What layer in the atmosphere is the least dense?

The least dense layer of the atmosphere is the exosphere. It is the outermost layer, extending from about 600 kilometers (373 miles) above the Earth's surface to about 10,000 kilometers (6,200 miles). In this layer, the air is extremely thin, with particles so sparse that they can travel hundreds of kilometers without colliding with one another. The exosphere gradually transitions into outer space.