Ion-Dipole and Interparticle Forces

Drop NaCl into water and the ions don't dissolve because water is friendly. They dissolve because water is polar. Each water molecule has a partial negative charge on its oxygen and a partial positive charge on its hydrogens. When Na+ enters the water, the oxygen ends of nearby water molecules swing around to face it, since opposite charges attract. When Cl- enters, it's the hydrogen ends that turn toward it. This orientation is the ion-dipole force: a full charge on the ion pulling on a partial charge on the molecule.

Once enough water molecules surround an ion in this oriented arrangement, you've got a hydration shell. For Mg2+, which has a small radius and a 2+ charge, the shell is tight and strongly held: six or more water molecules point their oxygens directly at the ion, and that first shell is organized enough to drag a second, looser shell of water around it too. The stronger the ion's charge and the smaller its radius, the more tightly packed and energetically stable that shell becomes. This is why hydration energy depends on charge squared divided by radius: double the charge and you quadruple the attraction, because the force itself follows the same inverse pattern as Coulomb's law.

It's tempting to call this an ionic bond, since both involve charges attracting, but they're not the same kind of event. An ionic bond forms when electrons actually transfer from one atom to another, creating two new ions locked together by that full-charge-to-full-charge pull. An ion-dipole force transfers nothing. It's an intermolecular attraction between an ion that already exists and a molecule that already has its own partial charges. This distinction sets up the rest of the unit, where you'll compare ion-dipole forces to weaker intermolecular forces like hydrogen bonding and London dispersion, all of which govern solubility and solution behavior without any electrons changing hands.