Protactinium (Pa)
Protactinium (Pa) is a rare and radioactive element with the atomic number 91, belonging to the actinide series of the periodic table, which includes other notable elements like thorium and uranium. It was first predicted by Dmitri Mendeleev in the 1800s but was not isolated until the early 20th century, with significant contributions from chemists Lise Meitner and Otto Hahn. Protactinium appears as a silvery-gray metal, highly reactive with oxygen and water, and is typically found in a solid state at room temperature. It has several isotopes, the most stable being protactinium-231, which has a half-life of 32,700 years.
Due to its extreme rarity—occurring in only a few parts per trillion in the Earth's crust—protactinium is highly expensive to obtain and is primarily sourced from uranium ores or as a byproduct from nuclear reactors. While it has no significant biological function and poses health risks due to its radioactivity, it is mainly utilized in scientific research, necessitating careful handling similar to that of other radioactive materials. Overall, protactinium’s unique properties and scarcity make it a subject of interest in nuclear science and research.
Subject Terms
Protactinium (Pa)
- Element Symbol: Pa
- Atomic Number: 91
- Atomic Mass: 231.0359
- Group # in Periodic Table: n/a
- Group Name: Actinides
- Period in Periodic Table: 7
- Block of Periodic Table: f-block
- Discovered by: Kasimir Fajans, Oswald Helmuth Göhring (1913)
Protactinium is a rare, radioactive element in the periodic table. It is a metal and a member of the actinide group of elements, which includes elements such as thorium, uranium, and plutonium. All fifteen actinides are radioactive with large atomic radii, and they generally have good compound-forming abilities, a characteristic known as valence.
The existence of protactinium was predicted long before it was actually discovered. Dmitri Mendeleev, a Russian chemist, developed the modern periodic table in the mid-1800s. His version of the table contained many gaps between elements. Mendeleev surmised that there should be an element between the elements thorium and radium, but he never discovered such an element for himself. He predicted the existence of several other elements too, believing that they filled the holes in his table. Though correct, his predictions were not confirmed until after his death.
Protactinium continued to elude scientists until 1900, when British chemist William Crookes isolated a mystery element from uranium. However, he could not confirm its identity. In 1913 chemists Kasimir Fajans and Oswald Göhring were studying the products of uranium’s decay chain when they discovered what was later known to be protactinium-234, a short-lived isotope of protactinium that they named "brevium" due to its brief half-life.
It was not until 1918 that protactinium was identified for what it was. In that year two groups of scientists—Lise Meitner and Otto Hahn in Germany and Frederick Soddy and John Cranston in Great Britain—independently isolated the isotope protactinium-231. The discovery of protactinium is officially attributed to Meitner and Hahn, who separated the isotope from uraninite (formerly known as pitchblende), a mineral ore also containing uranium and oxygen. It was not until 1934, however, that protactinium was completely isolated by Aristid von Grosse, who decomposed protactinium iodide into a metal by heating it on a filament.
Physical Properties
Protactinium has many physical properties in common with its fellow actinides uranium and thorium. Protactinium is silvery-gray, with a bright metallic sheen. Its standard state at 298 kelvins (K) is solid. In this state protactinium is denser than thorium, with a density of 15.37 grams per cubic centimeter (g/cm3), but less dense than uranium. Protactinium is highly reactive with oxygen, water vapor, and inorganic acids, and although it can form compounds, it does not react with alkalis. The melting point of protactinium is 1572 degrees Celsius (°C). Its boiling point is 4000 °C. The specific heat of protactinium is 99.1 joules per kilogram-kelvin (J/kg·K). Its electrical conductivity is 5.6 × 106 siemens per meter (S/m), and its thermal conductivity is 47 watts per meter-kelvin (W/m·K). Protactinium is considered a superconductor at temperatures below 1.4 K. It is paramagnetic, which means that it is magnetized when placed within a magnetic field, but it does not retain this property upon removal.
Chemical Properties
The most common oxidation state of protactinium is +5, but it can also assume states of +3 and +4. Protactinium has twenty-nine known isotopes, all of which are radioactive. The most stable of these isotopes is protactinium-231, which accounts for almost all of the naturally occurring protactinium that exists. Protactinium-231 is a product in the decay chain of uranium-251, and it is long lasting, with a half-life of 32,700 years. Proctactinium-233 and proctactinium-230 are the next most stable isotopes, with half-lives of 27 and 17.4 days, respectively. The remaining twenty-six isotopes all have half-lives shorter than 1.6 days, and in most cases shorter than 1.8 seconds.
The protactinium isotopes with mass numbers less than 231 (protactinium’s most stable isotope) are alpha emitters. This means that they undergo a form of radioactive decay known as alpha decay by emitting alpha particles (two protons and two neutrons bonded together), primarily resulting in the production of isotopes of the element actinium. Protactinium isotopes that are heavier than 231 undergo beta decay, which is a form of radioactive decay in which a proton is converted into a neutron (or vice versa) inside the nucleus of an atom. When the atom goes through this process, energy is released in the form of a beta particle (a fast-moving electron or positron). This process usually results in isotopes of uranium. Protactinium has a tetragonal crystal structure.
Applications
Protactinium is one of the rarest of all naturally occurring elements. In Earth’s crust, only a few parts of protactinium per trillion exist, on average, but it can be found in greater quantities in uraninite ores, especially those from Zambia. Protactinium’s scarcity also makes the element one of the most expensive to obtain. In 1961 the United Kingdom Atomic Energy Authority extracted 125 grams of 99.9 percent pure protactinium, and this quantity was the only source of the element for many years. This extraction cost around $500,000, and the protactinium that was obtained was sold for use in scientific research labs. Since then, protactinium is most commonly extracted from the waste of nuclear reactors.
Protactinium serves no major biological role, but it can be ingested in trace amounts by consuming food, drinking water, or breathing in air. The gastrointestinal tract can absorb small amounts of protactinium from these sources; however, most of what enters the body is expelled as waste. Only a small amount (about 0.05 percent) of the protactinium consumed makes its way into the bloodstream. This absorption poses a potential threat to the body, such as cancer, due to the element’s highly radioactive nature. To protect against potential harm when coming in contact with protactinium, those working with the element should exercise caution and follow the same procedures used in the handling of uranium and plutonium; that is, heavy rubber gloves are recommended.
Due to protactinium’s rare, expensive, and radioactive nature, there are no common uses for the element outside of basic scientific research.
Bibliography
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