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: SI unit of Z is the volumetric collision rate (unit m3⋅s−1). where:
* is the number of A molecules in the gas,
* is the number of B molecules in the gas,
* is the collision cross section (unit m2), the area when two molecules collide with each other, simplified to , where the radius of A and the radius of B.
* is the Boltzmann constant (unit m2⋅kg⋅s−2⋅K−1),
* is the temperature (unit K),
* is the reduced mass of the reactants A and B, (unit kg)
