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.
Index

Bond concepts

Reference
Tipler
Elem. Modern Physics, Ch 5
HyperPhysics***** Quantum Physics ***** Chemistry  R Nave
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