To understand the molecular and electronic structure of alkali metal ions, we carry out the MP2 calculation and demonstrate that the maximal coordinator numbers for the hydrated K+ and Rb+ are 8, while those for the hydrated Cs+ and Fr+ are 10. Furthermore, on the basis of the binding energy, the HOMO-LUMO gap and the electron affinity, the stability of the molecular and electronic structures of M+ (H2O)8 (M = K, Rb, Cs, Fr) decreases with the increasing alkali metal atomic number and the stability of the molecular structures of M+ (H2O)8-10 (M = Cs, Fr) decreases with the increasing cluster size. [ABSTRACT FROM AUTHOR]