The kinetic diameter is not the same as atomic diameter defined in terms of the size of the atom's electron shell, which is generally a lot smaller, depending on the exact definition used.
[1] Kinetic diameter is related to the mean free path of molecules in a gas.
Mean free path is the average distance that a particle will travel without collision.
Here, the Maxwell–Boltzmann distribution of energies must be considered, which leads to the modified expression,[3] The following table lists the kinetic diameters of some common molecules; Collisions between two dissimilar particles occur when a beam of fast particles is fired into a gas consisting of another type of particle, or two dissimilar molecules randomly collide in a gas mixture.
The scattering cross section, σ, in a collision between two dissimilar particles or molecules is defined by the sum of the kinetic diameters of the two particles, We define an intensive quantity, the scattering coefficient α, as the product of the gas number density and the scattering cross section, The mean free path is the inverse of the scattering coefficient, For similar particles, r1 = r2 and, as before.