The sequence of ionic mobility in aqueous solution is:

In aqueous solution, smaller ions have higher charge density and undergo greater hydration. ${\text{Na}}^{+}$ is the smallest and most hydrated, while ${\text{Cs}}^{+}$ is the largest and least hydrated. A larger hydrated radius leads to lower mobility.Order of hydrated radius: ${\text{Na}}^{+}>{\text{K}}^{+}>{\text{Rb}}^{+}>{\text{Cs}}^{+}$.Order of mobility: ${\text{Cs}}^{+}>{\text{Rb}}^{+}>{\text{K}}^{+}>{\text{Na}}^{+}$.