Astatine

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85 poloniumastatineradon
I

At

(Uus)
At-TableImage.png
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 the rarest naturally occurring chemical element. It is a member of the halogen family of elements and is the heaviest halogen. Its chemical properties appear to resemble those of iodine. All its isotopes are radioactive, and a few of them are produced by the natural radioactive decay of uranium-235 and uranium-238. Various compounds of astatine have been prepared in minute amounts, and the possibility of their use for nuclear medicine is being studied.

Contents

Occurrence and production

The total amount of astatine in the Earth's crust has been estimated to be less than one ounce (28 grams) at any given time—corresponding to no more than one teaspoonful in volume. It is produced in nature by the radioactive decay of uranium and thorium, and it is therefore present in trace amounts in minerals of these elements.

Astatine can be artificially produced by bombarding bismuth with energetic alpha particles. This method generates 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 the mixture in the presence of air, and condensing the vapors in a separate container.

History

The name astatine was derived from the Greek word αστατος (astatos), meaning "unsteady." Long before it was discovered, Dmitri Mendeleev had predicted its existence based on his analysis of the periodic table. He called it "eka-iodine."

This element was discovered in 1940 by Dale R. Corson, K. R. MacKenzie, and Emilio Segrè at the University of California, Berkeley. They found it during experiments in which they subjected bismuth to a barrage of alpha particles. An earlier name for the element was alabamine (Ab).

Notable characteristics

In the periodic table, astatine is located in group 17 (former group 7A), the halogen family, below iodine. In addition, it lies in period six, between polonium and radon.

According to experiments done with a mass spectrometer, the chemical properties of this highly radioactive element probably resemble those of the other halogens, especially iodine. (Like iodine, it would be expected to accumulate in the thyroid gland.) Yet astatine is thought to be more metallic than iodine and is classified as a metalloid. Researchers at the Brookhaven National Laboratory have been able to study elementary reactions that involve astatine, but chemical research into this element is limited by its extreme rarity, which is a result of its extremely short half-life.

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. They are, however, also being studied for their potential use in nuclear medicine.

See also

References

  • Astatine Los Alamos National Laboratory. Retrieved October 3, 2007.
  • Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry, 4th ed. New York: Wiley, 1980. ISBN 0-471-02775-8
  • Chang, Raymond. Chemistry, ninth ed. New York: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031
  • Greenwood, N.N., and A. Earnshaw. Chemistry of the Elements, 2nd Edition. Oxford, U.K.; Burlington, Massachusetts: Butterworth-Heinemann, Elsevier Science, 1998. ISBN 0750633654 Online Version Retrieved October 3, 2007.

External links

All links retrieved November 19, 2012.

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