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protactinium (Pa)
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) |
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