|Molecular Shapes and the Polarity of Molecules
|One of the main reasons you should be concerned about the polarity
of molecules is because many physical properties, such as melting point and
boiling point, are affected by it. This is because polar molecules attract
each other - the positive end of one polar molecule attracts the negative
end of another.
|The strength of the attraction depends both on the amount of charge
on either end of the molecule and on the distance between the charges. There
are many molecules that are not dipoles even though they contain bonds that
are polar. The reason for this is that can be seen if you look at the key
role that molecular structure plays in determining molecular polarity.
|Lets start with the H-Cl molecule, which has only two atoms and therefore
only one bond. This bond is polar, and opposite ends of the bond carry partial
charges of opposite sign. Because there are only two atoms in this molecule,
which are located at the ends of the bond, the molecule as a whole has ends
with equal but opposite charges. A molecule with equal but opposite charges
on opposite poles is polar, so HCl is a polar molecule. In fact, any molecule
composed of just two atoms that differ in electronegativity must be polar.
|For molecules that contain more than two atoms, consider the combined
effects of all the polar bonds. Sometimes, when all the atoms attached to
the central atom are the same, the effects of the individual polar bonds cancel
and the molecule as a whole is non-polar. In the diagram below the +--->
sign is used to show the direction of the dipole. The arrow head points to
the negative end of the bond and the crossed end is the positive end.
|In CO2 both bonds are identical, so each bond dipole is
of the same magnitude. Because CO2 is a linear molecule, these
bond dipoles point in opposite directions and work against each other. The
net result is that their effects cancel, and CO2 is a non-polar
molecule. Although it is not as easy to visualize, the same thing also happens
in BCl3 and CCl4. In each of these molecules the effects
of one of the bond dipoles is cancelled by the effects of the others.
|If all the atoms attached to the central atom are not the same, or
if there are lone pairs in the valence shell of the central atom, the molecule
will usually be polar.
|Not every structure that contains lone pairs on the central atom produces
polar molecules. The following are two exceptions.