How Do Van Der Waals Forces Hold Molecules Together

The Intermolecular forces are the interactive forces acting between neighboring molecules. There are several types of intermolecular forces such as strong ion-dipole interactions, dipole-dipole interactions, London dispersion interactions or induced dipole bonds. Among these intermolecular forces, London dispersion forces and dipole-dipole forces fall under the category of Van Der Waals forces. 

This article looks at,

1. What are Dipole-Dipole Interactions
2. What are London Dispersion Interactions
3. How Do Van Der Waals Forces Hold Molecules Together

What are Dipole-Dipole Interactions

When two atoms of different electronegativities share a pair of electrons, the more electronegative atom pulls the pair of electrons towards itself. Therefore, it becomes slightly negative (δ-), inducing a slightly positive charge (δ+) on the less electronegative atom. For this to happen, the electronegativity difference between two atoms should be >0.4. A typical example is given below:

Figure 1: Example of Dipole-Dipole Interactions

Cl is more electronegative than H (electronegativity difference 1.5). Therefore, the pair of electrons is more biased towards Cl and becomes δ-. This δ- end of the molecule attracts the δ+ end of another molecule, forming an electrostatic bond between the two. This kind of bonding is called dipole-dipole bonds. These bonds are the result of asymmetric electric clouds around the molecule.

Hydrogen bonds are a special kind of dipole-dipole bonds. For a hydrogen bond to occur, there should be a highly electronegative atom attached to a hydrogen atom. Then the pair of electrons shared will be pulled towards the more electronegative atom. There should be a neighboring molecule with a highly electronegative atom which has a lone pair of electrons on it. This is called the hydrogen acceptor which accepts electrons from a hydrogen donor.

Figure 2: Hydrogen bond

In the above example, the oxygen atom of the water molecule behaves as the hydrogen donor. The nitrogen atom of the ammonia molecule is the hydrogen acceptor. The oxygen atom in the water molecule donates a hydrogen to the ammonia molecule and makes a dipole bond with it. These types of bonds are called hydrogen bonds.

What are London Dispersion Interactions

London dispersion forces are mostly associated with non-polar molecules. It means that the atoms participating in forming the molecule are of similar electronegativity. Hence, there is no charge formed on atoms.

The reason for London dispersions is the random movement of electrons in a molecule. The electrons can be found at any end of the molecule at any time, making that end δ-. This makes the other end of the molecule δ+. This appearance of dipoles in a molecule can induce dipoles in another molecule as well.

Figure 3: Example of London Dispersion Forces

The picture above shows that the δ- end of the molecule on the left hand repels electrons of the nearby molecule hence, inducing a slight positivity at that end of the molecules. This leads an attraction between the oppositely charged ends of two molecules. These types of bonds are called London dispersion bonds. These are considered the weakest type of molecular interactions and may be temporary. The solvation of non-polar molecules in non-polar solvents are due to the presence of London dispersion bonds.

How Do Van Der Waals Forces Hold Molecules Together

The Van Der Waals forces mentioned above are considered somewhat weaker than ionic forces. Hydrogen bonds are considered much stronger than other Van Der Waals forces. London dispersion forces are the weakest type of Van Der Waals forces. London dispersion forces are often present in halogens or noble gasses. The molecules freely float away since the forces that keep them together are not strong. This makes them take up a large volume.

Dipole-dipole interactions are stronger than London dispersion forces and often present in liquids. The substances that have molecules which are kept together by dipole interactions are considered polar. Polar substances can only be dissolved in another polar solvent.

The following table compares and contrasts the two types of Van Der Waals forces.

However, Van Der Waals forces are weaker compared to ionic and covalent bonds. So it doesn’t need much energy supply to be broken.

Reference:
1. “Dipole-Dipole Interactions – Chemistry. ” Socratic.org. N.p., n.d. Web. 16 Feb. 2017.
2. “Van der Waals Forces.” Chemistry LibreTexts. Libretexts, 21 July 2016. Web. 16 Feb. 2017.

Image Courtesy:
1. “Dipole-dipole-interaction-in-HCl-2D”By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia
2. “Wikipedia HDonor Acceptor”By Mcpazzo – Own work (Public Domain) via Commons Wikimedia