# 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?

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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.

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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