston in Britain, found the long-lived isotope protactinium-231. Hahn and Meitner named it protoactinium, 'before actinium', because it decays into actinium; the name was shortened to protactinium in 1949.