- noun a natural radioactive element
- A rare radioactive element with atomic number 85. There are about 30 known isotopes, all of which are unstable. Naturally occurring astatine amounts to about one ounce spread over the earth's crust, so when needed it is synthesized by bombarding bismuth with alpha particles. It is used as a radioactive tracer. It is a halogen, and its chemical symbol is At.
- chemical symbolAt