Protactinium
Protactinium, a metallic, radioactive chemical element found in all uranium ores. The longest-lived isotope of protactinium has a half-life of 34,300 years; the shortest-lived, a half-life of two seconds. Protactinium was discovered in 1917 by two separate groups of scientists—by Frederick Soddy and John A. Cranston and by Otto Hahn and Lise Meitner. The element was originally called protoactinium (meaning parent of actinium") because it decays into actinium.
Symbol: Pa. Atomic number: 91. Atomic weight of most stable isotope: 231. Melting point: about 2,700 F. (1,500 C.). Specific gravity: 15.4. Protactinium has 13 known isotopes: Pa-225 to Pa-237. It belongs to the actinide series of the Periodic Table and may have a valence of +4 or +5.