## Wikipedia - Cammino libero medio mean free path, $$\lambda$$

https://doi.org/10.1351/goldbook.M03778
The average distance a molecule travels between collisions. For a molecule, $$\lambda = (\sqrt{2}\ \pi \ n\ d_{\text{m}}^{2})^{-1}$$, where $$n$$ is the number of molecules per unit volume and $$d_{\text{m}}$$ is their mean diameter. For O2 at one atmosphere and $$25\ ^{\,\unicode{x26ac}}\text{C}$$, this distance is only $$9.7\times 10^{-6}\ \text{cm}$$; at $$10^{-6}$$ atmospheres and $$25\ ^{\,\unicode{x26ac}}\text{C}$$ it is $$9.7\ \text{cm}$$. For an @A00176@ particle, the mean free path, $$\lambda _{\text{B}}$$ in the @S06027@ region (see @S06028@) is given by: $$\lambda _{\text{B}} = \sqrt{\frac{3\ k\ T}{m}}\ m\ B$$ where $$m$$ is the mass of the particle, $$k$$ is the @B00695@ ($$1.381\times 10^{-23}\ \text{J K}^{-1}$$), $$T$$ is the temperature ($$\text{K}$$) and $$B$$ is the @M03955@.
Sources:
Green Book, 2nd ed., p. 56 [Terms] [Book]
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2201 [Terms] [Paper]