Protactinium . Chemical element of the periodic table whose symbol is Pa and its atomic number is 91. Protactinium is a silvery metallic element that belongs to the group of actinides , which has an intense metallic luster.

Protactinium is, from the formal point of view, the third member of the actinides and the first in which 5f electrons appear, but its chemical behavior in aqueous solution resembles that of tantalum and niobium more than that of the other actinides.


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  • 1 Discovery
  • 2 Abundance and natural state
  • 3 Preparation
  • 4 Properties
    • 1 Property Values
  • 5 Isotopes
  • 6 Health effects
  • 7 Environmental effects
  • 8 Applications
  • 9 Compounds
  • 10 See also
  • 11 External links
  • 12 Sources


In 1913 Fajans and Göhring discovered isotope 234 with a half-life of 77 seconds, which is why they called it a brevium.

In 1917 isotope 231 was obtained, with a period of 32,700 years, by two independent teams, Otto Hahn and Lise Meitner in Germany and JA Cranston, A. Fleck and F. Soddy in England .

This isotope emits alpha particles, forming actinium, which is why at first it was given the name of protoactinium (“progenitor of actinium”), since when the isotope 231Pa was radioactively decomposed, actinium was obtained, until in 1949 it was adopted the current name of protactinium.

Abundance and natural state

It is one of the rarest elements in nature and more than 20 isotopes are known, the most common of which is 231 Pa.

Its presence in the earth’s crust is estimated to be around 0.00001 ppb.

It is formed by transmutation of thorium into the natural radioactive family of 235U, which is why it is found in the minerals of this element.

It is found in pitchblende in a proportion of 0.1 ppm while the concentration of 231 Pa Pa in the minerals of Zaire is 30 times higher.


In 1927, Grosse prepared 2 mg of a white powder, which turned out to be Pa 2 O 5 . Seven years later, starting from 0.1 g of pure Pa 2 O 5 , he isolated the element by two methods, one of which consisted in converting the oxide to iodide and treating it with an incandescent filament and high vacuum:

2PaI 5 -> 2Pa + 5I 2

It is normally obtained as a product of the fission of uranium, thorium and plutonium . It is also recovered from uranium ores by solvent extraction and the P to F 4 produced is reduced with barium .


It is a shiny, metallic, radioactive solid. Its shine slowly disappears if exposed to air. Above 1.4 K it behaves as a superconductor. Isotopes with a mass number between 215 and 238 are known. The most stable isotope, 231Pa, has a half-life of 32,700 years and evolves to actinium by emission of alpha particles.

It emits alpha particles and its dangerousness from the radiological point of view is similar to that of polonium, so it requires careful handling similar to that used to handle other radioactive elements such as plutonium. Isotopes with mass numbers 216, 217, and 222-238 are radioactive. Only 231 Pa, 234 Pa and 234m Pa are present in nature. The most important of them is 231 Pa, an alpha emitter with a half-life of 32,500 years. The artificial isotope 233Pa is an important intermediate in the production of the fissile 233 U. Both 231 Pa and 233Pa can be synthesized by neutron irradiation of thorium. Metallic protactinium is silvery, malleable, and ductile. Samples exposed to air at room temperature show little or no tarnish after several months. The many protactinium compounds that have been prepared and characterized are binary and polinary oxides, halides, oxyhalogenides, sulfates, oxysulfates, double sulfates, oxynitrates, selenates, carbides, organometallic compounds, and noble metal alloys. It is superconductive below 1.4 K .

