Intermolecular Forces in Chemistry

Chemical molecules are under the constant influence of various chemical forces. They dictate the chemical and physical characteristics of chemical substances. All forces in a chemical can be divided into two major categories: Intermolecular and Intramolecular forces. The former exists between molecules, while the latter acts within a molecule.

Intermolecular forces are secondary forces that control molecule-molecule interactions. There are three contributors to molecular interactions. And they are:

1. Hydrogen bonding
2. Ion-dipole and Ion-induced dipole
3. Van der Waal forces

On the other side, intramolecular forces are chemical bonds, which include ionic, covalent, and metallic bonds. They glue all the atoms of a molecule together. Since these forces are responsible for making a chemical molecule itself, they contribute to the chemical properties. Intramolecular forces are much stronger than intermolecular and of most interest to chemists.

In contrast, intermolecular forces are outside a molecule and mediate molecular movements as a whole. So, they control the physical aspects of chemical substances, such as boiling point, melting point, physical phases, viscosity, vapor pressure, and other bulk properties.

In gas, intermolecular forces of attraction are very weak and cannot compete against thermal (kinetic) energy associated with individual molecules. That is why gas is characterized by the free movement of its molecules because intermolecular forces are too weak to hinder free motion. However, with increasing pressure/decreasing temperature, the influence of intermolecular forces is more pronounced. And it is also possible to liquefy or solidify a real gas.

When it comes to liquids and solids, intermolecular forces are predominant but weaker than intramolecular forces. The molecules in liquids and solids are much closer to one another and are strongly held by intermolecular forces. Though the molecules possess kinetic energy and have some degree of free motion, it is not enough to overcome molecular forces.

Origin of intermolecular forces

Now, we have some ideas about intermolecular forces. But from where do they originate? Are intermolecular forces a fundamental force in nature? The answer is no; they are not a fundamental force.

In the beginning, we mentioned various contributors of intermolecular forces. All of them have a common origin: electrostatics.

Molecules are made of atoms; atoms consist of equal positive (protons) and negative (electrons) charges. So, molecules can be thought of as a bubble of an equal number of positive and negative charges. The distribution of charges within a molecule may be symmetrical or not. But it is possible to influence the distribution of charges in a molecule. For example, one side of a molecule may become more negative (increase in electron probability), while the other suffers from a decrease in electron probability—which is more positive.

There are many factors that can influence the charges in a molecule. As an example, a large electronegativity difference between two bonded atoms in a molecule can create a permanent dipole. It is an intrinsic factor—it arises from the molecule itself. Sometimes, perturbations in a molecule might result from the neighboring environment. Or sometimes, it may be because of random fluctuations in a molecule. We will cover all these in the next section.

In short, we can say all molecules, including monoatomic, can be polarized to some extent. The polarization may be permanent or last for an instant.

When we have two partially-polarized molecules close to each other, they experience an electrostatic force. It could be the electrostatic force of repulsion if the same-charge sides (positive-positive or negative-negative) of two molecules face each other or the electrostatic force of attraction if opposite-charge sides align face-to-face.

These forces of attraction and repulsion are termed intermolecular forces.

It is the natural tendency of any system to minimize its energy and reach a stable state. In our case, it is electrostatic potential energy. Thus, the molecules always align themselves to reduce the repulsion among them. And it is achieved by aligning themselves in such a manner opposite-charge sides of molecules face each other.

Various types of intermolecular forces

There are three major types of contributors to intermolecular forces. They are discussed below:

Hydrogen bonding

Hydrogen bonding is one of the strongest intermolecular forces. When an electronegative atom (O, N, F) is covalently bonded to hydrogen, it experiences partial polarization. The electronegative atom, say oxygen, pulls bonding electrons toward it, making hydrogen electron deficient. When this electron deficient hydrogen comes in contact with an electronegative atom bearing a lone pair of electrons, a hydrogen bond is formed. 

Let us take the example of water to explain this. Water has an angular geometry. Oxygen is a central atom with two hydrogen atoms attached to it, making a bond angle (H-O-H) of 104.5°. There are two lone pairs of electrons on oxygen.

Since oxygen is far more electronegative than hydrogen, the bonding electrons are shifted toward oxygen, creating a partial negative charge on oxygen and a partial positive on hydrogen.

These forces of attraction and repulsion are termed intermolecular forces.

It is the natural tendency of any system to minimize its energy and reach a stable state. In our case, it is electrostatic potential energy. Thus, the molecules always align themselves to reduce the repulsion among them. And it is achieved by aligning themselves in such a manner opposite-charge sides of molecules face each other.

Various types of intermolecular forces

There are three major types of contributors to intermolecular forces. They are discussed below:

Hydrogen bonding

Hydrogen bonding is one of the strongest intermolecular forces. When an electronegative atom (O, N, F) is covalently bonded to hydrogen, it experiences partial polarization. The electronegative atom, say oxygen, pulls bonding electrons toward it, making hydrogen electron deficient. When this electron deficient hydrogen comes in contact with an electronegative atom bearing a lone pair of electrons, a hydrogen bond is formed.

Let us take the example of water to explain this. Water has an angular geometry. Oxygen is a central atom with two hydrogen atoms attached to it, making a bond angle (H-O-H) of 104.5°. There are two lone pairs of electrons on oxygen.

Since oxygen is far more electronegative than hydrogen, the bonding electrons are shifted toward oxygen, creating a partial negative charge on oxygen and a partial positive on hydrogen.

Ion-dipole forces are stronger than hydrogen bonding because of the involvement of an ion (completely polarized species). And the strength of the interaction will grow with the charge of an ion and/or with the dipole moment of a polar molecule.

An ion-induced dipole is much similar to an ion-dipole interaction. The only difference is we have a nonpolar species instead of a polar. In the previous section, we have mentioned that any species can be polarized to some extent. So, when an ion approaches a nonpolar molecule, it creates distortions in the electronic cloud of the nonpolar molecule, creating a non-permanent dipole.

Ion-dipole forces are stronger than hydrogen bonding because of the involvement of an ion (completely polarized species). And the strength of the interaction will grow with the charge of an ion and/or with the dipole moment of a polar molecule.

An ion-induced dipole is much similar to an ion-dipole interaction. The only difference is we have a nonpolar species instead of a polar. In the previous section, we have mentioned that any species can be polarized to some extent. So, when an ion approaches a nonpolar molecule, it creates distortions in the electronic cloud of the nonpolar molecule, creating a non-permanent dipole.