Protactinium is one of the rarest naturally occuring elements with an approximate abundance in the earth's crust of 1.4 parts per trillion. Originally named "brevium" by Kasimar Fajans and Oswald Helmut Göhring from the Latin word brevis meaning brief, due to the very short half life of the protactinium-234 isotope they observed, the IUPAC name "protactinium" was chosen after Otto Hahn and Lise Meitner discovered the more stable protactinium-231 isotope. This name was derived from the Greek word protos meaning first or before, therefore the name protactinium refers to this element being the parent of actinium in the nuclear decay chain of uranium-235. There are 29 known radioisotopes of protactinium with half lifes ranging from 53 nanoseconds for protactinium-219 to 32,760 years for protactinium-231. Protactinium primarily exhibits the +4 and +5 oxidation states (although +2 and +3 are also possible) and forms a variety of inorganic and organometallic compounds. Protactinium-231 is also used in conjunction with thorium-230 as a double-isotope tracer for radiometric dating, particularly for marine sediment.
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