MadSci Network: Chemistry |
Why doesn't a DISPERSION force repel as strongly as it attracts
If they are due to the random unsymmetrical distribution of electrons around a molecule and thus creating a temporary dipole, why don't you get a (random) distribution that causes molecules to repel. Which would then cancel the intermolecular attraction? This is a very good question, Andrew. You are missing one thing, and that is that the temporary dipoles in neighboring molecules are not independent. Instead, temporary dipoles induce temporary dipoles in neighboring molecules in such a way that an attractive interaction results. Think of it this way: consider three nonpolar molecules. In my diagrams, the crosses are the nuclei and the circles represent the electron clouds. Suppose all the electrons rush to one side of one of the molecules. They will exert a powerful repulsive force on the electrons in the neighboring molecule on that side, which will push them to the other side of the neighboring molecule. This leaves exposed nuclei on the neighbor, and the positive charge is attracted to the negatively-charged electrons of the first molecule. Meanwhile, the exposed nuclei of the first molecule are busy attracting electrons on a neighboring molecule on the other side. The resulting imbalances in electron distributions are then aligned in such a way as to create attractive forces between all three molecules.While this does not happen sequentially, and indeed is likely to be destroyed by electron motions in the next moment, the overall result is a situation in which attractive forces slightly outnumber the repulsive ones. Another, more complicated, way to think of this is as follows:Remember that dispersion forces are weak, and they fall off very rapidly with distance. Because of this, I usually compare dispersion forces to friction, because they are related to molecular surface area. I usually compare interactions between nonpolar molecules to those between pieces of cooked pasta. It's harder to separate large pasta shells than small ones, and much harder to separate cooked spaghetti strands from each other than it is to separate individual pieces of cooked macaroni. Some of this answer may be found in P.W. Atkins' Physical Chemistry. Some of it is my own opinion, bolstered by my knowledge of physics.
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