where Ds,i (SI unit: m
2/s) is the surface diffusion coefficient for species
i.
where Rs,i (SI unit: mol/(m
2·s)) is the sum of all sources due to surface reactions and adsorption/desorption phenomena.
Writing the density of sites of the surface as Γs (SI unit: mol/m
2) and the site occupancy number for each species as
σi (dimensionless), the surface coverages can be calculated from: