Astatine (as-tə-tin)
is a radioactive chemical element with the symbol At and it
has an atomic number of 85. The atomic weight is
210. 
Things you
might not know about Astatine:
- Astatine is the
rarest naturally occurring element, with less than 30 grams
estimated to be contained in the entire Earth's crust.
- While compounds
of Astatine are primarily of theoretical interest, they are being
studied for potential use in nuclear medicine.
- Since astatine
is extremely radioactive, it should be handled with extreme
care.
See GC2XAY2 -
Hydrogen, for an explanation about this series of
caches.