At home in America's capital, say (8)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

even though I have a degree in chemistry, it took me a while to spot the setter’s tricky definition, a chemical symbol, as we more often see with As for arsenic. A[merica’s}, STATE = say, insert IN for at home. Astatine being below iodine in the halogens group, element 85, and the rarest in the earth’s crust as its longest lived isotope has a half-life of only 8 hours. If you (theoretically) had enough of it to form a solid piece, it would immediately evaporate because of the heat of its intrinsic radioactivity.