Final answer:
Rubidium (Rb) exhibits a stronger ion-dipole attraction to water than sodium (Na). This is due to the larger size of Rb ions compared to Na ions, which results in a lower charge density and thus a stronger attraction to the polar water molecules.
Step-by-step explanation:
The strength of the ion-dipole attraction between an alkali metal ion and water depends on the charge density of the ion, which is, in turn, inversely related to the size of the ion. In this case, Rb (rubidium) exhibits a stronger ion-dipole attraction to water than Na (sodium) because Rb has a larger atomic radius and is lower on the periodic table, making its ion larger and thus reducing its charge density compared to Na.
This is backed by the concept that ions with lower charge densities will typically be less strongly solvated than those with higher charge densities. Given that charge density is calculated as the charge of the ion divided by its volume, and that Rb+ is larger than Na+, Rb+ ions will have a lower charge density and, therefore, experience a stronger ion-dipole attraction when solvated by water molecules.