The London Dispersion force is caused by the formation of temporary dipoles in the electron cloud of neutral atoms that attract each other.
Why is this only attractive? Why don't the like poles repel, thereby nullifying the net effect?
I fail to understand the answer (the only one and unaccepted) in this post.
A simple demonstration in electrostatics can convince that London dispersion forces are attractive. Has your physics teacher shown "charging by induction"?
Charging by induction video
Now replace the rods by molecules. Note the rods always attract. If one molecule develops a dipole or negative charge (on one side), it will induce an oppositive dipole in its neighbour.
Second demo, which you should yourself. Open tap in such a way that only a thin stream of water is flowing, but not dripping. Charge a comb with your hair and bring it close to the water stream. You will see water is attracted to the comb. No matter what you use for charging, water always gets attracted. There is no repulsion, because the permanent dipole of water always orients itself in such a way that its end is opposite in sign to whatever is the charge on the comb.
Water attracted to a charged comb
Now van der Waals attraction is not due to permanent dipoles but transient dipoles.
BTW, atoms and molecules do repel each other if you try to bring them very close. Can you compress water easily? Why and why not?
P.S. Please don't consider the answers on the web as answers from the Heavens. Online answers are written by students, teachers, researchers, former chemists etc. and all of us can be right or wrong.
I would add two points to MFarooqs' answer.
First, I would emphasize that the basis for attractive van der Waals interactions is the polarizability of molecules. Polarizability is the property of having flexible charge distributions which may be distorted by interacting with charges outside of a molecule, leading to a more stable electronic state. Polarizability is responsible for macroscopic phenomena such as the response of an electrically neutral material to an applied electric field.
The second point is that repulsive interactions are unsustainable, as they lead to separation of the interacting particles, whereas attractive interactions are self-sustaining or even re-enforcing as they bring the particles closer together.
When two neutral atoms (noble gas atoms are a good example) approach each other, they are, at long distances, electrostatically "blind" to each other. If sufficiently close though, their outer electron clouds will begin to significantly repel each other and their nuclei will do the same, and simultaneously opposite charges will attract. This does not seem to bode well for the prospects of forming a net attractive bond. However, since the electron clouds can be distorted (polarized), if a spontaneous asymmetry exists or arises in the charge distribution (easier to visualize if one atom is larger than the other, for instance He approaching Ar), then a net attractive force can develop and become reinforced. This is an induced (as opposed to permanent) type of polarization responsible for attractive van der Waals interactions.
