Collision frequency

Main article: Collision theory

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 Z = N_\text{A} N_\text{B} \sigma_\text{AB} \sqrt\frac{8 k_\text{B} T}{\pi \mu_\text{AB}}, where : N_\text{A} is the number of A particles in the volume, : N_\text{B} is the number of B particles in the volume, : \sigma_\text{AB} is the collision cross section, the "effective area" seen by two colliding molecules (for hard spheres, \sigma_\text{AB} = \pi(r_\text{A} + r_\text{B})^2, where r_\text{A} is the radius of A, and r_\text{B} is the radius of B), : k_\text{B} is the Boltzmann constant, : T is the thermodynamic temperature, : \mu_\text{AB} = \frac{m_\text{A} m_\text{B}}{m_\text{A} + m_\text{B}} is the reduced mass of A and B particles.