What type of molecules are primarily involved in dipole-dipole interactions?

Dipole-dipole interactions occur between molecules that have permanent dipoles, which are established when there is a significant difference in electronegativity between the atoms in a bond, leading to a distribution of charge within the molecule. This results in regions of partial positive and negative charges.

Polar molecules are characterized by this uneven charge distribution, which makes them capable of aligning themselves in such a way that the positive end of one polar molecule is attracted to the negative end of another polar molecule. This attraction between oppositely charged ends of the dipoles is what defines dipole-dipole interactions.

In contrast, ionic compounds consist of charged ions held together by ionic bonds and do not participate in dipole-dipole interactions. Nonpolar molecules, with their uniform charge distribution, lack permanent dipoles and do not exhibit these interactions either. Similarly, metallic compounds are characterized by a lattice of positive ions in a sea of delocalized electrons, leading to metallic bonding rather than interactions based on permanent dipoles. Therefore, the involvement of polar molecules in dipole-dipole interactions makes them the correct answer.