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 yr). Protactinium is pentavalent, resembling niobium and tantalum.
|relative atomic mass||231.036|
|melting point||1,200°C (2,192°F)|
|boiling point||4,000°C (7,232°F)|