1. Collision diameter
The closest distance of approach between the centres of the molecules taking part in a collision is called collision diameter.it is usally represented by σ. σ

2. Collision Number and Collision frequency
The no of collision which a single molecules makes with other molecules in one second is called collision number.
On the basis of kinectic theory of gases,it can be shown that in gas containing n identical molecules per cm3,the no of collision which a single molecules will undergo with other molecules in one sec is given by
Nc =√2πνσ2n
Where v=average velocity of the gas molecules in cm per sec
σ=molecular diameter in cm
n=no of molecules/cm3

3. Movement of molecules A assuming other molecules to be stationary
Consider a particular molecule A moving in a particular direction .if average speed of the molecule is v cm/sec,I twill travel a distance of v cm in one second.if only A is supposed to move and all other molecules are supposed to be stationary, A will collide in 1 sec with all the molecules whose centres lie within the cylinder of length v cm and radius equal to molecular diameter σ as shown in fiqure.

4. Types of molecular collisions
(a) (b) (c)
Relative velocity=0 Relative velocity=2v Relative velocity=√2ν