at

Science Dictionary
At  
The symbol for astatine.
American Heritage
Science Dictionary
astatine   (ās'tə-tēn')  Pronunciation Key 
Symbol At
A highly unstable, rare, radioactive element that is the heaviest of the halogen elements. Its most stable isotope has a half-life of 8.3 hours. Atomic number 85; melting point 302°C; boiling point 337°C; valence probably 1, 3, 5, 7. See Periodic Table.
The American Heritage® Science Dictionary
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