What does the kinetic theory say about the motion of atoms

The kinetic theory of gases assumes that the molecules are very small relative to the distance between molecules. The molecules are in constant, random motion and frequently collide with each other and with the walls of any container.

Another assumption is that each particle is in constant motion:

In gases, the movement of the particles is assumed to be random and free.

In liquids, the movement is somewhat constrained by the volume of the liquid.

In solids, the motion of the particles is severely constrained to a small area, in order for the solid to maintain its shape.