mean free path, λ

https://doi.org/10.1351/goldbook.M03778
The average distance a molecule travels between collisions. For a molecule, λ=(2 π n dm2)−1, where n is the number of molecules per unit volume and dm is their mean diameter. For O2 at one atmosphere and 25 ⚬C, this distance is only 9.7×10−6 cm; at 10−6 atmospheres and 25 ⚬C it is 9.7 cm. For an aerosol particle, the mean free path, λB in the Stokes region (see Stokes law) is given by: λB=3 k Tm m B where m is the mass of the particle, k is the Boltzmann constant (1.381×10−23 J K−1), T is the temperature (K) and B is the mobility.
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]
Citation: 'mean free path' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.M03778 RIS BibTex EndNote
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