Air is matter that has weight and contains 78 percent nitrogen and 21 percent oxygen. Air also has small amounts of argon, carbon dioxide and other ingredients.
Air is a mixture of gases, which is why it has weight. Gases are a composition of several molecules that remain in constant motion. It occupies volume in space, but how much space it takes up depends on its temperature. Normally, gases at a higher temperature take up more space.
Air's density varies in different parts of the atmosphere. At the edge of space, its density is zero. Air density, temperature and pressure can change several times per day and varies from region to region.