Gas-phase front-side attack identity S(N)2(C) and S(N)2(Si) reactions, CH(3)X1 + X2(-) -> CH(3)X2 + X1(-) and SiH(3)X1 + X2(-) -> SiH(3)X2 + X1(-) (X = F, Cl), are investigated by the ab initio method and molecular face (MF) theory. The computations have been performed at the CCSD(T)/aug-cc-pVTZ//MP2/6-311++G(3df,3pd) and CISD/aug-cc-pVDZ levels. Front-side attack identity S(N)2 reactions for both SiH3X and CH3X have double-well potential energy surfaces (PESs), but their transition-state positions are different relative to the positions of reactants and products: it is lower for SiH3X, and it is higher for CH3X. The minimum energy path for an S(N)2(Si) reaction with retention of configuration proceeds from a stable pentacoordinated anion intermediate of C-s symmetry (TBP) via a C-s transition state (SP) to a complementary pentacoordinated intermediate (TBP) and finally up to separate products. Berry pseudorotation has been observed in the front-side attack identity S(N)2(Si) reactions with F- and Cl- along the intrinsic reaction coordinate (IRC) routes. In addition, the geometrical transformations of front-side attack identity S(N)2(C) and S(N)2(Si) reactions based on the IRC calculations at the MP2/6-311++G(3df, 3pd) level of theory are described compared with those of corresponding back-side attack reactions. The difference between front-side attack identity S(N)2(C) and S(N)2(Si) reactions has been demonstrated. In MF theory, the potential acting on an electron in a molecule (PAEM) is all important quantity; in particular, its D-pb can measure the strength of a chemical bond in a molecule. It is found that the difference between D-pb values of reactant and transition state may be related to the activation energy for front-side and back-side attack S(N)2(C) and S(N)2(Si) reactions, and the D-pb curves along the IRC routes have features similar to those of the potential energy profiles for all of the back-side attack S(N)2 reactions and front-side attack S(N)2(Si) reaction with F-. Furthermore, according to the MF theory, the spatial dynamic chaging features of the molecular shapes and the face electron density are vividly depicted for the course of the reactions.