Ionic mobility in an aqueous solution depends on the size of the hydrated ion, not the gaseous ion radius.

1. Hydration Concept: Smaller gaseous ions have a higher charge density and attract more water molecules, resulting in a larger hydrated radius. Conversely, larger gaseous ions have a smaller hydrated radius.
2. Trends:

• Gaseous Radii: $Na^+ < K^+ < Rb^+ < Cs^+$
• Hydrated Radii: $Na^+_{(aq)} > K^+_{(aq)} > Rb^+_{(aq)} > Cs^+_{(aq)}$

3. Mobility: Mobility is inversely proportional to the size of the moving species (the hydrated ion). Therefore, the smallest hydrated ion ($Cs^+$) moves the fastest, and the largest hydrated ion ($Na^+$) moves the slowest.
4. Conclusion: The order of ionic mobility is $Cs^+ > Rb^+ > K^+ > Na^+$. This is supported by conductivity data where $\lambda^0$ for $K^+$ (73.5) is greater than $Na^+$ (50.1) .