Property Values

  • Atomic Mass: 231.03588 amu
  • Melting Point: 1841 K
  • Boiling Point: 4300 K
  • Density: 15370 kg / m 3
  • Normal Reduction Potential: – 1.20 V Pa5 + | Pa
  • Specific Heat: 120.00 J / kg ºK
  • Heat of Fusion: 16.7 kJ / mol
  • Heat of Vaporization: 481.0 kJ / mol
  • Heat of Atomization: 527.0 kJ / mol of atoms
  • Oxidation States: +3, +4, +5
  • 1st Ionization Energy: 568 kJ / mol
  • Atomic Volume: 15 cm 3/ mol
  • Polarizability: 25.4 Å 3
  • Electronegativity (Pauling): 1.5
  • Melting point (ºC): 1572
  • Boiling point (ºC): 4200
  • Valencia: 4.5
  • Electronegativity: 1.5
  • Atomic Radius: 1.63 Å
  • Ionic Radius: Pa + 3 = 1.08 Å Pa + 4 = 0.91 Å
  • Ionization Energy (kJ / mol): 568
  • Oxidation state: +4

It is reactive, giving hydrogen when it reacts with water vapor. It also reacts with oxygen and acids. It acts with the oxidation states +4 and +5 and various halides of the element and other compounds have been prepared, some of which are colored.


29 radioisotopes of protactinium have been characterized, the most stable being 231 Pa, with a half-life of 32,760 years; the 233 Pa, with a half life of 26.967 days; and 230 Pa with a half-life of 17.4 days. The rest of the radioactive isotopes have half-lives of less than 1.6 days and most have half-lives of less than 1.8 seconds. This element also has two metastates , 217m Pa (half life of 1.15 milliseconds) and 234m Pa (half life of 1.17 minutes).

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

Health effects

Protactinium is generally a health hazard only if it enters the body, although there is a small external risk associated with the gamma rays emitted by protactinium 231 and a number of short-lived products of the decay of protactinium 227. Protactinium can be taken into the body by ingesting food, water, or breathing air. When protactinium is inhaled, a significant fraction can move from the lungs through the blood to other organs, depending on the solubility of the compound. Gastrointestinal absorption from food or water is a likely source of internal protactinium deposition in the general population. Most of the protactinium taken by ingestion will quickly leave the body with the feces; only around 0, 05% of the ingested amount is absorbed from the intestinal tract into the bloodstream. After leaving the intestine or lung, about 40% of the protactinium that enters the bloodstream is deposited in the skeleton, about 15% in the liver, about 2% in the kidneys, and the rest is excreted. The biological half-life in the skeleton is around 50 years. Of the protactinium deposited in the liver, it is assumed that 70% is retained with a mean biological life of 10 days, with the remaining 30% having a mean biological life of 60 days. Of the protactinium deposited in the kidneys, it is assumed that 20% is retained with a biological life of 10 days, with the remaining 80% having a medium biological redistribution. The greatest health concern is cancer resulting from ionizing radiation emitted by protactinium deposited in the skeleton, liver, and kidneys. The health risks associated with protactinium 234m are included with those of uranium 238. Protactinium 234m disintegrates emitting a high energy beta particle so precautions against this radiation must be taken when handling uranium; for example, hard rubber gloves are used to protect the hands and arms.

The risk of inhalation of protactinium 231 is one of the highest of all radionuclides. Actin 227 and its decay products are responsible for more than 80% of this inhalation risk. While the risk factor for ingestion is much lower than for inhalation, ingestion is generally the most common form of entry into the body. Similar to other radionuclides, the risk coefficient for tap water is 75% of that for ingestion with the diet. In addition to the risk from internal exposure, there is a risk from external exposure to protactinium 231 gamma rays.

Environmental effects

Protactinium occurs naturally in soil, rocks, surface and groundwater, plants and animals in very low concentrations, on the order of one part per trillion, or 0.1 picocuries (pCi) / g. Higher levels are present in uranium ores and other geological materials. Essentially all naturally occurring protactinium is present as protactinium 231. Protactinium preferentially adheres well to soil, and the concentration associated with sandy soil particles is normally 550 times higher than in interstitial water (the water in the space that exists between soil particles); concentration ratios are even higher (over 2,000 and more) in loam and clay soils.

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



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 a 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 231 Pa.


Known protactinium compounds:

  • Fluorides
    • PaF 4
    • PaF 5
  • Chlorides
    • PaCl 4
    • PaCl 5
  • Bromides
    • PaBr 4
    • PaBr 5
  • Iodides
    • PI 3
    • PI 4
    • PI 5
  • Oxides
    • PaO
    • PaO 2
    • Pa 25


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