Astatine Facts (Element 85 or At)

Astatine is a radioactive element with symbol At and atomic number 85. It has the distinction of being the rarest natural element found in the Earth's crust, as it is only produced from radioactive decay of even heavier elements. The element is similar to its lighter congener, iodine. While it is a halogen (a nonmetal), it has more metallic character than other elements than the group and most likely behaves as a metalloid or even a metal. However, sufficient quantities of the element have not been produced, so its appearance and behavior as a bulk element have yet to be characterized.

Atomic Number: 85

Symbol: At

Atomic Weight: 209.9871

Discovery: D.R. Corson, K.R. MacKenzie, E. Segre 1940 (United States). Dmitri Mendeleev's 1869 periodic table left a space below iodine, predicting the presence of astatine. Over the years, many researchers attempted to find natural astatine, but their claims were largely falsified. However, in 1936, Romanian physicist Horia Hulubei and French physicist Yvette Cauchois claimed to discover the element. Eventually, their samples were found to have contain astatine, but (partly because Hulubei had issued a false claim for element 87 discovery) their work was downplayed and they never received official credit for the discovery.

Electron Configuration: [Xe] 6s² 4f¹⁴ 5d¹⁰ 6p⁵

Word Origin: Greek astatos, unstable. The name refers to the element's radioactive decay. Like other halogen names, astatine's name reflects a property of the element, with the characteristic "-ine" ending.

Isotopes: Astatine-210 is the longest-lived isotope, with a half-life of 8.3 hours. Twenty isotopes are known.

Properties: Astatine has a melting point of 302°C, an estimated boiling point of 337°C, with probable valences of 1, 3, 5, or 7. Astatine possesses characteristics common to other halogens. It behaves most similarly to iodine, except that At exhibits more metallic properties. The interhalogen molecules AtI, AtBr, and AtCl are known, although it has not been determined whether or not astatine forms diatomic At₂. HAt and CH₃At have been detected. Astatine probably is capable of accumulating in the human thyroid gland.

Sources: Astatine was first synthesized by Corson, MacKenzie, and Segre at the University of California in 1940 by bombarding bismuth with alpha particles. Astatine may be produced by bombarding bismuth with energetic alpha particles to produce At-209, At-210, and At-211. These isotopes can be distilled from the target upon heating it in air. Small quantities of At-215, At-218, and At-219 occur naturally with uranium and thorium isotopes. Trace amounts of At-217 exist in equilibrium with U-233 and Np-239, resulting from the interaction between thorium and uranium with neutrons. The total amount of astatine present in the Earth's crust is less than 1 ounce.

Uses: Similar to iodine, astatine may be used as a radioisotope in nuclear medicine, mainly for cancer treatment. The most useful isotope maybe astatine-211. Although its half-life is only 7.2 hours, it may be used for targeted alpha particle therapy. Astatine-210 is more stable, but it decays into deadly polonium-210. In animals, astatine is known to concentrate (like iodine) in the thyroid gland. Additionally, the element becomes concentrated in the lungs, spleen, and liver. The element's use is controversial, as it has been shown to cause breast tissue changes in rodents. While researchers may safely handle trace quantities of astatine in well-ventilated fume hoods, working with the element is extremely dangerous.

Element Classification: Halogen

Melting Point (K): 575

Boiling Point (K): 610

Appearance: Presumed to be a solid metal

Covalent Radius (pm): (145)

Ionic Radius: 62 (+7e)

Pauling Negativity Number: 2.2

First Ionizing Energy (kJ/mol): 916.3

Oxidation States: 7, 5, 3, 1, -1

• Corson, D. R.; MacKenzie, K. R.; Segrè, E. (1940). "Artificially Radioactive Element 85." Physical Review. 58 (8): 672–678.
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