London Dispersion Forces Explained

London dispersion forces, also known as van der Waals forces or dispersion forces, are a type of intermolecular force that arises between non-polar molecules. These forces are responsible for the physical properties of substances, such as melting and boiling points, viscosity, and surface tension. In this article, we will delve into the world of London dispersion forces, exploring their definition, causes, and effects on the physical properties of substances.

Definition and Causes of London Dispersion Forces

London dispersion forces are a type of intermolecular force that occurs between non-polar molecules, which are molecules that do not have a permanent electric dipole moment. These forces are caused by the temporary dipoles that form in non-polar molecules due to the movement of electrons. As electrons move around the nucleus, they create temporary dipoles, which are regions of partial positive and negative charge. These temporary dipoles induce dipoles in neighboring molecules, resulting in a weak attractive force between the molecules.

Theoretical Background

The London dispersion forces were first described by German physicist Fritz London in 1930. London’s theory states that the temporary dipoles in non-polar molecules are responsible for the attractive forces between molecules. The theory is based on the concept of quantum mechanics, which describes the behavior of electrons in atoms and molecules. According to the theory, the temporary dipoles in non-polar molecules are caused by the fluctuations in the electron cloud, which create temporary regions of partial positive and negative charge.

The strength of London dispersion forces depends on several factors, including the size of the molecules, the number of electrons, and the polarizability of the molecules. Polarizability refers to the ability of a molecule to deform its electron cloud in response to an external electric field. Molecules with high polarizability tend to have stronger London dispersion forces.

Effects of London Dispersion Forces on Physical Properties

London dispersion forces have a significant impact on the physical properties of substances, including their melting and boiling points, viscosity, and surface tension. The strength of the London dispersion forces determines the magnitude of these properties. For example, substances with strong London dispersion forces tend to have higher melting and boiling points, as the molecules are more strongly attracted to each other.

Melting and Boiling Points

The melting and boiling points of a substance are directly related to the strength of the London dispersion forces between its molecules. Substances with strong London dispersion forces require more energy to overcome the attractive forces between the molecules, resulting in higher melting and boiling points. For example, the melting point of xenon is -111.75°C, while the melting point of helium is -272.2°C, due to the stronger London dispersion forces in xenon.

The boiling point of a substance is also affected by the London dispersion forces. Substances with strong London dispersion forces tend to have higher boiling points, as the molecules are more strongly attracted to each other, requiring more energy to separate them. For example, the boiling point of argon is -185.85°C, while the boiling point of neon is -246.08°C, due to the stronger London dispersion forces in argon.

Viscosity and Surface Tension

London dispersion forces also affect the viscosity and surface tension of a substance. Viscosity refers to the resistance of a fluid to flow, while surface tension refers to the energy required to increase the surface area of a liquid. Substances with strong London dispersion forces tend to have higher viscosities and surface tensions, as the molecules are more strongly attracted to each other, making it more difficult for them to flow or change shape.

Real-World Applications of London Dispersion Forces

London dispersion forces have numerous real-world applications, including the development of nanomaterials, biomaterials, and energy storage devices. For example, the unique properties of graphene, a nanomaterial composed of carbon atoms, are due in part to the strong London dispersion forces between the carbon atoms. These forces allow graphene to exhibit exceptional strength, conductivity, and thermal properties.

In addition, London dispersion forces play a crucial role in the development of biomaterials, such as proteins and lipids. The strength of the London dispersion forces between these molecules determines their structure and function, which is essential for their biological activity. For example, the folding of proteins is influenced by the London dispersion forces between the amino acid residues, which determines the protein's overall structure and function.