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How do London Dispersion Forces and Dipole-Dipole forces exist at the same time between polar molecules?

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closed as off-topic by M.A.R. ಠ_ಠ, airhuff, Gaurang Tandon, Jannis Andreska, aventurin Mar 18 '18 at 15:56

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Let's take a polar, covalently bonded molecule, like $\ce{HCl}$.

In this molecule, chlorine is more electronegative than hydrogen, so chlorine attracts the bonding electrons more than hydrogen in the covalent bond. This means that chlorine will always be slightly more negatively charged than hydrogen. This gives way to permanent dipole-dipole interactions between other molecules, since slightly negative chlorine atoms will attract slightly positive hydrogen atoms in other molecules.

However, the electrons are constantly moving around both atoms in the molecule. This means that at any point in time, there may be an imbalance of electrons on one side of a molecule, meaning that side of the molecule will be slightly negatively charged, whilst the other side will be slightly positively charged. This gives way to a temporary dipole that's induced between molecules, and it works in the same way as the permanent dipole-dipole interactions, where the slightly negative side of a molecule will attract the slightly positive side of another molecule. Since this dipole is temporary, the electrons may redistribute themselves so that there's a new temporary dipole, or no temporary dipole at all.

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