Ion-dipole effects vs. atomic radius

[Bipolarity]

When dissolved in water, which of the following ions will form stronger ion-dipole bonds with the water molecules? Li+ or Na+?

Both have roughly the same charge... Na has greater radius, but I don't see why or how that has any bearing on the problem.

[Borek]

Radius changes distance between charges.

[Bipolarity]

Radius changes distance between charges.

I'm sorry but could you please elaborate a little more on that?
Radius I guess does decrease the force between the metal ion and each water molecule (due to Coulomb's law), but it turns out that Li+ actually forms more bonds with water. Why is that?

[Borek]

Smaller ion means dipoles are closer to the charge, so the Coulomb force is larger.

[dizam]

I believe this has to do with the difference in electronegativity. Lithium has a higher electronegativity.

[DrDu]

Solvation is often treated within a simple model, the Born model, which treats the atom as a charged sphere inside the medium assumed to be continuous and described by its dielectric constant. The model is still used a lot to describe the solvation of proteins even today.
Confer e.g.
http://pchemandyou.blogspot.com/2008/01/born-model-of-solvation.html