An electric monopole is a single charge, and a dipole is two opposite charges closely spaced to each other, or something which looks like that electrically. Dipoles are actually very abundant in nature. For example, a water molecule has a large permanent electric dipole moment. Its positive and negative charges are not centered at the same point; it behaves like two equal and opposite charges separated by a small distance. Another phenomenon happens to uncharged pith balls. In the presence of a charged object, the uncharged pith ball will be attracted to the charged object because the little dipoles have responded to the electric field of the rod.
Dipole–dipole interactions are electrostatic interactions of permanent dipoles in molecules. These interactions tend to align the molecules to increase the attraction (reducing potential energy). An example of a dipole–dipole interaction can be seen in hydrogen chloride (HCl): the positive end of a polar molecule will attract the negative end of the other molecule and influence their arrangement . Polar molecules have a net attraction between them.
Keesom interactions (named after Willem Hendrik Keesom) are attractive interactions of dipoles that are Boltzmann-averaged over different rotational orientations of the dipoles. The energy of a Keesom interaction depends on the inverse sixth power of the distance, unlike the interaction energy of two spatially fixed dipoles, which depends on the inverse third power of the distance. Molecules often contain dipolar groups but have no overall dipole moment. This occurs if there is symmetry within the molecule that causes the dipoles to cancel each other out. This occurs in molecules such as tetrachloromethane. Note that the dipole–dipole interaction between two atoms is usually zero, because atoms rarely carry a permanent dipole.