noun. a highly unstable radioactive element (the heaviest of the halogen series); a decay product of uranium and thorium.
1. Noun, singular or mass At present, it is not necessary to remember a fifth halogen called astatine. 2. Verb, base form To find metals on the periodic table, look at the ladder created from boron to astatine. 3. Adjective The six non-metallic elements are fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (As) and tennessine (Ts).