Ion-dipole effects vs. atomic radius

  1. 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.
     
  2. jcsd
  3. Borek

    Staff: Mentor

    Radius changes distance between charges.
     
  4. 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?
     
  5. Borek

    Staff: Mentor

    Smaller ion means dipoles are closer to the charge, so the Coulomb force is larger.
     
  6. I believe this has to do with the difference in electronegativity. Lithium has a higher electronegativity.
     
  7. DrDu

    DrDu 4,421
    Science Advisor

    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
     
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