Relativistic and nonrelativistic differential cross sections for argon, krypton, and iodine are computed for the energy range 20 eV-150 keV. The atomic fields as determined by Clementi (Hartree-Fock) and Liberman (Dirac) are used interchangeably in the two calculations. Contributions to the cross section arising from exchange between incident and atomic electrons are also considered. Relativistic and nonrelativistic comparisons are made, and where available, the experimental data are also reproduced. A number of conclusions regarding relativistic and exchange effects are reached, most noteworthy of which are the low-energy exchange effects.