Air is a homogeneous mixture, not a compound. The components of a mixture can be separated once they have been combined; a compound is a new substance formed by a reaction between components.
Air consists mostly of nitrogen, oxygen, carbon dioxide, water vapor and argon, as well as smaller amounts of hydrogen, xenon and other elements. Everyday chemical reactions, such as oxidization and combustion, depend on elements present in air. For example, oxygen separates from the air mixture and combines with iron and water to create iron oxide or rust. Iron oxide is a compound because it is a new substance formed through a reaction between its components, and its components can't be broken down into oxygen, iron and water once the reaction has taken place. Air, like saturated sugar water, is a homogeneous mixture with uniformly distributed components.