Question

May I have some examples of van der waals forces in chemistry please?

Answer 1

Van der Waals forces are attractive forces that hold molecules close together. These attractive forces are more commonly referred to as intermolecular forces (IMF). The three most common types of Van der Waals forces (intermolecular forces), listed below from weakest to strongest, are:

 1. London Dispersion forces
 2. Dipole-Dipole forces
 3. Hydrogen

Dipole-Dipole forces are one of van der Waals' three forces. Dipole Dipole forces occur in polar molecules, that is, molecules that have an unequal sharing of electrons. For example, HCl comprised of the atom Hydrogen and Chlorine is polar. The Chlorine atom has an extra electron, which came from the hydrogen atom. Because of this, the chlorine part of the molecule is negatively charged, and the hydrogen side of the molecule is positively charged.

ie. H - Cl
    +    -

So in a solution where there are thousands of these molecules around that are slightly charged on each side, the molecules naturally orient themselves the accommodate the charge. The positive part of one molecule will move until it is next to the negative part of a neighboring molecule. These forces between molecules tend to make them 'stick' together.

Dispersion forces are another of van der Waals' three forces. They exist between nonpolar molecules. For example, chlorine gas is made up of two chlorine atoms. In this bond, the electrons are equally shared and are not dominant on one side of the molecule as is the case in HCl. The atom looks like this

Cl - Cl

no overall charge on either side but, it is important to remember that within a bond, electrons are constantly MOVING. They zoom around the atoms really quickly. As a result, there may be a tiny instant in time where the electrons happen to be dominant on one side, creating a situation like this,

Cl - Cl
+    -

However, this temporary charge disappears as quickly as it appeared because the electrons are moving so fast. These temporary dipoles allow the temporarily negative side of one molecule to attract the temporarily positive side of another molecule, which is the intermolecular force.

Hydrogen bonding is the third type of van der Waals' forces. It is exactly the same as dipole-dipole interaction, it just gets a special name. A hydrogen bond is a dipole dipole interaction that occurs between any molecule with a bond between a hydrogen atom and any of oxygen/fluorine/nitrogen. So, Hydrogen Fluoride (HF), Water (H2O), ammonia (NH3)....any kind of substance that has a hydrogen bonded to either an oxygen, fluorine or nitrogen atom, exhibits hydrogen bonding. The hydrogen bond is just the dipole dipole force but it is extremely strong compared to either dipole dipole forces like HCl. It is extremely strong because F N and O are extremely good at attracting electrons and H is extremely good at losing them. So basically, the bond is EXTREMELY a one-sided affair, resulting in an extreme dipole situation, thus named, a hydrogen bond. The extremely positive side of the molecule will orient itself with the extremely negative side of another molecule.

The van der Waals' forces are very weak. I said the hydrogen bond is extremely strong, but that is only compared to the other van der Waals' forces. Compared to say, a covalent bond, a hydrogen bond is approximately one tenth of that strength. The dipole-dipole bond is weaker still, and the dispersion forces are the weakest of Van De Waals' forces. That is demonstrated in the fact that, take for example, Cl2. Chlorine gas exhibits dispersion forces, the weakest of van der Waals' forces. Cl2 is a GASEOUS compound, because the dispersion forces are not strong enough to pull the molecules together as a solid. The dispersion forces can only suffice to keep the substance as a gas, because the forces between molecules are so weak that they can float about all over the place and exist as a gas.