Protactinium

Protactinium is a chemical element on the periodic table whose symbol is Pa and its atomic number is 91.

The element was first identified in 1913 by Kazimierz Fajans and Oswald Helmuth Göhring and named brevium due to the short half-life of the specific isotope studied, ie protactinium-234. A more stable isotope of protactinium, ²³¹Pa, was discovered in 1917/18 by Lise Meitner in collaboration with Otto Hahn, and they chose the name protactinium. IUPAC chose the name "protactinium& #3. 4; in 1949 and confirmed Hahn and Meitner as discoverers. The new name meant "(nuclear) precursor of actinium" and reflected that actinium is a product of the radioactive decay of protactinium. John Arnold Cranston (working with Frederick Soddy and Ada Hitchins) is also credited with discovering the more stable isotope in 1915, but he delayed his announcement due to being called up for service in World War I.

The longest-lived and most abundant (nearly 100%) naturally occurring isotope of protactinium, protactinium-231, has a half-life of 32,760 years and is a decay product of uranium-235. Much smaller trace amounts of the short-lived protactinium-234 and its nuclear isomer protactinium-234m occur in the uranium-238 decay chain. Protactinium-233 results from the decay of thorium-233 as part of the chain of events used to produce uranium-233 by neutron irradiation of thorium-232. It is an unwanted intermediate in thorium-based nuclear reactors and is therefore removed from the active zone of the reactor during the breeding process. Ocean science uses the element to understand the ancient ocean. Analysis of the relative concentrations of various isotopes of uranium, thorium, and protactinium in water and minerals is used in radiometric dating of sediments up to 175,000 years old and in modeling various geological processes.

Notable Features

Protactinium is a silvery metallic element that belongs to the group of actinides, which has an intense metallic luster.

It is superconducting below 1.4 K.

Applications

Due to its scarcity, high radioactivity, and toxicity, there are currently no uses for protactinium outside of basic scientific research.

Protactinium-231 (formed by alpha decay of uranium-235 followed by beta decay of thorium-231) could perhaps sustain a nuclear chain reaction and, in principle, could be used to build a nuclear bomb. The critical mass, according to Walter Seifritz, is 750±180 kg. Other authors conclude that a chain reaction is not possible using ²³¹Pa.

History

Dimitri Ivanovic Mendeleev predicted in 1871 that there should be an element with an atomic number equal to 91 and that it would appear on the periodic table between thorium and uranium. In 1900 William Crookes isolated protactinium as a radioactive material formed from uranium, but he failed to identify it.

Protactinium was first identified in 1913 when Kasimir Fajans and O.H. Göhring found the short-lived isotope ^234mPa, with a half-life of about 1.17 minutes, during his studies of the decay chain of ²³⁸U. They gave the new element the name Brevium (Latin: brevis, meaning brief). The name was changed to Protoactinium (progenitor of actinium) in 1918 when two groups of scientists (Otto Hahn and Lise Meitner from Germany, and Frederick Soddy and John Cranston from the UK) independently discovered the ²³¹Pa, and shortened the name to protactinium (in Spanish, protoactinium) in 1949.

Aristid V. Grosse prepared 2 mg of Pa₂O₅ in 1927, and later succeeded in isolating protactinium for the first time in 1934 from 0.1 mg of Pa ₂O₅, first converting the oxide to an iodide and then breaking it under high vacuum using a filament heated by an electric current producing the reaction 2PaI₅ > → 2Pa + 5I₂ (Van Arkel-De Boer process).

In 1961, the United Kingdom Atomic Energy Authority (UKAEA) was able to produce 125 g of 99.9% pure protactinium by processing 60 tons of waste material in a process of 12 stages with an expense of $500,000. This was the only source of this element for many years and it is said to have been sold to laboratories at a cost of $2800/g in subsequent years.

Abundance and obtaining

Natural protactinium is produced by the radioactive disintegration of uranium and thorium, therefore it is found in all its minerals, its concentration being proportional to the percentage of said elements. In pitchblende it is found in an amount of about one part ²³¹Pa in 10 million ore (ie 0.1 ppm). Some mines in the Democratic Republic of the Congo have as low as 3 ppm.

Compounds

Known protactinium compounds:

• Fluoride
  • PaF₄
  • PaF₅
• Clotrines
  • PaCl₄
  • PaCl₅
• Bromurs
  • PaBr₄
  • PaBr₅
• Ioduros
  • PaI₃
  • PaI₄
  • PaI₅
• Oxidities
  • PaO
  • PaO₂
  • Pa₂O₅

Isotopes

29 protactinium radioisotopes have been characterized, the most stable being ²³¹Pa, with a half-life of 32,760 years; ²³³Pa, with a half-life of 26,967 days; and ²³⁰Pa with a half-life of 17.4 days. The rest of the radioactive isotopes have half-lives less than 1.6 days, and most have half-lives less than 1.8 seconds. This element also has two metastates, ^217mPa (half-life 1.15 milliseconds) and ^234mPa (half-life 1.17 minutes).

The primary decay mode of the more stable isotope ²³¹Pa and of those lighter ones is alpha decay while for the heavier isotopes it is beta decay. The primary decay products of the lighter isotopes (²³¹Pa or lighter) are actinium (Ac) isotopes while the heavier isotopes produce uranium (U) isotopes.

List of isotopes

Table references:

Precautions

Protactinium is toxic and highly radioactive. For this reason, it requires precautions similar to those used when handling plutonium.