513.001 Molecular and Solid State Physics

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Physical
Constants

Periodic System
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Ibach and Lüth, 1.3

It is well known that common salt (NaCl) is easily dissolved in water. In the solution Na and Cl atoms are present as positive and negative ("solvated") ions. Show that, due to the high dielectric constant of water and the resulting screening of the Coulomb potential, the binding of a NaCl crystal in water is smaller than the mean thermal energy of the free ions. Calculate the equilibrium separation of the ions of a hypothetical NaCl crystal in water and show that this separation is larger than the van der Waals radius of a water molecule, thereby justifying the approximate approach used here to discuss the solubility of NaCl.