van der Waals forces
/ van der VAHLS /
Have you noticed how two perfectly smooth, clean glass surfaces stick faintly when pressed together? That barely-there cling is a glimpse of van der Waals forces — the weakest of all the everyday attractions, present between any two atoms that come close enough to touch.
Every atom's electrons are in constant motion, and at any instant they may bunch slightly to one side, giving the atom a fleeting, tiny plus-and-minus (a momentary dipole). That flicker tugs the electrons of a neighboring atom into a matching arrangement, so the two briefly attract. The effect lasts only an instant and is extraordinarily weak — far weaker than a hydrogen bond — and it has a strict requirement: the atoms must be very close, almost touching, because the force fades sharply with distance. Push them too close, though, and their electron clouds repel. So there is a 'just right' distance (the van der Waals contact distance) where the gentle attraction peaks.
Because van der Waals forces demand near-perfect closeness, they reward good shape-fit above all. When two molecular surfaces are exactly complementary — every bump fitting a hollow — a great many atoms touch at once, and all those tiny attractions sum to something significant. This is why molecular recognition is so shape-dependent: a snugly fitting partner racks up van der Waals contacts that a mismatched one cannot. They also help stack the flat bases inside DNA, contributing to the stability of the double helix alongside hydrogen bonds.
A gecko walks up glass because the millions of microscopic hairs on its toes make so many van der Waals contacts that, summed together, they support the animal's weight — the same all-from-many principle that lets a well-fitted drug molecule cling to its target protein.
The weakest force of all, made powerful by perfect fit.
Van der Waals attraction works only at very short range and turns into strong repulsion if atoms are forced too close — so it is as much about defining how close atoms can pack as about gluing them together.