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:  
which has units of [volume][time].
Here,
*  is the number of A molecules in the gas,
*  is the number of B molecules in the gas,
*  is the collision cross section, the "effective area" seen by two colliding molecules, simplified to , where  the radius of A and  the radius of B.
*  is the Boltzmann constant,
*  is the temperature,
*  is the reduced mass of the reactants A and B,