Ionic mobility in aqueous solution is inversely proportional to the size of the hydrated ion. Smaller ions have a higher charge density, which leads to stronger hydration (attraction to water molecules), forming a larger hydrated ion. Larger hydrated ions move slower through the solution, resulting in lower ionic mobility.

Why Option (D) is correct:

For isoelectronic ions or ions in the same period, as the charge increases, the charge density increases, leading to stronger hydration and a larger hydrated radius. Therefore, the ionic mobility decreases. For ${\mathrm{Na}}^{+}$, ${\mathrm{Mg}}^{2+}$, and ${\mathrm{Al}}^{3+}$, all are isoelectronic with 10 electrons. The order of charge is ${\mathrm{Na}}^{+}<{\mathrm{Mg}}^{2+}<{\mathrm{Al}}^{3+}$. This means the order of hydration is ${\mathrm{Na}}^{+}<{\mathrm{Mg}}^{2+}<{\mathrm{Al}}^{3+}$ (increasing hydrated radius). Consequently, the order of ionic mobility is ${\mathrm{Na}}^{+}>{\mathrm{Mg}}^{2+}>{\mathrm{Al}}^{3+}$.

Option Analysis:

• A) ${\mathrm{Be}}^{2+}>{\mathrm{Ba}}^{2+}$: ${\mathrm{Be}}^{2+}$ is much smaller than ${\mathrm{Ba}}^{2+}$. Due to its high charge density, ${\mathrm{Be}}^{2+}$ is extensively hydrated, forming a very large hydrated ion. ${\mathrm{Ba}}^{2+}$ is larger and less hydrated. Therefore, hydrated ${\mathrm{Be}}^{2+}$ is larger than hydrated ${\mathrm{Ba}}^{2+}$, meaning ${\mathrm{Be}}^{2+}$ has lower ionic mobility than ${\mathrm{Ba}}^{2+}$. So, ${\mathrm{Be}}^{2+}<{\mathrm{Ba}}^{2+}$. This option is incorrect.
• B) ${\mathrm{Li}}^{+}>{\mathrm{Rb}}^{+}$: ${\mathrm{Li}}^{+}$ is the smallest alkali metal ion, leading to the highest charge density and most extensive hydration. ${\mathrm{Rb}}^{+}$ is much larger and less hydrated. Therefore, hydrated ${\mathrm{Li}}^{+}$ is larger than hydrated ${\mathrm{Rb}}^{+}$, meaning ${\mathrm{Li}}^{+}$ has lower ionic mobility than ${\mathrm{Rb}}^{+}$. So, ${\mathrm{Li}}^{+}<{\mathrm{Rb}}^{+}$. This option is incorrect.
• C) ${\mathrm{I}}^{-}<{\mathrm{Cl}}^{-}$: ${\mathrm{I}}^{-}$ is a larger ion than ${\mathrm{Cl}}^{-}$. Larger ions have lower charge density and are less hydrated. Therefore, hydrated ${\mathrm{I}}^{-}$ is smaller than hydrated ${\mathrm{Cl}}^{-}$ (or at least, ${\mathrm{I}}^{-}$ is less hydrated and thus moves faster). So, ${\mathrm{I}}^{-}>{\mathrm{Cl}}^{-}$. This option is incorrect.

Correct Answer: (D)