The earth's atmosphere is a complex mixture of several "gases", either atomic or molecular in nature. Air consists primarily of N2 (78%) and O2 (21%), with small amounts of several other substances, including Ar (0.9%).
Gases differ from solids and liquids in several significant ways:
-A gas expands spontaneously to fill its container. The volume of a gas equals the volume of the container in which it is held. 
-A gas is highly compressible. When pressure is applied to a gas, its volume readily decreases. 
-Gases form homogeneous mixtures with each other regardless of the identities or relative proportions of the component gases. 
-Compared to solids and liquids, the molecules of gases are relatively far apart. In air, the molecules take up only about 0.1% of the total volume - compared to the individual molecules of a liquid that occupy about 70% of the total space.

The most important measurable properties of gases are:
amount of gas