Question

What are dipoles? how are they related to van der waals forces?

i don't understand what the dipoles are and how they relate to the vanderwaals forces.
Please help me!

Answer 1

`"Dipoles are essentially instances of electric charge separation"`
`"at a molecular level."`

`"They give rise to intermolecular Van der Waals forces of interaction."`

Explanation:

A dipole forms when electric charge is separated, i.e. polarized. The modern covalent bond is considered to be a region of high electron density BETWEEN two positively charged nuclei

The covalent bond is thus conceived to be the sharing of two electrons: the bond is a region of high electron density that negates the electrostatic repulsion between 2 positively-charged atomic nuclei, such that a net attractive force results.

Because homonuclear diatomic molecules, i.e. `X_2(X=H, F, N, etc.)`, are formed by 2 of the SAME atoms the electron density of the bond cannot be polarized to either side of the bond. But form a `C-Cl`, or a `C-N`, or a `C-O`, or a `Si-X` bond, the electrons cloud distort towards the MOST ELECTRONEGATIVE ATOM:

The illustration tries to represent non-polar, versus polar covalent, versus ionic interaction (where the electron transfers). (And their cut-off points are reasonably arbitrary). The important points to realize that where 2 atoms are covalently bound, if one atom is more electronegative than the other, charge separation occurs, i.e. to give `""^(delta+)X-Y^(delta-)`. In the bulk phase of the material, the dipole line up to form `""^(delta+)X-Y^(delta-)*""^(delta+)X-Y^(delta-)`. Importantly note that this is an EXTRA INTERMOLECULAR force, a Van der Waals interaction, that has consequences on the boiling point of the substance.

As an undergraduate you have to know that molecular polarity is addressed as the vector sum of the individual bond dipoles: `"CCl"_4` is a NON-POLAR molecule (because the `C-Cl` bond dipoles sum to zero) but `"HCCl"_3` has some polarity, because the vector sum is non-zero.

Hydrogen bonding could be considered another example of dipole-dipole interaction, and certainly such interactions is conceived to elevate the boiling points of `OH_2`, `HF`, and `NH_3`. Because this dipole-dipole interaction is exceptionally strong, (as witness the elevated boiling points of these hydrides with respect to lower Group hydrides), such `"hydrogen bonding"` is considered under its own umbrella.