van der Waals attraction or repulsion?

Loudon mentions two van der Waals forces in Chapter 2, an attractive force and a repulsive force. This is confusing because, other than the fact that both forces have something to do with electrons, they are not related.

The attractive van der Waals force is described nicely in Figure 2.8. It appears when the distance between two atoms (or molecules) is short, but not too short (if the atoms’ space-filling models are just touching or are separated by a small gap, then that’s perfect). The force is created by temporary imbalances in each atom’s electron distribution caused by electron movement around the nucleus. The imbalances produce small temporary electrostatic fields and two atoms will naturally correlate their imbalances (fields) so that they attract one another. The attractive force is very weak.

The repulsive force is another beast entirely. It suddenly replaces the attractive force when the distance between the atoms is less than the sum of the atoms’ van der Waals radii. The repulsive force is very strong and we blame it on Pauli repulsion, the tendency of same-spin electrons to avoid each other (or “die” trying).

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