Alloying: When an impurity atom of a different size compared to the host atom is placed into the crystal lattice, the impurity atom distorts the resin around it, either by pushing the host atoms farther away, or by pulling them in, as depicted. The cross section that scatters the electron is the lattice region that has been elasticity distorted by the impurity (the impurity atom itself and its neighbouring host atoms), so that in this zone, the electron suddenly experiences a force due to sudden change in PE. This region has a large scattering event from impurity only. We define τ_t as the mean time between scattering events arising from thermal vibrations alone and as the mean time between scatterings events arising from collisions with impurity alone. In, general an electron may be scattered by both processes, so effective mean time τ between any which scattering events will be less than the individual scattering time.