van der Waals Bonding

Water molecules in liquid water are attracted to each other by electrostatic forces, and these forces have been described as van der Waals forces or van der Waals bonds. Even though the water molecule as a whole is electrically neutral, the distribution of charge in the molecule is not symmetrical and leads to a dipole moment - a microscopic separation of the positive and negative charge centers. This leads to a net attraction between such polar molecules which finds expression in the cohesion of water molecules and contributes to viscosity and surface tension. Perhaps it is fair to say that van der Waals forces are what holds water in the liquid state until thermal agitation becomes violent enough to break those van der Waal bonds at 100°C. With cooling, residual electrostatic forces between molecules cause most substances to liquify and eventually solidfy (with the exception of helium, which never becomes a solid at atmospheric pressure).

Even nonpolar molecules experience some van der Waals bonding, which can be attributed to their being polarizable. Even though the molecules don't have permanent dipole moments, they can have instantaneous dipole moments which change or oscillate with time. These fluctuations of molecular dipole moments lead to a net attraction between molecules which allow nonpolar substances like carbon tetrachloride to form liquids. Examination of the dipole electric field shows that the electric field from one instantaneous dipole will tend to polarize a neighboring molecule such that it will be attracted - sort of the electrical analog to a bar magnet magnetizing a paper clip so that it will be attracted to the magnet. (This happens regardless of which pole of the magnet is brought close to the paper clip.) The weaker van der Waals forces in nonpolar liquids may be manifested in low surface tension and low boiling points.


Bond concepts

Elem. Modern Physics, Ch 5
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