Van der Waals bonding

From Academic Kids

Van der Waals bonding is the sole process by which noble gasses are attracted to each other, and the dominant form of interaction between electrically neutral species with all of their bonds satisfied. These intermolecular attractions are weaker than both ionic and covalent bonding, and often are weaker than even hydrogen bonds. However, they play a major role in such fields as organic chemistry and surface science.

Bonding by van der Waals forces occurs through induced dipole interactions. By contrast to dipolar molecules, which posess a small static dipole due to electronegativity differences between covalently bonded atoms, dipole oscillations are observed in all atoms and molecules.

These oscillations create a dipole field that varies regularly in time, so that a nearby observer would see a positive charge at one moment, and a negative charge at the next. When two bodies have the same (or a similar) oscillation frequency, the positive charge on one body is able to synchronize with the negative charge on the next, so that electrostatic attraction occurs even though both bodies have an average charge of zero. In this way, even neutral atoms or molecules with satisfied orbitals can be bonded to one another. It is this bonding that constitutes van der Waals bonding.

Among the weakest bonds of this type are those which hold liquid helium together. Even very small amounts of energy are enough to break these bonds, which causes helium to boil at a very low temperature (-268.93 degrees celsius). The tiny amount of energy absorbed makes helium an extremely inefficient coolant. Fundamentally, this weak bonding is due to the tiny amount of charge present in helium (two electrons, two protons) and the stiffness with which the electron shell is held in place. These two effects combine to prevent any strong dipole oscillations.

By contrast, benzene and related compounds form extremely strong van der Waals bonds between the flat surfaces of their rings (phenyl stacking). The strength of such bonds is on par with hydrogen bonding: benzene boils at 80.1 degrees celsius, compared to water at 100 degrees. This is because aromatic compounds have a large and relatively dense electron cloud situated away from the plane of bonding. This makes it both more polarizable and more accessible to nearby phenyl groups.

In plastics without strong dipoles, van der Waals bonding is what holds adjacent polymer chains together into a coherent material. Compared to the covalent bonds which form the backbone of a polymer, van der Waals bonds are generally much less directional and universally more likely to re-form when broken. This gives polymers their ductility and toughness. This can be understood by analogy with fibre reinforced plastics, with covalent bonds in the role of fibers and van der Waals bonds corresponding to the matrix.

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