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.Learn more about Atmosphere