Hey, I learned this in science a few weeks ago!!
The least dense layer is the exosphere which is the outermost layer of the atmosphere.
In my science book it says, "it is least dense because it is farthest from the earth, whose gravitation force decreases in accordance with the inverse-square law. Since molecules and ions in this region have a better-than-even chance of escaping from the gravity of the earth, many of them do escape, to be replace by infalling matter from space itself, or by atmospheric matter from the layers below."
Hope this helps!!!!!!!!!