Astatine is a highly radioactive halogen. All isotopes decay quickly; only trace amounts naturally exist. It likely forms a diatomic solid with metallic to metalloid-like character.
Astatine is a rare, highly radioactive element in the halogen group (Group 17). It was first synthesized in 1940 by Dale R. Corson, Kenneth MacKenzie, and Emilio Segrè at the University of California by bombarding bismuth with alpha particles:
\(\mathrm{^{209}Bi + ^4He \rightarrow ^{211}At + 2n}\)
The name ‘Astatine’ comes from the Greek word ‘astatos’, meaning unstable.
Astatine belongs to the halogen group (Group 17) because it has seven valence electrons (ns2np5) like fluorine, chlorine, bromine, and iodine. However, due to its large atomic size and metallic character, its behavior differs significantly from lighter halogens.
Astatine is extremely rare because all of its isotopes are radioactive and decay quickly. The most stable isotope, At-210, has a half-life of only 8.1 hours. It is continuously produced and destroyed in trace amounts during the decay of heavier elements like uranium and thorium.
Astatine has over 30 known isotopes, all radioactive. The most notable are:
Because of these short half-lives, astatine can only be studied in small, synthetic quantities.
Astatine is expected to behave chemically similar to iodine but with more metallic properties. It can form diatomic molecules (At2) and ionic compounds like astatides (At−). It may also form oxides and halides such as AtCl and AtO3:
\(\mathrm{2\,At + Cl_2 \rightarrow 2\,AtCl}\)
Astatine is often classified as a metalloid-like halogen. It shows both metallic and nonmetallic behavior. While it chemically resembles iodine, its physical properties such as conductivity and luster are closer to metals.
Astatine can exhibit multiple oxidation states: −1, +1, +3, +5, and +7. The −1 state occurs in astatide salts like NaAt, while positive states appear in compounds like AtCl, AtCl3, and AtF7.
\(\mathrm{Na + At \rightarrow NaAt}\)
Due to its short half-life, astatine has no large-scale industrial use. However, astatine-211 is being studied in targeted alpha-particle cancer therapy, where it can deliver localized radiation to destroy cancer cells without damaging surrounding tissue.
Astatine is produced by bombarding bismuth-209 with alpha particles in a cyclotron:
\(\mathrm{^{209}Bi + ^4He \rightarrow ^{211}At + 2n}\)
This reaction creates astatine isotopes that are then separated from the target using chemical methods.
Astatine is believed to be a dark, metallic-looking solid at room temperature. It likely forms diatomic molecules (At2), has a melting point around 302 °C, and a boiling point near 337 °C, though these values are theoretical because of its scarcity.
Astatine’s extreme radioactivity and short half-life make it difficult to produce and study in large quantities. Only picogram amounts can be synthesized at a time, and it decays quickly, making direct physical measurements nearly impossible.