The composition of gases in exosphere is

The exosphere is the outermost layer of Earth's atmosphere, beginning above the thermosphere at approximately 480 km to 600 km altitude [1]. In this region, the air is extremely rarefied, resembling a vacuum where individual atoms can travel hundreds of kilometers without colliding [1]. Due to the effects of gravity and the heterospheric nature of the upper atmosphere, gases are sorted by their atomic weight [1]. Heavier elements like nitrogen and oxygen remain in the lower layers, while the lightest gases—specifically hydrogen and helium—dominate the exosphere [3]. These light gases are weakly bonded by gravity and frequently achieve escape velocity, drifting off into outer space [2]. Consequently, the primary composition of the exosphere consists of individual atoms of helium and hydrogen [3].

Sources

1. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Dirunion of Atmosphere > p. 6
2. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Permanent Gases of The Atmosphere > p. 271
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Atmospheric Escape > p. 280