protactinium (Pa)

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.


atomic number 91
relative atomic mass 231.036
relative density 15.4
melting point 1,200°C (2,192°F)
boiling point 4,000°C (7,232°F)