astatine: a highly unstable radioactive element (the heaviest of the halogen series); a decay product of uranium and thorium
chemical element: any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter
protoctista: in most modern classifications, replacement for the Protista; includes: Protozoa; Euglenophyta; Chlorophyta; Cryptophyta; Heterokontophyta; Rhodophyta; unicellular protists and their descendant multicellular organisms: regarded as distinct from plants and animals