Protactinium

Protactinium is a chemical element with the symbol Pa and atomic number 91. Its longest-lived isotope has a half-life of 32,760 years.

Characteristics

Protactinium is a silver metallic element that belongs to the actinoid group, with a bright metallic luster that it retains for some time in the air. It is superconductive at temperatures 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 (which is formed by the alpha decay of uranium-235 followed by beta decay of thorium-231) could possibly sustain a nuclear chain reaction; Walter Seifritz has estimated that it might in principle be used to build a nuclear weapon with a critical mass of 750±180 kg. Other authors have concluded that such chain reactions would not be possible.

History

In 1890, Mendeleev predicted the existence of an element between thorium and uranium. In 1900, William Crookes isolated protactinium as a radioactive material from uranium; however, he did not identify it as a new element.

Protactinium was first identified in 1913, when Kasimir Fajans and O. H. Göhring encountered the short-lived isotope 234m-Pa (half-life of about 1.17 minute), during their studies of the decay chain of 238-U. They gave the new element the name Brevium (Latin brevis, brief, short); the name was changed to Protoactinium in 1918 when two groups of scientists (Otto Hahn and Lise Meitner of Germany and Frederick Soddy and John Cranston of the UK) independently discovered 231-Pa. The name was shortened to Protactinium in 1949.

Aristid von Grosse prepared 2 mg of Pa₂O₅ in 1927, and in 1934 performed the first isolation of elemental protactinium from 0.1 mg of Pa₂O₅, by converting the oxide to an iodide and then cracking it in a high vacuum with an electrically heated filament by the reaction 2PaI₅ → 2Pa + 5I₂ (iodide process).

In 1961, the United Kingdom Atomic Energy Authority was able to produce 125 g of 99.9% pure protactinium, processing 60 tons of waste material in a 12-stage process and spending 500,000 USD. For many years, this was the world's only supply of the element. It is reported that the metal was sold to laboratories for a cost of 2,800 USD / g in the following years.

Occurrence

Protactinium occurs in pitchblende to the extent of about 1 part ²³¹Pa per 10 million parts of ore (i.e., 0.1 ppm). Some ores from the Democratic Republic of the Congo have about 3 ppm.

Compounds

Known Protactinium compounds:

• Fluorides: PaF₄, PaF₅
• Chlorides: PaCl₄, PaCl₅
• Bromides: PaBr₄, PaBr₅
• Iodides: PaI₃, PaI₄, PaI₅
• Oxides: PaO, PaO₂, Pa₂O₅

See also Protactinium compounds.

Isotopes

29 radioisotopes of protactinium have been characterized, with the most stable being 231-Pa with a half life of 32760 years, 233-Pa with a half-life of 26.967 days, and 230-Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lifes that are less than 1.6 days, and the majority of these have half lifes that are less than 1.8 seconds. This element also has 2 meta states, 217m-Pa (t_½ 1.15 milliseconds) and 234m-Pa (t_½ 1.17 minutes).

The primary decay mode for isotopes of Pa lighter than (and including) the most stable isotope 231-Pa (ie, ²¹²Pa to ²³¹Pa) is alpha decay and the primary mode for the heavier isotopes (ie, ²³²Pa to ²⁴⁰Pa) is beta minus (β⁻) decay. The primary decay products of isotopes of Pa lighter than (and including) 231-Pa are element Ac (actinium) isotopes and the primary decay products for the heavier isotopes of Pa are element U (uranium) isotopes.

Precautions

Protactinium is both toxic and highly radioactive. It requires precautions similar to those used when handling plutonium.

References

External links

• ANL factsheet
• WebElements.com - Protactinium
• It's Elemental - Protactinium
• InfoHaunter