well lets see.

from my knowledge its (basically)
oxygen, nitrogen and carbon dioxide(very small amount)

from a NASA website its..
The atmosphere is primarily composed of Nitrogen (N2, 78%), Oxygen (O2, 21%), and Argon (Ar, 1%). A myriad of other very influential components are also present which include the water (H2O, 0 - 7%), "greenhouse" gases or Ozone (O, 0 - 0.01%), and Carbon Dioxide (CO2, 0.01-0.1%).

EDIT: the exosphere starts at the top of the thermosphere (53 miles high - 372 miles high) and its main composition is hydrogen and helium. (and we all know why its full of helium when we think of all those balloons we let go of XD LOL)