Astatine (At)

• Element Symbol: At
• Atomic Number: 85
• Atomic Mass: 210
• Group # in Periodic Table: 17
• Group Name: Halogens
• Period in Periodic Table: 6
• Block of Periodic Table: p-block
• Discovered by: Dale R. Corson, Kenneth Ross MacKenzie, Emilio Segrè (1940)

Astatine is an extremely rare, radioactive element of the periodic table. It is part of the group of halogen elements. This group also includes fluorine, chlorine, bromine, and iodine. Astatine is the heaviest halogen. Like the other halogen elements, astatine is unstable in its pure form.

Scientists inferred the existence of astatine before the element was actually discovered. A space was left below iodine in the eight-column periodic table created by Russian chemist Dmitri Mendeleev in 1871. This space, for an element that had eighty-five protons, is also seen in the reordered version of the periodic table from 1915. In the 1920s scientists began trying to find or make element 85. This missing element was given the name eka-iodine, based on terminology used by Mendeleev. Eka means "one" in Sanskrit; the name refers to the fact that element 85 is one position below iodine in the periodic table.

There were many attempts to discover or produce this element in the lab. Some of them were considered successful at first. One attempt at discovery occurred in 1931. Fred Allison and other scientists from Alabama Polytechnic Institute used a magneto-optic method to observe compounds that contained halogens. His group claimed to have observed element 85 using this method and even named it, calling it "alabamine." However, the team’s results were thrown out when Allison’s magneto-optic machine was found to be faulty.

In 1937 Indian chemist Rajendralal De purified an unknown black substance from monazite sand that he claimed was eka-iodine. De named this substance "dakin" but later changed the name to "dekhine." His results were questioned, however, because the substance he claimed to have purified would have been too dangerously radioactive to handle.

A number of European scientists reported preliminary evidence of element 85 in the 1930s. Two of these scientists were Romanian physicist Horia Hulubei and French physicist Yvette Cauchois. Working together, these scientists observed spectral lines of the elusive element using an x-ray spectrometer. World War II interfered with their work, however, before they could verify and report their findings.

In 1940 a team at the Berkeley campus of the University of California finally produced the element. This team consisted of American physicists Dale R. Corson and Kenneth R. MacKenzie as well as Italian physicist Emilio Segrè. These scientists used a cyclotron to shoot accelerated alpha particles at a bismuth target. When the particles hit the target, the bismuth took up alpha particles and released neutrons. This process created isotopes of an element with eighty-five protons. The scientists named this element astatine, which comes from the Greek word astasos, meaning "unstable."

Physical Properties

Astatine is so rare and unstable that very few of its physical properties have been observed. Scientists have used their knowledge of similar elements to predict many of the physical properties of astatine. Astatine is solid in its standard state at 298 kelvins (K). The melting point of astatine is 302 degrees Celsius (°C). Its boiling point is 337 °C. The thermal conductivity of astatine is 2 watts per meter-kelvin (W/m·K). Thermal conductivity is a measure of how well a substance conducts heat. At 1.6 W/m·K, the thermal conductivity of ice is similar to that of astatine. The color, density, hardness, specific heat, and specific gravity of astatine are unknown.

Chemical Properties

Astatine’s chemical composition is similar to that of iodine. Its crystal structure is unknown. Common oxidation states of astatine are 7, 5, 3, 1, and −1. Astatine has thirty-two known isotopes. All of these isotopes are highly radioactive and unstable. The longest-lived isotope of astatine is astatine-210, with a half-life of around eight hours. Both astatine-210 and astatine-211 have been produced within particle accelerators. Other isotopes are produced naturally during one of three radioactive decay series. Astatine-218 is produced in the uranium series, astatine-216 in the thorium series, and astatine-215 and astatine-219 in the actinium series.

Applications

Although some astatine isotopes are produced naturally, the half-lives of these isotopes are extremely short. This makes astatine nearly impossible to find in nature. Astatine is so rare that only about twenty-eight grams of this element are likely to exist in Earth’s crust at any point in time. The only natural element that is rarer than astatine is berkelium.

Nuclear reactors are used to synthesize astatine. Scientists accelerate alpha particles at a bismuth-209 metallic target. The bismuth-209 takes up one of the alpha particles and releases neutrons. This process creates isotopes of astatine. The astatine isotopes can be distilled from the air within the reactor. They can also be gathered by dissolving the target in nitric acid or hydrochloric acid.

Because astatine is highly radioactive, short-lived, and extremely rare, there have been few practical uses for the element. Astatine-211has been tried in radiation therapy to treat melanomas and prostate cancer. Further research is being done on astatine-211 to determine a way to bind the isotope to a carrier molecule that will target cancer cells. If astatine-211 can be delivered directly to a cancer cell, its high radioactivity will quickly destroy the cell.

Although astatine was discovered during World War II, the progression of the war effectively stopped most research on this element because laboratories in Europe were destroyed by bombs. Many European physicists had to flee their countries, while many American physicists shifted their focus to work on the Manhattan Project, the American effort to develop the atomic bomb. One of the physicists who discovered astatine, Emilio Segrè, became a group leader on the Manhattan Project. After the war, few resources were available for the study of astatine.

Astatine remains difficult to study. Anyone working with astatine must analyze traces of the element, not the element itself. Astatine only exists at ultralow concentrations, making it impossible for spectroscopic equipment to be used to determine the molecular structure of the element.

Bibliography

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Scerri, Eric. "Mendeleev’s Periodic Table Is Finally Completed and What to Do about Group 3?" Chemistry International 34.4 (2012): 28–31. Web. 24 July 2015.

Schneider, Stefan. "Astatine (At)." Encyclopædia Britannica. Encyclopædia Britannica, 18 Oct. 2013. Web. 24 July 2015.

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Thornton, Brett F., and Shawn C. Burdette. "Finding Eka-Iodine: Discovery Priority in Modern Times." Bulletin for the History of Chemistry 35.2 (2010): 86–96. Web. 25 Aug. 2015.