Box 47 Van der Waals forces

Non-polar molecules have no permanent dipole and cannot form normal bonds. The non-polar noble gases, however, condense to liquid and ultimately form solids if cooled sufficiently. This suggests that some form of intermolecular force holds the molecules together in the liquid and solid state. The amount of energy (Box 4.8) required to melt solid xenon is 14.9kJmol-1, demonstrating that cohesive forces operate between the molecules.

Weak, short-range forces of attraction, independent of normal bonding forces, are known as van der Waals' forces, after the 19th-century Dutch physicist. These forces arise because, at any particular moment, the electron cloud around a molecule is not perfectly symmetrical. In other words, there are more electrons (thus net negative charge) on one side of a molecule than on the other, generating an instantaneous electrical dipole.

This dipole induces dipoles in neighbouring molecules, the negative pole of the original molecule attracting the positive pole of the neighbour. In this way, weak induced dipole-induced dipole attractions exist between molecules.

Induced dipoles continually arise and disappear as a result of electron movement, but the force between neighbouring dipoles is always attractive. Thus, although the average dipole on each molecule measured over time is zero, the resultant forces between molecules at any instant are not zero.

As the size of molecules increase, so do the number of constituent electrons. As a result, larger molecules have stronger induced dipole-induced dipole attractions. It must be stressed, however, that van der Waals' forces are much weaker than both covalent and ionic bonds.

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