Protactinium Pa

Pa, protactinium, was first identified in 1913 in the decay products of U-238 as the Pa-234 isotope (6.7 h) by Kasimir Fajans and Otto H. Gohring. In 1916, two groups, Otto Hahn and Lisa Meitner, and Frederick Soddy and John A. Cranston, found Pa-231 (10 years) as a decay product of U-235. This isotope is the parent of Ac-227 in the U-235 decay series, hence it was named protactinium (before actinium). Isolation from U extraction sludges yielded over 100 g in 1960. 

Th Thorium 232.03S1 91 Pa Protactinium 92 U Uranium 93 Np Neptunium 94 Pu Plutonium 95 Am Americium 96 Cm Curium 97 Bk Berkelium 98 Cf Californium 99 ES Einsteinium 100 Fm Fermium 101 Md Vlendelevium 102 No Nobelium Lr Lawrencium... 

Ho Holmium Er Erbium Tm Thulium Yb Ytterbium Lu l.utetium f Hf Hafnium Ta Tantalum W Wolfram f Re Rhenium Os Osmium Ir Iridium Pt Platinum Au Gold Hg Mercury Tl Thallium Pb Lead Bi Bismuth Po Polonium At Astatine f Rn Radon Fr Francium f Ra Radium Ac Actinium Th Thorium Pa Protactinium U Uranium Np Neptunium f Pu Plutonium f Am Americium f Cm Curium f Bk Berkelium f Cf Californium i... 

Pa protactinium, 91, 1917 from proto (first) and actinium, because it disintegrates into actinium. 

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