The basic structure of the air we breath is:
Nitrogen 78.084%
Oxygen 20.964%
Carbon dioxide 0.033%
Other gases 0.919%
Other gases consisting of Neon, Argon, Helium, Hydrogen, Carbon Monoxide etc, etc..
The percentages will vary slightly from place to place depending on
such things as smog / airpollution, foliage etc. Other than that the
percentages will remain pretty much close to a constant.
Although, The volume and density and movement will vary with altitude and temperature.
Air is denser at sea level where atmospheric pressure is greater. And
less dense at higher altitudes. therefore at high altitudes the air
is thinner (spread over a greater area) where the atmospheric
pressure is less.
There are some physical laws you can reference to determine the behavior of air/gases..
Boyles Law:
Demonstrates that if the temperature remains constant, the volume
of a given mass of gas is inversely proportional to the absulute
pressure.
Charles Law:
The amount of change in either volume or pressure of a given
volume of gas is directly proportional to the change in the absolute
temperature.
The volume would increase (spread / thin out) as the temperature increases and vise versa..
Daltons Law:
The total pressure exerted by a mixture of gases is equal to the sum of
the pressures of each of the different gases making up the mixture -
each gas acting as if it alone were present and occupied the total
volume.
Henry's Law:
This law has to do with gases disolving into liquid
To find greater detail on this yo can google these laws..