Intermolecular forces (IMFs) are the interactions that hold the physical world together, acting as a molecular “glue” between individual molecules. The dispersion force is the weakest and most universal type of attraction. Unlike other IMFs, such as dipole-dipole interactions, the dispersion force is present in all atoms and molecules, regardless of their polarity or chemical composition. This attractive force is responsible for the transition of nonpolar substances from gas to liquid and solid states.
Defining the Dispersion Force
The dispersion force, also known as the London Dispersion Force (LDF), is a temporary attractive force that arises between adjacent atoms or molecules. This attraction is based on fleeting shifts in the distribution of electrons within a molecule’s electron cloud. Because electrons are present in all matter, the dispersion force is the only intermolecular force that is absolutely universal. Dispersion forces are the only intermolecular forces operating between nonpolar molecules, such as noble gases or nonpolar diatomic molecules like \(\text{O}_2\). It contributes to the overall attraction between all molecules, even those that also exhibit stronger attractions like hydrogen bonds. The force is named after the physicist Fritz London, who provided the first quantum-mechanical explanation for this type of intermolecular attraction in 1930.
The Origin of Instantaneous Attraction
The mechanism of the dispersion force begins with the random movement of electrons within an atom’s electron cloud. Although the average distribution of electrons over time is symmetrical, at any instant, electrons might cluster on one side of the nucleus. This momentary, uneven distribution of negative charge creates a temporary separation of charge, forming an “instantaneous dipole.” The side with the electron surplus becomes momentarily negative, and the opposite side becomes positive.
This instantaneous dipole influences a neighboring atom or molecule. The temporary charge imbalance distorts the electron cloud of the second molecule, pushing its electrons away from the negative end of the first dipole. This action creates an “induced dipole” in the neighbor, and the resulting attraction between the instantaneous and induced dipoles is the dispersion force. This attraction is very short-lived, constantly forming and disappearing as electrons continue their motion.
Factors That Determine Strength
The magnitude of the dispersion force is determined by two main molecular properties: molecular size and shape. Generally, larger atoms and molecules exhibit stronger dispersion forces. This is because larger molecules possess a greater number of electrons, and their valence electrons are, on average, farther from the nucleus. Electrons that are less tightly held can be more easily shifted or distorted to form the temporary dipoles, a property known as high polarizability.
The second factor is the molecule’s shape, specifically the surface area available for interaction. Molecules with elongated or linear shapes can maximize their contact area with neighboring molecules, which allows for more extensive and stronger dispersion interactions. For example, the linear molecule \(n\)-pentane has stronger dispersion forces than its more spherical isomer, neopentane, even though both have the same number of atoms and electrons. The compact, spherical shape of neopentane minimizes the surface area of contact, resulting in weaker overall forces.
Real-World Manifestations
The strength of the dispersion force has a direct influence on the physical properties of substances, most notably their boiling and melting points. For nonpolar molecules, overcoming the dispersion forces is the only energy required to separate the molecules and change the substance’s phase. Substances with stronger dispersion forces, such as larger hydrocarbon chains, require more energy to boil, leading to higher boiling points. This explains why small molecules like methane (\(\text{CH}_4\)) are gases at room temperature, while much larger hydrocarbon chains, like those in motor oil, are liquids.
Dispersion forces are also responsible for the physical state of the halogens, where fluorine (\(\text{F}_2\)) and chlorine (\(\text{Cl}_2\)) are gases, bromine (\(\text{Br}_2\)) is a liquid, and iodine (\(\text{I}_2\)) is a solid at standard conditions. The increasing size and number of electrons from fluorine to iodine results in progressively stronger dispersion forces, making it harder to separate the molecules. Furthermore, these forces play a role in biological and materials science, such as the adhesion of gecko feet to surfaces or the binding of polymer chains in plastics.