The S(N)2 reactions at C and Si have been investigated by ABEEM-sigma pi method compared with the potential acting on an electron in a molecule (PAEM). The ABEEM-sigma pi model demonstrates the charge distributions of the atom region, bond region, and lone-pair electron region during the course of the reaction. The differences between title reactions have been demonstrated. Studies indicate that there are good linear correlations among the charges obtained by ABEEM-sigma pi method, the force constants, the relative energies, the ionization potentials, etc. This study enhances our understanding of the electron transfer from the nucleophile to the leaving group.