A theoretical model, based on the Galitskii-Migdal-Feynman formalism, is introduced for determining the scattering properties of argon gas, especially the "effective" total, viscosity and average cross-sections. The effective phase shifts are used to compute the quantum second virial coefficient in the temperature range 87.3-120 K. The sole input is the Hartree-Fock dispersion (HFD-B3) potential. The thermophysical properties of the gas are then calculated. The results are in good agreement with experimental data.