Chemistry

An Overview about the Chemical Element Astatine



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Astatine

Symbol: At

Atomic Number: 85

Atomic Mass: 210.0 amu (atomic mass units)

Melting Point: 302.0 C (575.15 K, 575.6 F)

Boiling Point: unknown

Number of Protons: 85

Number of Electrons: 85

Number of Neutrons: 125

Classification: Semi Metal

Group Name: Halogen

Crystal Structure: Unknown

Density @ 293 K: approximately 7 grams per cubic centimeter

Color: Unknown

Astatine is a radioactive halogen and was first produced at the University of California in 1940 by R. Carson, K.R. MacKenzie and Emilio Segre. This team of scientists used a cyclotron to accelerate alpha particles which they then aimed at a target made of the isotope bismuth-209. The resulting collision yielded atoms of the isotope astatine-211 with two free neutrons being released for each atom of astatine formed. This isotope has a half-life of 7.214 hours and decays by either alpha decay or electron capture. The element gets its name from the Greek word "astatos" which means unstable.

There is a very small quantity of the element present in the earth's crust where it is produced by the decay of uranium and thorium atoms. The total amount present in the earth crust at any time has been estimated at less than 30 grams. All work on the element has been done on man-made isotopes. Only 0.05 micrograms of astatine have ever been produced so very little is known about the chemical interactions of the element. The element is in Group Number 17 and Period Number 6 in the periodic table. Astatine has the ionization energy of 9.5 eV. Its oxidation state, like its boiling point and exact density, is unknown. From knowledge of the other stable halogens it can be assumed that if visible quantities of the element were ever produced it would be a solid at room temperature, dark colored and have a metallic sheen.

A number of different isotopes of astatine have been produced with atomic mass numbers ranging from 193 to 223. Astatine-210 has the longest half-life of the isotopes produced to date at 8.1 hours. This isotope will decay by two different methods: by alpha decay it will form bismuth-206 and by electron capture it forms polonium-210. The shortest half-life of 125 nanoseconds belongs to the isotope astatine-213 which decays by alpha decay.

As only a very small amount of astatine has ever been produced and all of its known isotopes have short half-lives no industrial uses for the element have been found. It is used only in scientific research.

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