Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.
Protactinium is one of the rarest elements on Earth. It is formed when uranium and other radioactive elements break down. For many years, the only supply of protactinium of any size was kept in Great Britain. The British government had spent $500,000 to extract 125 grams (about four ounces) of the element from 60 tonnes (60 tons) of radioactive waste. Relatively little is known about the properties of the element, and it has no commercial uses.
Discovery and naming
Scientists first learned about radioactive elements toward the end of the nineteenth century. Radioactive elements are elements that break apart all by themselves. They give off radiation—somewhat similar to light or X rays—and change into new elements. Radiation is energy transmitted in the form of electromagnetic waves or subatomic particles.
For example, the element uranium is radioactive. It emits radiation over very long periods of time. It begins to change into other elements. One of those elements is protactinium.
Many naturally occurring isotopes are radioactive. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.
Many of the radioactive isotopes that occur in nature are related to each other. For example, when uranium-238 breaks apart, it forms a new isotope, thorium -238. But thorium-234 is radioactive also. It breaks apart to form radium -230. And radium-230 is also radioactive. It breaks apart to form actinium -230.
This process often goes on for a dozen steps or more. Finally, an isotope is formed that is not radioactive. The chain—or "family" of radioactive isotopes—comes to an end.
During the early 1900s, scientists were trying to understand these radioactive families. They were trying to identify all the elements found in a family. In doing so, they sometimes found new elements. Such was the case with element number 91. Many scientists had been looking for element number 91 for some time. There was an empty box in the periodic table for element 91. That meant that a new element was yet to be found. Some scientists decided to look in the radioactive families for that element.
In 1913 German-American physicist Kasimir Fajans (1887-1975) and his colleague, O. H. Göhring, claimed to have found element number 91. They suggested the name brevium for the element. They chose the name because the half life of the isotope they found was very short ("brief"). It was only 1.175 minutes.
The half life of a radioactive element is the time it takes for half of a sample of the element to break down. That means that 10 grams of the isotope they studied would break down very quickly. Only 5 grams would be left after 1.175 minutes. Then 2.5 grams (half of 5 grams) would be left after another 1.175 minutes, and 1.25 grams (half of 2.5 grams) after another 1.175 minutes, and so on. Until the discovery by Fajans and Göhring, the element had been known as uranium-X2. That name came from the element's position in one of the radioactive families.
In 1918, another isotope of element number 91 was discovered by German physicists Lise Meitner (1878-1968) and Otto Hahn (1879-1968). This isotope had a half life of 32,500 years. It was much easier to study than the isotope discovered by Fajans and Göhring.
The new element was originally given the name protoactinium, meaning "first actinium." It comes from the way the element breaks down. Its first product is the element actinium. In 1949, the element's name was changed slightly to its current form, protactinium.
Protactinium is a bright shiny metal. When exposed to air, it combines easily with oxygen to form a whitish coating of protactinium oxide. Its melting point is thought to be about 1,560°C (2,840°F) and its density about 15.37 grams per cubic centimeter.
Protactinium forms compounds with the halogens (fluorine, chlorine, bromine, and iodine ) and with hydrogen. But these compounds have not been studied in detail.
Occurrence in nature
The amount of protactinium in the Earth's crust is too small to estimate accurately. Its most common ore, pitchblende, contains about 0.1 part per million of protactinium.
About 20 isotopes of protactinium are known. All are radioactive. (See "Discovery and naming" for a more detailed explanation of isotopes.)
Protactinium does not occur naturally.
Lise Meitner | Austrian physicist
U ntil recently, science has often been a difficult occupation for women. Male scientists once believed that women did not have the mental powers to do good research. Women who became famous scientists usually had to be outstanding in their own field, and they had to overcome the strange prejudices of their male colleagues.
No one knew more about discrimination in science than Lise Meitner (1878-1968). Meitner was born in Vienna, Austria, on November 7, 1878. She learned about the work of Marie Curie while in high school and decided to pursue a career in science. She earned her Ph.D. degree in physics in 1906.
After working as a nurse during World War I (1914-18), Meitner took a job at the University of Berlin. At first, she had to overcome huge obstacles. Her superior would not allow her to work in a laboratory if men were present. He had a tiny laboratory built for her in a closet.
Meitner persevered, however. She eventually became a professor of physics at the school and also served as co-director of the Kaiser Wilhelm Institute in Berlin. The other co-director at this famous research institution was Otto Hahn (1879-1968), a physicist with whom Meitner worked throughout most of her career.
