Do movements of electrons and of nuclei affect each other?

When an atom or molecule is undergoing acceleration in translational motion, does the nuclei ever deviate from the exact centroid of the atom?

In other words, do the electron shell arrangments act more like a cage to confine the nucleus (much like a pebble in a beachball), or does the larger moving momentum of the nucleus cause deformations of the electron shell...?
Michael Prescott (eximer@netcom.com)

Yes, the electrons tug on the nucleus, and the nucleus tugs on the electrons, so their motions affect each other. But it's a very small effect because the nuclei are at least 3 orders of magnitude more massive than the electrons. We usually get away with assuming that the nuclei are clamped in place, while the electrons roam freely around and among them.

The assumption is called the "Born-Oppenheimer approximation". It makes calculating the energy of an atom or a molecule much simpler. You can ignore the kinetic energy of the nuclei and assume that the sum of all the internuclear repulsions is a constant. Then the nuclear and the electron motions can be treated as two separate (and easier) problems.

Are you looking for situations where the motions aren't separable? It can and does happen, whenever the electrons can't adjust instantaneously to movements of the nuclei. Here are a few references that might be helpful (they're far above the general chemistry level!)

  • B. T. Sutcliffe, "Fundamentals of computational quantum chemistry", in Computational Techniques in Quantum Chemistry, G. H. F. Diercksen, B. T. Sutcliffe, and A. Veillard (Eds.), Reidel, Boston (1975), p. 1.
  • Born, M., Oppenheimer, J. R., Ann. Phys. Liepzig, 84, 457 (1927).
  • Born, M., Huang, K., "Dynamical Theory of Crystal Lattices", Oxford Press, Oxford (1954).
  • Longuet-Higgens, H. C., Adv. Spectr.,, 2, 429 (1961).

Author: Fred Senese senese@antoine.frostburg.edu



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