Protactinium (Pa) is 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 years). Protactinium is pentavalent, resembling niobium and tantalum.