Collision frequency

Collision frequency describes the rate of collisions between two atomic or molecular species in a given volume, per unit time.

In an ideal gas, assuming that the species behave like hard spheres, the collision frequency between entities of species A and species B is:[1] which has units of [volume][time]−1.

Here, In the case of equal-size particles at a concentration

is the number of collisions per second, can be written as:[2] Where: Here the frequency is independent of particle size, a result noted as counter-intuitive.

For particles of different size, more elaborate expressions can be derived for estimating