Astatine

Astatine is an extremely rare, highly radioactive element in the halogen family. Tiny amounts form naturally as radioactive elements decay, but all astatine isotopes are short-lived, and no visible sample of the element has ever been produced. Its physical appearance and many bulk properties remain uncertain; a dark or metallic-looking solid has been predicted, not confirmed. Astatine-211 is made artificially and is being studied for targeted alpha-particle cancer therapy.