The S(N)2 reactions X- + CH3Y --> Y- + CH3X have been investigated in the gas phase and in solution using the Hartree-Fock level with 6-31+G* and 3-21G* bases, respectively. In the gas phase the interpretation has been done in the context of the HSAB principle using Gazquez's formalism using the polarizability (proportional to softness) of the two minima corresponding to the two ion-molecule complexes, and the TS, the softness of the nucleophile and the charge on the leaving group. The thermodynamic study shows that the reaction energy, DeltaE(r-p) and the energy differences between the two ion-molecule complexes, DeltaE(m), are close to the experimental data available. The application of the MHP indicates that in these reactions the molecules arrange themselves to be as hard as possible. The polarizable continuum model (PCM) has been used to study the influence of the solvent on the kinetics of the S(N)2 reaction. The application of Gazquez's formula provides fundamental information in a case study, with X = Cl and Y = I: the activation energy calculated with the PCM model yields a very good correlation with the values obtained with Gazquez's formula. Finally, we concentrate on the influence of solvation on the nucleophilicity and the kinetics using the (PCM) model with X = F, I and Y = Br and compare the results with the gas phase. The order of reactivity in the gas phase is F- < I-, which is the same order as in the solvent using the polarizable continuum model, but opposed to experiment. If, however, we take into consideration the interaction energy caused by the hydrogen bond, the order of reactivity in solution is reversed yielding the same results as experiment.