astatine

Etymology

From Ancient Greek ἄστατος (ástatos, “unstable”) + -ine.

noun

1. A highly radioactive chemical element (symbol At), one of the halogens, with atomic number 85.
   Astatine, for instance, is practically unstudied. It has a name and a place on the periodic table (next to Marie Curie's polonium), but almost nothing else. 2003, Bill Bryson, A Short History of Nearly Everything, BCA, page 221