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Author Topic: Ionic Compounds and van der waals forces  (Read 232 times)

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bigchungus

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Ionic Compounds and van der waals forces
« on: February 04, 2019, 02:39:25 PM »

Okay, so 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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Enthalpy

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Re: Ionic Compounds and van der waals forces
« Reply #1 on: February 05, 2019, 05:39:01 AM »

Graphite is one example of covalent molecule with no clear limits. Or polyethylene. Or metals.

I leave the vocabulary questions to other people.
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Corribus

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Re: Ionic Compounds and van der waals forces
« Reply #2 on: February 05, 2019, 10:32:17 AM »

Van der Waals forces (usually) collectively refer to interactions between molecules and atoms that aren't permanent covalent or ionic bonds. Most of them are electrostatic in nature. London forces are one particular type of VdW force that arise due to instantaneous fluctuations in the charge density on adjacent molecules. A way to think of London forces is the following. Even in a neutral, nonpolar molecule (think propane), there are short-lived fluctuations in the electric fields created by the molecular electrons. These fluctuations create instantaneous positive or negative charge distributions that cause molecules to stick together, because nearby positive and negative charges exhibit an attractive electrostatic force. Not all VdW forces are London forces, but there is a tendency to use them interchangeably at times. Other classes of VdW (intermolecular) forces include those between permanent dipoles, induced dipoles (e.g., a polar molecule next to a nonpolar molecule will induce a slight dipole in the nonpolar molecule because of the electric field surrounding the dipolar molecule), and natural repulsion due to Pauli exclusion pressure.

There's a lot of ambiguity with nomenclature along these lines so it's important to clarify what a person means when they speak of van der Waals forces, and you shouldn't be too surprised if textbooks, articles, and instructors use slightly different definitions. It does cause some confusion.

Ionic compounds would technically have London dispersion forces, but they are so weak compared to the permanent ionic bonds that they really don't have much of an influence over the properties of the compound. For this reason, you really only talk about dispersion forces in the context of covalent molecules.
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