아스타틴(At) is a rare and radioactive element classified as a halogen in group 17 of the periodic table. It is the heaviest stable isotope of the halogens and is known for its short half-life, making it highly unstable. Due to its scarcity, it is found in trace amounts in nature, primarily produced through the decay of heavier elements like uranium and thorium. This element was first synthesized in 1940 by a team of scientists including Dale R. Corson, Emilio Segrè, and Arthur E. Gall. Although it has limited practical applications due to its radioactivity, research is ongoing into its potential use in targeted alpha-particle cancer therapy.