Name Protactinium
Symbol Pa
Atomic Number 91
Atomic Mass 231.036 atomic mass units
Number of Protons 91
Number of Neutrons 140
Number of Electrons 91
Melting Point 1600.0 C
Boiling Point Unknown
Density 15.4 grams per cubic centimeter
Normal Phase Solid
Family Rare Earth Metals
Period 7
Cost $280 per gram



Origin of Name From the Greek protos, meaning first, combined with actinium
Date and Place of Discovery In 1917 (see below)
Discovered by Otto Hahn and Lise Meitner in Berlin
Kasimir Fajans in Germany and
John Cranson, A. Fleck and Fredrich Soddy in Glasgow.
Common Compounds
  • Protactinium bromide
  • Protactinium chloride
  • Protactinium fluoride
  • Protactinium iodine
  • Protactinium oxide
Interesting facts
  • It is one of the most rare and expensive elements.
  • It is superconductive at temperatures below 1.4 K.
  • It has 29 radioisotopes which have been characterized.
  • The isotope, Protactinium-231, formed by alpha decay of Uranium-235, might create a nuclear chain reaction which might be used in a nuclear weapon.
Common Uses There are no known uses outside of the laboratory because of its high radioactivity and high toxicity.


Photo Courtesy of
Chemical Elements
Jefferson Lab
Mrs. Purdy's Web Page
Web Elements

Protactinium Atomic Structure Elements by Name Elements by Number Home