Where is air least dense in this layer of atmosphere?

Air is least dense in the exosphere, which is the outermost layer of Earth's atmosphere. In this layer, the density of air molecules decreases significantly with altitude, as they are spread far apart due to the extremely low pressure. The exosphere extends from about 600 kilometers (373 miles) above sea level to about 10,000 kilometers (6,200 miles), where the atmosphere gradually transitions into outer space.