Van Der Waals Forces/intermolecular Forces

In all A-level specifications students are required to learn about "Inter-molecular Forces". As a former teacher and tutor this is one of the common areas I get asked to explain.

Firstly, lets look at the term "Inter-molecular Force". Literally this can be translated as the "forces between molecules". They are not chemical bonds which are strong and occur between atoms within a molecule, they are weak forces of attraction between the molecules themselves. Now, here is where many students encounter difficulty and this is the reason why.

"Inter-molecular forces (IMF)" are ONLY relevant to molecules that have simple covalent bonding.

IMF are not relevant to metals (metallic bonding), ionic compounds (ionic bonding and giant ionic lattice structure) and giant covalent structures such as diamond and silicon dioxide (this substances do not exist as simple molecules).

Let me say it again, you only encounter IMF between simple covalent molecules.

Secondly, there are 3 types of IMF. (not explained in detail though)

1) Hydrogen bonding (note: this is not a bond!)

2) Permanent dipole-dipole interactions (caused by differences in electronegativity of atoms covalently bonded to each other)

3) Temporary/Induced/Instantaneous dipole-dipole interactions or London (dispersion) Forces.

Now all 3 of these are examples of Van der Waals forces or Inter-molecular Forces.

Too often students are being taught that the term Van der Waals forces is only equivalent to describing IMF 3) Temporary/Induced/Instantaneous dipole-dipole interactions or London (dispersion) Forces.

This is because over many years it was an accepted definition allow by exam boards.

However with the new A-level specifications from 2018, things are changing. It may we be the case that exam mark schemes continue to allow the use of this incorrect terminology, and so teachers and student incorrectly learn the same.

So to conclude, I urge students and teachers to relearn that the term Van der Waals (VDW) forces describes all 3 examples of IMF.