Protactinium, element 91, is one of the most rare of the naturally occurring elements and may well be the most difficult of all to extract from natural sources. Protactinium is, formally, the third element of the actinide series and the first having a 5f electron. The superconducting properties of protactinium metal provide clear evidence that Pa is a true actinide element (Smith et al., 1979). Its chemical behavior in aqueous solution, however, would seem to place it in group VB of the Mendeleev's table, below Ta and Nb.
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