Carbon dioxide, also known by the chemical formula CO2, has a higher density than the other gases found in air, which makes CO2 heavier than air. Air is composed of approximately 78 percent nitrogen, 21 percent oxygen, and less than 1 percent of other gases.
Density is defined as mass per unit volume of a substance, expressed in kilograms per cubic meter. At standard temperature and pressure, the combined density of air is 1.29 kilograms per cubic meter. By contrast, carbon dioxide has a density of 1.97 kilograms per cubic meter - the highest density of all the constituent gases. Atmospheric mixing keeps the chemicals in air aloft.