Yes, ar is a Scrabble word!
...and is worth 2 points in Scrabble, and 2 points in Words with Friends
ar
Noun
1. A colorless and odorless inert gas; one of the six inert gases; comprises approximately 1% of the earth's atmosphere
2. A unit of surface area equal to 100 square meters
3. A state in south central United States; one of the Confederate states during the American Civil War