Protactinium, Physical and Chemical Properties

Protactinium is a radioactive metal formed in the decay chain of uranium but not thorium (Fig. 1). It exists in two crystalline forms: α-Protactinium is tetragonal body-centered (a = 393.1 pm, c = 323.6 pm) and β protactinium is cubic face-centered (a = 501.9 pm). Protactinium isotope ²³¹Pa (t _1/2 = 32,480 years) is the most important of all 24 known isotopes. The isotope ²³³Pa is of special importance because it is formed as an intermediate in the production of fissile ²³³U from non-fissile ²³²Th by neutron capture:

Protactinium, Physical and Chemical Properties, Fig. 1

Protactinium in the decay scheme of uranium