Permanent dipole-dipole forces arise between molecules due to what?

• A. Permanent dipole-dipole forces
• B. Hydrogen bonding
• C. London dispersion forces
• D. Ionic bonding

Answer: (A)

When a molecule has a permanent dipole moment, parts of the molecule carry partial charges: one end is slightly negative and the other slightly positive. This happens when polar covalent bonds and an asymmetric shape prevent an even distribution of electron density. Those partial charges create electrostatic attractions with the partial charges on nearby molecules—the positive end of one molecule is drawn to the negative end of another. That specific, persistent attraction is what we call permanent dipole-dipole forces.

This is stronger than London dispersion forces, which arise from temporary, momentary dipoles that occur in all molecules, and it’s different from hydrogen bonding, which is a particular strong type of dipole-dipole interaction involving hydrogen bonded to N, O, or F. It’s also not ionic bonding, which involves full charges and transfer of electrons to form ions.