The earth is the largest of the inner or terrestrial planets. It is nearly 8,000 miles in diameter and nearly spherical. Its oblateness is minimal.   It can be seen as a series of 3 concentric spheres, increasingly more dense inwards: 
                                            1. the gaseous atmosphere, 
                                            2. the liquid hydrosphere and 
                                            3. the solid earth, whose outer layer is called the lithosphere.

Description of the atmosphere 

Composed of about 78% Nitrogen, 21% Oxygen, 1% Argon and traces of carbon dioxide, water and other gases, the atmosphere is the least dense and outermost of the earth's layers. Various thicknesses are given for the atmosphere. In reality there is no clear upward boundary and the atmosphere becomes less dense as we go outward. Somewhere near 100 miles in height it is so thin that its atoms and molecules do not significantly affect the orbit of satellites, so we can consider that a very approximate but reasonable outer limit for the atmosphere. Relative to the hydrosphere and lithosphere, the mass of the atmosphere is trivial and amounts to some 0.01% of the total mass of the planet.