Astatine

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

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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^[1]), though 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 consequence of its extremely short half-life. Its most stable isotope has a half-life of around 8.3 hours. The final product of the decay of astatine is an isotope of lead.

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. Guinness World 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, in a 1957 essay on large numbers, scientific notation, and the size of the atom, wrote that in "all of North and South America to a depth of ten miles", the number of astatine atoms at any time was "only a trillion".^[2]

History

The existence of "eka-iodine" had been predicted by Mendeleev. Astatine (after Greek αστατος astatos, meaning "unstable") 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.

Compounds

Multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. While these compounds are primarily of theoretical interest, they are being studied for potential use in nuclear medicine.[1]

Isotopes

Main article: isotopes of astatine

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 8.3 hours; the shortest-lived known isotope is ²¹³At, which has a half-life of 125 nanoseconds.

References

• Los Alamos National Laboratory - Astatine