Re: why do non-polar molecules have smaller forces of attraction between them

First of all, a correction to your question: polar molecules are different from ionic substances.

Ionic substances are held together by electrostatic attraction between oppositely charged ions. This is a very strong interaction called ionic bonding. There are no molecules in ionic compounds - just a 3D lattice of ions.

When you get to covalent compounds, you get discrete molecules - atoms bonded strongly to each other within molecules, but with much weaker forces holding the molecules together.

The strongest of these intermolecular forces is between polar molecules containing either N,O or F, which must have both a lone pair of electrons and a H bonded to it. If these condidtions are satisfied, you get the intermolecular force named (slightly misleadingly) hydrogen bonding.
Examples are water, ammonia and ethanol.

The unusual properties of water (high m.pt. and b.pt. for its mass, surface tension and capillary action, solid state less dense than the liquid, etc.) are all due to its extensive hydrogen bonding (each O atom has 2 lone pairs and 2 H-atoms bonded to it, so you get the maximum amount of hydrogen bonding possible)

Next in strength is the intermolecular force between polar molecules which don't satisfy the conditions for hydrogen bonding. The attraction between molecules is electrostatic attraction between the opposite charges on neighbouring molecules. This is called dipole-dipole attraction. (it is important to note that this attraction is much less than ionic bonding because the charges are only due to uneven distribution of the electron cloud, and are only a fraction of a full +1 or -1 charge. Also they are distributed over a molecule of several atoms, so the charge density is very much less than that of an ion, as is the packing ability.
examples are chloromethane and hydrogen chloride.

Next comes the weakest intermolecular force, between non-polar molecules, and called Van der Waals forces or London forces. In fact this force exists between all molecules, but is overshadowed by the stronger forces of permanent dipoles. The attraction is again electrostatic, but is due only to temporary shifts in the electron cloud distribution in a non-polar molecule. At any given instant, there could easily be a slight dipole on a non-polar molecule, just because the electrons are in constant motion. This dipole can then influence a neighbouring molecule to adopt an opposite dipole, causing attraction. This is induced-dipole attraction and is temporary and shifting. It size depends on the number of electrons in the molecule, and how well neighbouring molecules can get near to each other (contact area). Although it is weak, it can become very significant in large molecules (for example it holds the sheets together in graphite)
other examples are hydrogen, carbon dioxide and the alkanes.

A look in any pre-university textbook will find you further examples and show diagrams of what I have explained.