Ion-dipole effects vs. atomic radius

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Discussion Overview

The discussion centers on the comparison of ion-dipole interactions formed by Li+ and Na+ ions when dissolved in water. Participants explore the implications of ionic radius and electronegativity on the strength of these interactions, touching on theoretical models of solvation.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions which ion, Li+ or Na+, forms stronger ion-dipole bonds with water, noting that both have similar charges but different radii.
  • Another participant states that the radius affects the distance between charges, implying a relationship to the strength of ion-dipole interactions.
  • A participant seeks clarification on how radius influences the interaction, suggesting that while radius may decrease the force due to Coulomb's law, Li+ forms more bonds with water.
  • It is proposed that a smaller ionic radius allows dipoles to be closer to the charge, resulting in a larger Coulomb force.
  • One participant introduces the idea that the difference in electronegativity may also play a role, asserting that lithium has a higher electronegativity.
  • A later reply references the Born model of solvation, which treats ions as charged spheres in a continuous medium, suggesting its relevance to understanding solvation in broader contexts.

Areas of Agreement / Disagreement

Participants express differing views on the factors influencing ion-dipole interactions, particularly regarding the roles of ionic radius and electronegativity. The discussion remains unresolved, with no consensus reached on the primary influences.

Contextual Notes

Participants note that the relationship between ionic radius and interaction strength may depend on specific assumptions and definitions, and the discussion references theoretical models that may have limitations in their applicability.

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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.
 
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Radius changes distance between charges.
 
Borek said:
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?
 
Smaller ion means dipoles are closer to the charge, so the Coulomb force is larger.
 
I believe this has to do with the difference in electronegativity. Lithium has a higher electronegativity.
 
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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