Kinetic-Molecular Theory of Gases
According to the Kinetic-Molecular Theory of matter, a substance in the gaseous phase is made up of atoms and/or molecules which are very far apart and which are constantly moving around. We say that the particles in a gas are so far apart that any forces they exert on each other are negligible.
The KMT of gases explains why a substance in the gaseous state has no definite volume and no definite shape and why gases are relatively compressible. A gas has no definite volume and no definite shape because the particles in a gas are far apart and are constantly moving. If particles in a gaseous state are put in a container, they will move around until they fill the container. In addition, gases are typically relatively compressible because the particles in gases are far apart and can be forced closer together.