Astatine

Related Topics:
Astatine

Astatine is a chemical element in the periodic table that has the symbol At and atomic number 85. This radioactive element occurs naturally from uranium-235 and uranium-238 decay and is the heaviest of the halogens.

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.

With the possible exception of francium, 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 one 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) occurs naturally in the Earth's crust."

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).

Occurrence

Astatine is produced by bombarding bismuth with energetic alpha particles to obtain relatively long-lived ²⁰⁹At - ²¹¹At, which can then be distilled from the target by heating in the presence of air.

Isotopes

Astatine has 33 known isotopes, all of which are radioactive; the range of their mass numbers is from 191 to 223. There exist also 23 metastable excited states. The longest-lived isotope is ²¹⁰At which has a half-life of 8.1 hours. The shortest-lived known isotope is ²¹³At which has a half-life of 125 nanoseconds.

References

Los Alamos National Laboratory - Astatine

External links

WebElements.com - Astatine