Astatine is a rare and highly radioactive element with the atomic number 85 and the chemical symbol At. Discovered in 1940 by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè at the University of California, Berkeley, it occurs naturally in minuscule amounts due to its short half-life, which ranges from seconds to hours. As part of the halogen group, astatine shares some chemical properties with iodine, but its extreme rarity and radioactivity have limited detailed study. It is primarily of interest in medical research, particularly in targeted alpha-particle cancer therapy.