Source of particular gases needed by plants and animals. The gases that form our atmosphere are not bound together by chemical bonds. This means that these gases can be separated by physical means. This is why we say that air is a mixture and not a compound. Gases are compressible, which means that the density of the atmosphere varies with altitude.
Air density is greatest at the surface of the Earth.
As distance above the surface increases, air density decreases. At sea level the air density is 1.3 kilograms per cubic meter. Five miles up the density has fallen to half the sea level value. At twelve miles air density is one-thirteenth the sea level value. At an altitude of 105 miles air density has dropped to one one millionth of a gram per cubic meter.