We present the effective optical potential of the interaction of an electron with an atom in dense semiclassical plasma of noble gases. This potential takes into account the collective screening effect and the quantum mechanical effect of diffraction. The influence of diffraction and screening effects on the characteristics of electron-atom collisions was investigated. Scattering phase shifts decrease with increase of the de Broglie wave. The electron-atom momentum-transfer cross-section at lambda(B) -> 0 tends to the data obtained earlier with a neglecting of the diffraction effect.