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I'm having trouble understanding exactly what intermolecular forces occur in ionic compounds.

What I understand is that ionic compounds are held together by electrostatic interactions between opposite charged ions and that they aren't "finite" in the way covalent molecules are, and therefore they do not have intermolecular forces.

However, while my textbook says that only covalent molecules have van der Waals forces, my professor said that all compounds, even ionic compounds have London dispersion interactions.

Isn't London dispersion a van der Waals force? How does that work?

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  • $\begingroup$ Generally while studying you tend to talk in absolutes: crystalline solids are fully ionic and molecules are fully covalent. However, often reality is a middle-ground; covalent bonds don't always mean 100% electron sharing (which is why we have the concept of polarized bonds), and ionic bonds don't always mean 100% electron abstraction (which is why we know the concept of polarizable anions and polarizing cations). The interaction between polarized ions is indeed the London force. $\endgroup$ – chemicalromance Feb 6 at 14:11
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London dispersion forces are instantaneous dipole-induced dipole interactions. All compounds can have an instantaneous dipole moment, including ions, so London dispersion forces exist for all compounds.

Unlike molecules, ions are charged. The electrostatic interactions between charges are far stronger than the London dispersion forces between ions, so normally dispersion forces are ignored when considering ions.

Therefore, both your textbook and your professor are correct.

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  • $\begingroup$ Just to help OP even more: while a covalent crystal relies only on dispersion forces, an ionic one would be hold even if the latter forces wouldn't exist. $\endgroup$ – Alchimista Feb 5 at 15:31

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