Astatine is a synthetically produced radioactive element designated the atomic symbol At and atomic number 85. Astatine derives its name from the greek word astatos which means unstable.

It was first synthesized in 1940 by D.R. Corson, K.R. MacKenzie, and E. Segre at the University of California. Corson and his colleagues essentially bombarded bismuth-209 with alpha particles, which produced astatine-211 as well as neutrons:

²⁰⁹Bi + He²⁺ ? ²¹¹At + 2¹n

Three years after it was first synthesized in a lab, trace amounts of astatine were discovered in some natural occurrences of luranium and thorium minerals. Naturally occurring astatine is considered the rarest element to be found in the Earth's crust. Its rarity is a consequence of the element's short half life. 

Properties

Astatine is a non metallic element that exists at room temperature in a solid state. It is the heaviest element of the halogen group (group 17). The other halogens, from lowest to highest atomic weight, are: fluorine, chlorine, bromine and iodine.

The word halogen is ultimately derived from the Greek words for salt, hals and the root word gen-, meaning to generate, hence generators of salt. The halogens are extremely reactive substances. When a halogen comes in contact with hydrogen, an acidic compound is formed, which then can be converted into a salt. 

Astatine is a highly radioactive element. All of its known isotopes (around 20) are radioactive. Its most stable isotopes ²¹⁰At and ²¹¹At have half-lives of 8.1 hours and 7.2 hours respectively.

Applications

Astatine can be used to treat certain thyroid conditions. Radio-iodine (iodine-131)  is more commonly or more widely used than astatine. However, astatine has some advantages over iodine, because, while it is similar to iodine in terms of chemical properties, astatine emits alpha particles, which are energetically stronger and travel shorter distances than the beta particles emitted by iodine-131. Because of this, astatine can be more effective in treating thyroid problems such as cancer while not affecting other nearby glands.

Other than the potential of it being used to treat thyroid cancer, astatine has no other known uses because of its short half-life.

Further reading

• Takahashi, N; Yano, D; Baba, H (1992). "Chemical behavior of astatine molecules". Proceedings of the international conference on evolution in beam applications, Takasaki, Japan, Nov 5‒8, 1991. pp. 536‒539..
• Meyers, Robert Allen (2001). "Halogen chemistry". Encyclopedia of physical science and technology (3rd ed.). Academic Press. pp. 197–222 (202). ISBN 978-0-12-227410-7.
• Astatine Reading Materials