Yes, at is a Scrabble word!
...and is worth 2 points in Scrabble, and 2 points in Words with Friends
at
Noun
1. A highly unstable radioactive element (the heaviest of the halogen series); a decay product of uranium and thorium
2. 100 at equal 1 kip in Laos
What Scrabble words can I make with the letters in at?
2 Letter Words