This theory was given by Clausius in 1857

Postulates:-  (1) Every gas is made up of extremely small particles called molecules. All the molecules of a gas are identical is shape size and mass.

(2) The actual volume of a gas is negligible in comparison to total volume of gas because the distance of separation bet them is very large.

(3) There is no force of attraction or repulsion between gas molecules.

(4) The force of gravitation is also supposed to be negligible on the gas molecules.

(5) The gas molecules follow zig-zag path (Brownian movement) because of their collision with one another and with the hence container.

(6) The pressure exerted by gas molecules is due to their collision with the wall of the container.

(7) Their collision is elastic and gas molecules are considered to be elastic hard spheres as no energy is wasted on collision.

If the collision is in elastic then on every collision, the molecules will lose their energy and finally settle down. i.e. there would have been no atmosphere. So during collision the energy is transferred from one molecule is another.

(8) Since molecules are moving with different velocities. They possess difference K.E however average K.E is same. Kinetic energy is only function of temperature.