London (Dispersion) Forces and Dipole-Dipole?

Question

These two forces confuse me a lot. From what I know, London forces occurs in ALL substances. However, they are most likely found in "neutral" (?) molecules. Dipole-dipole interactions are between polar molecules and are permanent. Can anyone clarify these two more for me? Also, given two different types of atoms and molecules, how do you differentiate the two? Thanks in advance!

Answer 1

You answered your own question in a way...
You will find dipole-dipole interactions in polar molecules. So if the electronegativity difference between the bonded atoms is sufficient to give a polar covalent bond and the geometry of the molecule is such that the polar bonds do not cancel out the dipole moment of each other, then you are looking for dipole dipole (or even dipole-induced dipole interaction depending on the molecules).

Dispersion forces are mainly induced dipole-induced dipole interactions; though the atom doesn't have a partial charge, there is a temporary change/perturbation in its electron cloud, producing a temporary dipole. This induces a respective change in the electron cloud of the atom it is interacting with and thus creates a dipole of opposite orientation.

If the other atom already has a partial or net charge then the effect of the atom with no partial charge is not strong enough to induce a dipole. On the other hand the atom with the partial or net charge will cause a much stronger polarization of the atom that has no charge (compared to the case where two atoms with no partial charge interact with each other) and then you have dipole-induced dipole or ion-induced dipole interactions respectively.