Van Der Waal’s Forces- A Detailed Overview

It is the short-lived attractive force between any two molecules, ion-molecules, non-polar – non-polar molecules, polar-polar molecules, and polar-non-polar molecules. It is an instantaneous force of attraction. It is weaker than a hydrogen bond.

Types of van der waal’s forces of attraction

1. Ion-dipole interactions
2. Dipole – Dipole interactions
3. Ion-induced dipole interactions
4. Dipole-induced dipole interactions
5. Instant dipole-induced dipole interactions/ London Dispersion

The order of strength of the forces in the above list is in decreasing order i.e. ion-dipole is the strongest out of all those forces whereas London dispersion is the weakest among all van der Waal’s forces.

Note: Dipole means polar. The dipole moment indicates whether a covalent compound or compound is polar or non-polar. Generally, compounds with different elements and with an electronegative difference are polar in nature. Due to E.N. difference, elements with higher electronegativity tend to draw electrons towards themselves. Due to this, they develop partial +ve and -ve charges and become polar in nature. For example, HCl is polar as H and Cl are two different elements with an electronegative difference. Similarly, H₂ is non-polar as there are two H atoms and there is no electronegative difference.

However, it is not always true that if in a compound, two elements have an electronegativity difference then it will be a polar molecule. For example, CO₂. Although there is an E.N difference between C and O and are two different elements, CO₂ is a non-polar molecule.

1. Ion-dipole interactions

It is the force of attraction between an ion(cation/anion) and a polar molecule.

\underset{anion}{Cl^\circleddash} --- \underset{dipole\ molecule}{H^{\delta^{+}}\rightarrow Cl^{\delta^{-}}}

Here, HCl is a polar molecule due to the electronegative difference between H and Cl as a result of which they develop partial positive and negative charge respectively. And, the bond/interaction between Cl^– anion and HCl molecule is ion-dipole interaction. This force of attraction is the strongest among van der Waal’s forces of attraction.

2. Dipole-Dipole interactions

The force of attraction between two polar molecules is dipole-dipole interaction.

H^{\delta^{+}}\rightarrow Cl^{\delta^{-}} \underset{\overbrace{\underset{interactions}{dipole\ dipole}}}{------}H^{\delta^{+}}\rightarrow Cl^{\delta^{-}}

In this, HCl is a polar molecule and the force of attraction between two polar molecules i.e. HCl and HCl are dipole-dipole interactions.

3. Ion-Induced Dipole interaction

The force of attraction between an ion (cation/anion) and a non-polar molecule is ion-induced dipole interaction. Here, induced dipole means the molecule is not dipole i.e. non-polar. And, dipole i.e. polarity is induced in that molecule in some way.

Br^{-}\underset{\overbrace{\underset{interactions}{I-I.D_{p}}}}{----}H^{\delta^{+}}\rightarrow H^{\delta^{-}}

In this type of interaction, when an ion reaches a non-polar molecule, it tries to repel the like charges and attracts the unlike charges due to which polarity is induced in the non-polar molecule. Here, as the bromide ion reaches the H₂ molecule, it repels the like charge i.e. negative charges and attracts the positive charges towards its nearest hydrogen. This resulted in a polarity even in a non-polar molecular. The attraction between such ions and molecule is ion-induced dipole interaction.

4. Dipole-Induced Dipole interaction

The force of attraction between polar and non-polar molecules is known as dipole-induced dipole interaction. For example,

H^{\delta^{+}}\rightarrow Cl^{\delta^{-}}\underset{\overbrace{\underset{interactions}{D_{p}-I.D_{p}}}}{-----}Cl^{\delta^{+}}\rightarrow Cl^{\delta^{-}}

In this case, HCl is a polar molecule and due to the E.N difference, partial positive and negative charges are developed. As it approached Cl2, it starts repelling like charges and attracting, unlike charges due to which polarity is generated in the Cl₂. The force between such molecules is dipole-induced dipole interaction.

5. Instantaneous Dipole-Induced Dipole interaction

The force of attraction between two non-polar molecules is instantaneous dipole-induced dipole interaction. This interaction is also known as London dispersion forces.

In this case, we assume that at a certain time t, all electrons are at one end of a molecule due to which it developed a partial negative charge because of high electron cloud density and the other end develops a partial positive charge. When such an instantaneous dipole molecule comes closer to a non-polar molecule, polarity is induced in the non-polar due to repelling and attracting of like and unlike charges. The bond/force between such molecules is London forces. They are the weakest among Van Der Waal’s forces of attraction.