Astatine

85 polonium ← astatine → radon I ↑ At ↓ (Uus) periodic table
General
Name, Symbol, Number	astatine, At, 85
Chemical series	halogens
Group, Period, Block	17, 6, p
Appearance	metallic (presumed)
Atomic mass	(210) g/mol
Electron configuration	[Xe] 4f14 5d10 6s2 6p5
Electrons per shell	2, 8, 18, 32, 18, 7
Physical properties
Phase	solid
Melting point	575 K (302 °C, 576 °F)
Boiling point	? 610 K (? 337 °C, ? 639 °F)
Heat of vaporization	ca. 40 kJ/mol
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 361 392 429 475 531 607
Atomic properties
Crystal structure	no data
Oxidation states	±1, 3, 5, 7
Electronegativity	2.2 (Pauling scale)
Ionization energies	1st: (est.) 920 kJ/mol
Miscellaneous
Magnetic ordering	no data
Thermal conductivity	(300 K) 1.7 W/(m·K)
CAS registry number	7440-68-8
Notable isotopes
Main article: Isotopes of astatine iso NA half-life DM DE (MeV) DP 210At 100% 8.1 h ε, β+ 3.981 210Po α 5.631 206Bi

Astatine (chemical symbol At, atomic number 85) is a radioactive element that occurs naturally from uranium-235 and uranium-238 decay. It is the heaviest of the halogens.

Occurrence and production

Astatine is produced by bombarding bismuth with energetic alpha particles to obtain the relatively long-lived isotopes ²⁰⁹At, ²¹⁰At, and ²¹¹At. These isotopes can then be separated from the bismuth by a process of distillation, which involves heating in the presence of air.

History

The existence of "eka-iodine" had been predicted by Mendeleev. Astatine (after Greek αστατος astatos, meaning "unsteady") was first synthesized in 1940 by Dale R. Corson, K. R. MacKenzie, and Emilio Segrè at the University of California, Berkeley by barraging bismuth with alpha particles. An earlier name for the element was alabamine (Ab).

Notable characteristics

This highly radioactive element has been confirmed by mass spectrometers to behave chemically much like other halogens, especially iodine (it would probably accumulate in the thyroid gland like iodine). Astatine is thought to be more metallic than iodine. Researchers at the Brookhaven National Laboratory have performed experiments that have identified and measured elementary reactions that involve astatine; however, chemical research into astatine is limited by its extreme rarity, which is a result of its extremely short half-life.

Astatine is the rarest naturally-occurring element, with the total amount in Earth's crust estimated to be less than 1 oz (28 g) at any given time; this amounts to less than one teaspoon of the element. The Guinness Book of Records has dubbed the element the rarest on Earth, stating: "Only around 0.9 oz (25 g) of the element astatine (At) occurring naturally"; Isaac Asimov wrote a 1955 essay on large numbers, scientific notation, and the size of the atom, in which he stated that the number of astatine atoms on Earth at any time was "only a trillion".

Isotopes

Astatine has many known isotopes, all of which are radioactive. Their mass numbers range from 191 to 223. There exist also 23 metastable excited states. Among the various known isotopes, the longest-lived member is ²¹⁰At, which has a half-life of 8.1 hours; the shortest-lived member is ²¹³At, which has a half-life of 125 nanoseconds.

Compounds

Multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. These compounds are primarily of theoretical interest; however, they are also being studied for potential use in nuclear medicine.

See also

• Chemical element
• Halogen
• Periodic table

References

ISBN links support NWE through referral fees

• Astatine Los Alamos National Laboratory. Accessed on December 10, 2006.
• Greenwood, N.N.; and Earnshaw, A. (1998). Chemistry of the Elements (2nd Edition). Oxford, U.K.; Burlington, Massachusetts: Butterworth-Heinemann, Elsevier Science. ISBN 0750633654. Online version available at [1]. Accessed on November 5, 2006.
• Cotton, F. Albert; and Wilkinson, Geoffrey (1980), Advanced Inorganic Chemistry (4th ed.), New York: Wiley. ISBN 0-471-02775-8.
• Chang, Raymond (2006). Chemistry (ninth ed.) New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031.

External links

• WebElements.com - Astatine

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