3. HYDROGEN BONDING:
Link to more extensive discussion: Hydrogen
The hydrogen bond is really a special case of dipole forces.
A hydrogen bond is the attractive force between the hydrogen
attached to an electronegative atom of one molecule and an electronegative
atom of a different molecule. Usually the electronegative atom
is oxygen, nitrogen, or fluorine.
In other words - The hydrogen on one molecule attached
to O or N that is attracted to an O or N of a different molecule.
In the graphhic on the left, the hydrogen is partially positive
and attracted to the partially negative charge on the oxygen
or nitrogen. Because oxygen has two lone pairs, two different
hydrogen bonds can be made to each oxygen.
This is a very specific bond as indicated. Some combinations
which are not hydrogen bonds include: hydrogen to another hydrogen
or hydrogen to a carbon.