#astatine

• Astatine — A highly unstable radioactive chemical element of the halogen group, with atomic number 85 and symbol At; it is the rarest naturally occurring element in the Earth's crust, produced only transiently through the decay of uranium and thorium, and due to its extremely short half-life (a maximum of 8.1 hours for its most stable isotope), its physical properties are studied almost exclusively through artificial synthesis techniques in particle accelerators.
• Astatine — An alternative, synonymous, and tolerated name within the scientific community for the chemical element 'astatin'; it follows the classical terminological pattern of metals and metalloids (with the suffix '-ium'), occasionally used in textbooks or older technical translations to denote the element with atomic number 85.
• At — The official chemical symbol for the element astatine, an extremely rare and unstable radioactive halogen with atomic number 85. In modern periodic tables, At represents the heaviest characterized element in the halogen group, whose physical and chemical properties are theoretically deduced or microscopically observed in high-energy nuclear physics laboratories due to its very short half-life.