Meitner's career took an unexpected turn in the 1930s. When Adolf Hitler (1889-1945) came to power in Germany, he began to rid the universities of anyone with a Jewish background. Although Meitner had been baptized as a Christian, she came from a Jewish family. She soon realized that her life would be in danger if she stayed in Berlin. So she escaped from Germany in 1938 and took a position in Copenhagen, Denmark.
One discovery for which Meitner and Hahn are known is the discovery of protactinium. They found the element while searching through the products of a nuclear reaction that had only recently been discovered. In fact, the ability of Hahn and Meitner to unravel the nature of that reaction proved to be even more important than the discovery of protactinium.
The reaction in question was one that occurs when neutrons (tiny particles that occur in atoms) are fired at uranium atoms. The reaction had been carried out by a number of scientists, but only Meitner and Hahn figured out what had actually taken place. In 1939, they wrote a paper explaining the reaction. They said that neutrons caused uranium atoms to fission, or split apart.
Meitner and Hahn had described for the first time one of the most important reactions in all of human history: nuclear fission. Nuclear fission later became the basis for weapons, such as the atomic bomb, and useful applications, such as nuclear power plants. For his role in this discovery, Hahn was awarded a share of the 1944 Nobel Prize in Chemistry. Meitner, who had contributed at least as much as Hahn, never received a Nobel Prize for her work. Scholars are still debating the reasons that Meitner's brilliant work was ignored by the Nobel Prize committee in 1944.
Uses and compounds
Neither protactinium nor its compounds have any commercial uses. It can be purchased in small amounts today from the Oak Ridge National Laboratory in Oak Ridge, Tennessee. It costs about $300 per gram.
Protactinium is very radioactive and highly dangerous. Researchers who work with it must take extreme cautions to protect themselves from its radiation.
An isotope of protactinium (having mass number 234 and a half-life of 1.1 minutes) was first identified by Kasimir Fajans and O. Gohring in 1913 as a short-lived member of the naturally occurring 238U decay series and was given the name brevium, meaning brief. The existence of protactinium was confirmed in 1918 when another isotope of protactinium (of mass 231 and a half-life of 3.3 × 104 years) was studied independently by Otto Hahn and Lise Meitner in Germany, and by Frederick Soddy and John Cranston in Great Britain. The current name of the element is a shortened version of the original protoactinium, derived in part from the Greek protos, meaning parent; protoactinium thus meant parent of actinium (its decay product). There are twenty-four known isotopes of Pa, having mass numbers ranging from 214 to 238, the most stable isotope being 231Pa. Protactinium metal is silvery and relatively nonreactive. It occurs at ppm levels in uranium ores and is extracted from these ores. There are about 125 grams (4.4 ounces) of protactinium in the world today. Its ground state electronic configuration is [Rn]5f 26d 17s 2, placing it in Group IIIB. Its principal oxidation state is +5, but there is no stable Pa5+ ion because it is hydrolyzed so quickly to species such as PaO(OH)2+ , or forms complexes with anions such as fluoride. Protactinium in its +4 state may exist in aqueous solution or in compounds. The most important solid compound of protactinium is Pa2O5.
see also Actinium; Berkelium; Einsteinium; Fermium; Lawrencium; Mendelevium; Neptunium; Nobelium; Plutonium; Rutherfordium; Thorium; Uranium.
Cotton, F. Albert, and Wilkinson, Geoffrey (1988). Advanced Inorganic Chemistry, 5th edition. New York: Wiley.
protactinium (prō´tăktĬn´ēəm), radioactive chemical element; symbol Pa; at. no. 91; mass number of most stable isotope 231; m.p. greater than 1,600°C; b.p. 4,026°C; sp. gr. 15.37 (calculated); valence +4, +5. Protactinium is a malleable, shiny silver-gray radioactive metal. It does not tarnish rapidly in air. Known compounds include a chloride (PaCl4), a fluoride (PaF4), a dioxide (PaO2), and a pentoxide (Pa2O5). Protactinium has 24 isotopes of which only three are found in nature. The most stable is protactinium-231 (half-life about 32,500 years); it is also the most common, being found in nature in all uranium ores in about the same abundance as radium.
Protactinium has been called the "mother" of actinium, which is formed by the alpha decay of protactinium. The first discovery of protactinium was in 1913 by Kasimir Fajans and O. Göhring, who found the isotope protactinium-234m (half-life 1.2 min), a decay product of uranium-238; they named it brevium for its short life. Protactinium-231 was first identified in 1918 by Otto Hahn and Lisa Meitner and independently by Frederick Soddy and John A. Cranston; the name protoactinium was adopted at this time. In 1927, Aristid V. Grosse prepared the pentoxide, and in 1934 isolated the metal from a purified sample of oxide. The name protactinium was adopted in 1949 by the International Union of Pure and Applied Chemistry.