Term: mean free path, \(\lambda\)
https://doi.org/10.1351/goldbook.M03778

Definition:
The average distance a molecule travels between collisions. For a molecule, λ = ( 2 π n d m 2 )-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 x 10E-6 cm; at 10E-6 atmospheres and 25°C it is 9.7 cm. For an aerosol particle, the mean free path, λ B in the Stokes region (see Stokes law) is given by: λ B = 3 k T m m B where m is the mass of the particle, k is the Boltzmann constant (1.381 x 10E-23 J K-1), T is the temperature (K) and B is the mobility.

Related Terms:
1) aerosol (http://doi.org/10.1351/goldbook.A00176).
2) stokes (http://doi.org/10.1351/goldbook.S06027).
3) boltzmann constant (http://doi.org/10.1351/goldbook.B00695).
4) mobility (http://doi.org/10.1351/goldbook.M03955).
5) stokes law (http://doi.org/10.1351/goldbook.S06028).

Source: Green Book, 2nd ed., p. 56 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)

Citation: 'mean free path, \(\lambda\)' 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

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