n. Symbol Pa A rare, extremely toxic, radioactive, lustrous, metallic element having more than 30 known isotopes and isomers, the most common of which is Pa-231 with a half-life of 32,760 years. Atomic number 91; atomic weight 231.036; melting point 1,572°C; specific gravity 15.37 (calculated); valence 2, 3, 4, 5. See Periodic Table. |
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