bonding, metals usually have high melting points, show great strength, and are
good conductors of electricity.
INTERMOLECULAR FORCES OF ATTRACTION
The term intermolecular forces refers to attractions between molecules.
Although it is proper to refer to all intermolecular forces as van der Waals
forces, named after Johannes van der Waals (Netherlands), this concept should be
expanded for clarity.
Dipole-Dipole Attraction
One type of van der Waals forces is dipole-dipole attraction. It was shown in the
discussion of polar covalent bonding that the unsymmetrical distribution of
electronic charges leads to positive and negative charges in the molecules, which
are referred to as dipoles. In polar molecular substances, the dipoles line up so
that the positive pole of one molecule attracts the negative pole of another. This is
much like the lineup of small bar magnets. The force of attraction between polar
molecules is called dipole-dipole attraction. These attractive forces are less than
the full charges carried by ions in ionic crystals.
TIP
Weakest of all, the London dispersion forces are one-tenth the force of most dipole
attractions.London Dispersion Forces
Another type of van der Waals forces is called London dispersion forces. Found
in both polar and nonpolar molecules, it can be attributed to the fact that a
molecule/atom that usually is nonpolar sometimes becomes polar because the
constant motion of its electrons may cause uneven charge distribution at any one
instant. When this occurs, the molecule/atom has a temporary dipole. This dipole
can then cause a second, adjacent atom to be distorted and to have its nucleus
attracted to the negative end of the first atom. London dispersion forces are about
one-tenth the force of most dipole interactions and are the weakest of all the
electrical forces that act between atoms or molecules. These forces help to
explain why nonpolar substances such as noble gases and the halogens condense
into liquids and then freeze into solids when the temperature is lowered
sufficiently. In general, they also explain why liquids composed of discrete
molecules with no permanent dipole attraction have low boiling points relative to
their molecular masses. It is also true that compounds in the solid state that are