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
existentialist: a philosopher who emphasizes freedom of choice and personal responsibility but who regards human existence in a hostile universe as unexplainable