- Astatine (At) is a rare, highly radioactive halogen with atomic number 85, belonging to Group 17 of the periodic table, alongside fluorine, chlorine, bromine, and iodine.
- Its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, giving it seven valence electrons. Like other halogens, astatine can exhibit oxidation states of –1, +1, +3, +5, and +7, but due to its radioactivity, its chemical behavior is less thoroughly studied. Astatine atoms contain eighty-five protons, eighty-five electrons, and varying numbers of neutrons depending on the isotope.
- The most stable isotope is astatine-210 (²¹⁰At), with a half-life of about 8.1 hours.
- Astatine was first synthesized in 1940 at the University of California, Berkeley, by physicists Dale R. Corson, Kenneth Ross MacKenzie, and Emilio G. Segrè. They produced it by bombarding bismuth with alpha particles, creating the isotope ²¹¹At. Its name comes from the Greek word astatos, meaning “unstable,” reflecting its extreme radioactivity. Although traces of astatine exist naturally as a product of uranium and thorium decay chains, it is estimated that at any given time only a few grams of astatine exist in Earth’s entire crust, making it the rarest naturally occurring element on the planet.
- Physically, astatine is difficult to study due to its scarcity and short half-lives. It is predicted to be a dark, metallic-looking solid under standard conditions, with properties intermediate between iodine and polonium. Astatine is expected to have a melting point around 302 °C (576 °F) and a boiling point near 337 °C (639 °F), though experimental confirmation is limited. It is thought to be more metallic than iodine, possibly forming a metalloid-like element.
- Chemically, astatine behaves like a heavier version of iodine but shows some metallic tendencies. It can form astatide salts (At⁻), similar to iodides, and can also create compounds in higher oxidation states, such as astatine oxyacids. Its halide chemistry suggests similarities to iodine, yet relativistic effects make it less electronegative and more prone to forming cationic species.
- Applications of astatine are almost exclusively in the field of nuclear medicine. One of its isotopes, astatine-211 (²¹¹At), is of great interest in targeted alpha-particle cancer therapy. Because it emits short-range but extremely destructive alpha radiation, it can be attached to molecules that selectively bind to cancer cells, delivering precise doses of radiation while minimizing damage to surrounding healthy tissues. However, its short half-life and difficulty in production limit widespread use.
- Biologically, astatine has no known essential role in living organisms. Like other halogens, it might mimic iodine in the body, concentrating in the thyroid gland if ingested, where its radioactivity would cause severe damage. Due to its scarcity, humans are not naturally exposed to significant amounts of astatine in the environment.
- Environmentally, astatine occurs only in trace amounts as part of natural radioactive decay chains of heavier elements, such as uranium and thorium. It is usually produced synthetically in particle accelerators or nuclear reactors for research or medical purposes. Because of its rarity, it does not pose environmental hazards in the way other radioactive elements do.
