6.2 van der Waals Interactions

An atom's electrons orbit its nucleus. Electrons have a negative electric charge, and nuclei have a positive charge.

One phenomenon that is observed between atoms happens when an electron of one atom is facing the other atom while the other atom's electrons have momentarily left a vacant spot that the nucleus can be seen through. These chance events allow the two atoms to weakly attract each other just for that instant. These are instances of van der Waals force.

But if left entirely to chance, the odds that one atom's electron would face the other atom's nucleus would be the same as the odds of two electrons facing each other, and the net force would be zero, neither attractive nor repulsive. However, the repulsion between electrons actually influences their orbits, causing one atom to open up a space in the direction of the other atom's electron. This is called polarization, and the degree to which an atom can repond in this way is called its polarizability.

This polarization of charge creates what's called a dipole, literally meaning "two poles", and its force is a dipole moment, using the term "moment" not to mean a point in time but rather in the mathematical sense. Van der Waals force is cause by temporary dipole moments between atoms that do not have any charge of their own. It's the only attractive force between saturated hydrocarbons, and it is the weakest of the intermolecular forces, generally between 0.4 to 4.0 kJ/mol or 0.1 to 1.0 kcal/mol. Yet this is the force that allows insects and geckos to climb up walls, which is really pretty impressive. I gotta hand it to lizards for managing to hold up their entire body weight with the weakest intermolecular force!

Most intermolecular forces are proportional to the inverse square of the distance between interacting atoms, but van der Waals forces are proportional to the inverse sixth power. Remember the two interacting atoms must polarize each other for the force to work. The effect of atom 1 polarizing atom 2 would follow the inverse square law, and atom 2 polarizing atom 1 would multiply by the inverse square again. The resulting attractive force between the temporary dipoles would contribute yet another inverse square. So the math works out to an inverse sixth, because 1/(r² r² r²) = 1/r⁶.

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