The kinetic-molecular theory says

1. A gas consists of molecules in constant random motion.

2. Gas molecules influence each other only by collision; they exert no other forces on each other.

3. All collisions between gas molecules are perfectly elastic; all kinetic energy is conserved.

4. The volume actually occupied by the molecules of a gas is negligibly small; the vast majority of the volume of the gas is empty space through which the gas molecules are moving.