The stability of M+ ions in aqueous medium depends on three factors: (i) Enthalpy of atomisation (ii) First ionisation enthalpies of isolated gaseous atom (iii) Hydration enthalpy.
An element in M+ state in aqueous medium is stabler if the electrode potential ( M+/M) value of more negative. Down the group, the tendency to form M+ ion increases. In this process, ionisation of solid metal is not involved in this process.