Astatine

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MELTING POINT: 302°C
BOILING POINT : 337°C
DENSITY : UNKNOWN
MOST COMMON IONS : At ⁻ , AtO ⁻ , AtO ₃ ⁻

Astatine is a radioactive halogen (the heaviest of the halogen elements) and is a solid at room temperature. Dale R. Carson, K. R. MacKenzie, and Emilio Segrè of the University of California produced the element in 1940 by bombarding an isotope of bismuth ( ²⁰⁹ Bi) with alpha particles. The origin of the name "astatine" is the Greek word astatos , which means "unstable."

Astatine is found in only vanishingly small amounts in nature—it is believed that only 30 grams (1 ounce) of the element are present in Earth's crust at any one time. It is produced naturally when the elements uranium and thorium decay. Astatine can also be produced in a nuclear reactor by the method used by its discoverers, according to the following reaction:

²⁰⁹ ₈₃ Bi + ⁴ ₂ He → ²¹¹ ₈₅ At + 2 ¹ ₀ n

The most stable isotope of astatine is ²¹⁰ At, which has a half-life of 8.1 hours. Other isotopes have mass numbers ranging from 193 to 223 and half-lives ranging from 125 nanoseconds ( ²¹³ At) to 7.2 hours ( ²¹¹ At). Astatine is known to form interhalogen compounds with bromine (AtBr), chlorine (AtCl), and iodine (AtI). Additional compounds (HAt and CH ₃ At) have also been detected.

Because of its scarcity and short half-life, there were no commercial uses for astatine as of 2003. Researchers are investigating astatine as a means of treating various cancers (e.g., lethal brain tumors) and diseases. Because of its similarities to iodine, which accumulates in the thyroid, it is believed that the element could be utilized to treat certain thyroid diseases.

SEE ALSO Halogens ; Radioactivity .

Stephanie Dionne Sherk

Bibliography

Lide, David R., ed. (2003). The CRC Handbook of Chemistry and Physics , 84th edition. Boca Raton, FL: CRC Press.

Internet Resources

Gagnon, Steve. "It's Elemental: Astatine." Jefferson Lab. Available from http://education.jlab.org/itselemental/iso085.html .