The molecule in a gas are constantly colliding with each other several times during the course of their movement. These collision do not change the net momentum of the colliding molecule but involve a transfer of momentum and energy from one part to another in the gas. Transport properties like heat, coduction, diffusion, viscosity and mean free path depend on the molecular collisions ; it is therefore essential to introduce the concept of molecular size. We shall derive an approximate relation for the frequency of intermolecular collisions. In the derivation, it will be assumed that molecules are of rigid spheres. When two identical molecules approach one another, there is certain beyond which they cannot come closer.
* Collision Diameter :- The distance of closest approach between the centres of molecules is known as the Collision Diameter.
* Collision Frequency :- It is the number of collisions per unit time per unit volume.
* Mean Free Path :- The average distance travelled by a molecule before colliding with other molecules is known as the Mean Free Path.
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