# Origin of Electromagnetic Interactions between Molecules

What is the origin of electromagnetic interaction between molecules? Anyway, it should have some relation with atoms. Also, These electromagnetic interactions are playing a major role in different properties of matter including the transition between solid-liquid-gas. Hence, what would be the source of these interactions..?

If these interactions originate from atoms, then some other questions come into focus:

• Are they related to the transition of electrons between various energy levels and emission of photons from an atom..?

• Are they related to cohesive forces between molecules in solids & liquids?

• note, that not all of the interactions (especially repelling) between molecules are electromagnetic in nature. – Yrogirg Aug 27 '12 at 7:52
• @Yrogig: Could you please provide a link for deeper study..? – Waffle's Crazy Peanut Aug 27 '12 at 12:45
• I meant en.wikipedia.org/wiki/Pauli_repulsion they are the forces that account for repelling between molecules at small distances. The book "Intermolecular and surface forces" by Israelachvili (Chapter 7) might be of interest. – Yrogirg Aug 27 '12 at 12:48

It is mainly an electrostatic interaction through a dipole-dipole interaction. However, the dipole moment can be permanent or induced. Depending on its nature the force has a different name:

You can also refer Vanderwaals Force

An atom or molecule is usually globaly neutral: ie there is exactly the same number of positive and negative charges. However the center of charge (barycenter of the qi) for the positive and negative charges does not always coincide. This gives rise to an electrostatic dipole that can interact with an external electric field. To answer to your first bullet, the electrostaic dipole is not linked to electrons changing orbital. The cohesive forces in solids have two different origin: orbital coupling (not of electromagnetic nature) or ionic bonds in ionic crystals (Na+, Cl-) for instance. In the latter the cohesion of the crystal is due to electrostaic interaction (not dipolar). For the liquids, the electrostatic forces are responsible for the observed properties. Polar solutions are made of molecules with a dipolar moment and are able to dissolve ionic crystals.

It looks like "electromagnetic interaction between molecules" is mostly due to Electrostatic force in the form of dipole-dipole forces and so on.

• This is not completely correct-- London forces are electrostatic, but Casimir forces are electrodynamic, and the transition is important at distances which are not infinite, but comparable to the wavelength of the atomic transitions. – Ron Maimon Aug 25 '12 at 21:48
• @Ron Maimon: I respectfully disagree, as I said "mostly due", and, as far as I know, Casimir forces are relatively very small, as evidenced by the fact that they were experimentally discovered decades after they were predicted. Therefore, I believe my answer is "completely correct", although it was not intended to be "complete". – akhmeteli Aug 26 '12 at 1:23
• Casimir forces are not so small, they are continuously linked to electrostatic London forces as the separation gets small. The measurements are only difficult for macroscopic objects. – Ron Maimon Aug 26 '12 at 4:48
• @Ron Maimon: So could you give a reference (earlier than, say, 1972 - remember, Casimir forces were discovered theoretically around 1948) to experimental evidence of Casimir forces for microscopic objects? Or otherwise explain in what sense Casimir forces are not so small? Or do you mean that electrostatic forces are in fact electrodynamic as they are photon-mediated? So far I just don't quite understand what you mean. – akhmeteli Aug 26 '12 at 5:17
• I mean that the Van-Der-Waals force, the coefficients "a" and "b" in the thermodynamic equation of state calculated by Van-Der-Waals, can be estimated from a ${1\over r^7}$ potential, not a ${1\over r^6}$ potential, with a cross over at some hundreds of atomic radii. The ${1\over r^6}$ force is electrostatic, and calculated by London from second order perturbation theory. I believe the discrepancy with experiment was noted by Casimir. The ${1\over r^7}$ force was calculated electrodynamically by Casimir, before Bohr suggested using vacuum energy, and the long-range part was known to be r^7. – Ron Maimon Aug 26 '12 at 5:49