A theory was developed to treat the formation of ion-molecule clusters in the gas phase as a discrete process and it includes ion-dipole and dipole-dipole interactions as a basis for depicting the electrostatic force between the central ion and the clustering molecules. All the energy of the interactions is summed over the total number of clustering molecules around a central ion. The total free energy of cluster formation is comprised of the electrostatic energy of the cluster formation, the free energy of vaporization of the condensing molecules, and the work done in changing the surface area through condensation of additional molecules about each ion cluster. A minimum in the total free energy with respect to the number of neutral molecules corresponds to the most stable ion-molecule cluster. Comparison between the theoretical predictions and experimental observations for both stable and radioactive ion clusters showed good agreement.