The basic assumptions of the kinetic-molecular theory are as follows:

  1. Gases consist of discrete molecules, which are very small compared with the spaces that exist between them.
  2. The gas molecules are all in motion. Their motion is continuous, progresses in a straight line, and can have a velocity that is different from those of the molecules around it.
  3. Collisions between molecules or between a molecule and the wall of its container are elastic. That is, no energy is lost in these collisions.
  4. Aside from collisions, molecules do not exert forces upon one another.

The kinetic-molecular theory is generally attributed to Rudolf Clasius, who published his theory in 1857. Kinetic-molecular theory attempted to explain Boyle's, Dalton's, Charles', and Avogadro's laws.