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.
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