intermolecular force

References:

• https://en.wikipedia.org/wiki/Intermolecular_force and linked pages
• conversations with GPT-4

A molecule is a set of atoms bonded together by chemical bonds (covalent or ionic bonds). Forces between molecules are weaker than chemical bonds, but have significant effects on properties and structure of the macro-scale material.

As far as I can tell, the major intermolecular 'forces' are all really just manifestations of electromagnetism as the fundamental force. They arise from a nonuniform charge distribution across a molecule creating a dipole (or sometimes higher-order poles due to more complex distributions).

Some molecular dipoles are 'permanent' due to the structure of the molecule containing electronegative atoms that tend to concentrate electrons. For example, the oxygen atom in a water molecule draws in the electrons from the hydrogen, creating a polar molecule and enabling hydrogen bonds (hydrogen bonds are considered the strongest of the intermolecular forces, because the polarization between a hydrogen and electronegative atom is quite strong compared to polarization in larger molecules).

Other dipoles are temporary or 'induced' by the presence of nearby dipoles. Totally neutral (nonpolar) atoms can even form transient dipoles, which induce dipoles in their neighbors, as in the London dispersion force.

Thus we can taxonimize dipole-dipole forces as:

1. induced-induced (London dispersion force)
2. permanent-induced (Debye forces)
3. permanent-permanent (includes Keesom interactions and hydrogen bonds)

For permanent dipoles, we also consider rotation. The "Keesom interaction" averages the dipole attraction between rotating molecules across all rotations, assuming that the attraction is not so strong as to fix the molecules into a single rotation. Naively, this average might include equal contributions from attractive and repulsive alignments, leading to a net zero force. But because the attractive alignments are somewhat more energetically favorable (if not enough so to fix the molecules in these alignments), the molecules spend more time in these alignments, leading to a net attractive force. This effect is stronger at low temperatures, and weakens at high temperatures where the increased energy is more likely to jostle molecules out of their 'preferred' attractive configurations.