As with hydrogen bonding, the positive and negative dipoles between different
molecules attract each other. However, the attraction is not as strong as that of a
hydrogen bond.
3) London Dispersion (aka Temporary Dipole)Every type of covalent compound exhibits the final type of intermolecular force,
known as London dispersion forces. For all molecules, the electrons are in
constant motion around the molecules. At any given moment in time, the
electron density will be greatest around one part of the molecule. That place
where the electron density is the highest will have a negative dipole, and the
place where the electron density is the lowest will have a positive dipole.
Over time, as the electrons keep moving around, these dipoles will shift position
and ultimately will cancel each other out. However, sufficient attraction exists
between these temporary dipoles while they exist to hold the molecules together,
even in a completely nonpolar substance such as O 2 . Without London dispersion
forces, nonpolar molecules would not be attracted to each other in any way.
Boiling Points
The most common way to quantitatively measure the strength of intermolecular
forces within a covalent compound is to measure the boiling point of the
compound. A substance boils when the intermolecular forces holding the
molecules together in a liquid phase are broken and the molecules move into a
gaseous phase. When water boils, you are NOT breaking the covalent bonds that
hold the hydrogen and oxygen atoms together within the molecule. If you were
to do so, the H 2 O would break down into H 2 and O 2 , both highly flammable
gases that would make cooking your dinner very interesting! Instead, when you
boil water, you are simply changing the phase of the water by increasing the
kinetic energy of the water molecules, to that point that they are energetic
enough to overcome the intermolecular forces between separate H 2 O molecules
(the boiling point).
Between compounds of similar mass, the stronger the intermolecular forces are,
the more energy that needs to be input to break them apart and the higher the
boiling point will be. This is why NH 3 (which has hydrogen bonding) has a
higher boiling point than PH 3 (which has permanent dipoles), which in turn has a