astatine

astatine
Name	astatine
Number	85
Category	halogen
Group	17
Standard atomic weight	[210]
Electron configuration	[Xe] 4f14 5d10 6s2 6p5
Phase	solid (predicted)
Melting point	575 K (302 °C, 576 °F) (estimated)
Boiling point	610 K (337 °C, 639 °F) (estimated)
Discovered by	Dale R. Corson, Kenneth Ross MacKenzie, Emilio Segrè
Discovery date	1940
Named after	Greek astatos (unstable)

astatine. It is the rarest naturally occurring element in the Earth's crust, existing only as a fleeting product of the radioactive decay of heavier elements like uranium and thorium. The element was first synthesized in 1940 at the University of California, Berkeley by a team led by Dale R. Corson. Due to its intense radioactivity and scarcity, the study of its chemistry is conducted with trace quantities and relies heavily on theoretical predictions.

Properties

Astatine exhibits properties consistent with its position in the halogen group on the periodic table, often described as a metallic-looking solid. Its estimated melting and boiling points are higher than those of lighter halogens like iodine, suggesting increased metallic character. The most stable isotope, astatine-210, has a half-life of only 8.1 hours, which severely limits experimental investigation. Its chemistry is primarily studied through radiochemical tracer techniques, often comparing its behavior to that of its periodic neighbors, polonium and radon. Theoretical models predict it can form the At^– anion but may also exhibit cationic behavior in compounds like AtO⁺.

History

The existence of element 85 was predicted by Dmitri Mendeleev, who termed it eka-iodine. Early, erroneous claims of discovery, such as alabamine by Fred Allison and dakin by Rajendralal De, were later disproven. The first confirmed synthesis was achieved at the University of California, Berkeley's cyclotron in 1940 by researchers Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè by bombarding bismuth-209 with alpha particles. The name, derived from the Greek word for unstable, was proposed by its discoverers and officially adopted by the International Union of Pure and Applied Chemistry. Minute natural occurrences were later confirmed in decay chains involving uranium-238 and thorium-232.

Occurrence and production

In nature, astatine is produced transiently in the uranium and thorium decay series, with total terrestrial inventory estimated at less than one gram at any moment. It is produced artificially by bombarding bismuth targets with high-energy alpha particles in particle accelerators like those at the Joint Institute for Nuclear Research in Dubna. Alternative production routes involve proton bombardment of thorium or uranium targets or the fission of plutonium in nuclear reactors. The resulting astatine must be separated rapidly from the target material and other radioisotopes using techniques such as dry distillation or solvent extraction.

Compounds

Due to its scarcity, few compounds have been characterized, but its chemistry is inferred to resemble iodine. Known hydrogen halides include hydrogen astatide, though it is highly unstable. It forms interhalogen compounds like astatine monochloride and likely astatine iodide. Studies using tracer chemistry indicate it can exist in multiple oxidation states, from –1 to +7. It forms salts with metals, such as sodium astatide, and complexes with organic molecules like methyl astatide. Theoretical work suggests possible polyatomic cations, including At₂⁺ and AtO₂⁺, in strongly acidic environments.

Applications

Its primary application is in nuclear medicine as a potential alpha-emitting radiotherapeutic agent. The isotope astatine-211 is of significant interest for targeted alpha-particle therapy due to its 7.2-hour half-life and decay via alpha emission to stable bismuth-207. Research institutions like the University of Washington and Brookhaven National Laboratory are investigating its use in treating microscopic cancers, such as residual disease after surgery for ovarian cancer or brain tumors. Its alpha particles have a very short range in tissue, which can minimize damage to surrounding healthy cells when the radionuclide is attached to a tumor-targeting molecule like a monoclonal antibody.

Precautions

All isotopes are intensely radioactive, presenting severe internal radiation hazards if ingested, inhaled, or absorbed. The primary risk arises from its alpha particle emissions, which can cause significant cellular damage and DNA strand breaks. Handling requires stringent radiation safety protocols, specialized glovebox or hot cell facilities, and extensive health physics monitoring. Research is conducted with microgram or nanogram quantities to minimize exposure. Due to its propensity to concentrate in the thyroid gland similar to iodine, particular precautions are taken to prevent incorporation.

Category:Chemical elements Category:Halogens Category:Synthetic elements