A rare, radioactive, metallic element of the actinide series, first identified in 1913. Its chief source is uraninite. More than 12 isotopes are known; all radioactive. Its most stable isotope is 231Pa (half-life 3.25 × 104 yr). Protactinium is pentavalent, resembling niobium and tantalum.
Related category INORGANIC CHEMISTRY
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