noun. 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. cross-reference: Periodic Table.

The radioactive chemical element, of atomic number 91; Atomic symbol Pa; at. wt. of longest-lived isotope, 231 (T1/2 = 32,500 yrs.) Also called brevium, Uranium X2 and UX2.

noun. A chemical element (symbol Pa) with atomic number 91.

noun. a short-lived radioactive metallic element formed from uranium and disintegrating into actinium and then into lead

"An element discovered in 1982 has been named meitnerium to honor Lise Meitner (1878-1968), the Austrian-Swedish physicist and mathematician who discovered the element protactinium and made major contributions to the understanding of nuclear fission."

actinium  

eka-protactinium  

metal  metallic element