The atmosphere of Earth is divided into five primary layers. From the surface out they are the troposphere, stratosphere, mesosphere, thermosphere, and the exosphere. The troposphere begins at ground level and the exosphere extends into space and marks the boundary of the atmosphere. Within these primary bands are other layers such as the ozone layer.
The exosphere extends from approximately 700 km above sea level to about 10,000 km above sea level. The atoms are so sparse in this layer that they no longer act as a gas, but rather as individual particles capable of escaping into the solar wind. The gas molecules sort themselves by weight, with the lightest at the top of the exosphere and the heavier particles at the lowest part or the exobase.
The exosphere is the part of the atmosphere where gases separate themselves by weight, and hydrogen is in the top layer of this layer.
The atmosphere has five layers. Starting from the layer that is closest to the earth to the one that is farthest away the layers are the troposphere, stratosphere, mesosphere, thermosphere, and the exosphere.
The exosphere is where molecules start to escape into space and that is where they tend to separate themselves by weight.
The gas in the top layer of the atmosphere is mainly hydrogen but also can include helium, carbon dioxide, and atomic oxygen.