Air is a mixture, not a compound. Scientists define a mixture as a heterogeneous blend of molecules and atoms in variable proportions, while a compound is a substance formed by the chemical combination of two or more elements in fixed proportions.
While air is a mixture of more than a dozen different gases, nearly 99 percent of air is made up of nitrogen and oxygen. Although the gases that make up air are thoroughly mixed, they do not generally combine with each other chemically to form new molecules. The nitrogen and the oxygen in air are still distinct molecules. By comparison, salt is a compound formed by the chemical combination of sodium and chlorine to form a new molecule called sodium chloride, which is distinctly different from either of its two constituent elements.