London dispersion forces are attractive forces between molecules. They are present in all substances, polar or nonpolar. They are the weakest intermolecular forces, yet universally present, influencing how all matter interacts at a molecular level.
How London Dispersion Works
London dispersion forces arise from the dynamic movement of electrons within atoms and molecules. Electron clouds, which represent the probability of finding electrons around a nucleus, are constantly in motion. This continuous movement can, at any given instant, lead to an uneven distribution of electrons, even in atoms or molecules that are normally electrically symmetrical.
When electrons momentarily shift to one side of an atom or molecule, a temporary dipole is created. One side becomes slightly negative due to higher electron density, while the opposite side becomes slightly positive due to a temporary electron deficiency. This fleeting dipole can then influence neighboring atoms or molecules.
This temporary dipole can induce a corresponding temporary dipole in an adjacent molecule. This occurs because the electrons in the second molecule are repelled by the negative end of the first molecule’s temporary dipole and attracted to its positive end. This synchronized shifting of electron clouds results in a weak, attractive electrostatic force between the two molecules. These attractions are often called instantaneous dipole-induced dipole attractions.
These dipoles are short-lived, forming and dissipating rapidly. However, because this process constantly occurs among countless molecules, an average attractive force emerges, holding molecules together. This explains how nonpolar substances can condense into liquids and then freeze into solids when temperatures are sufficiently lowered, overcoming the kinetic energy that keeps them apart.
What Influences London Dispersion Strength
The strength of London dispersion forces is primarily influenced by two molecular characteristics: molecular size and molecular shape. Larger molecules with more electrons exhibit stronger dispersion forces. This is because larger electron clouds are more diffuse and less tightly held by the nucleus, making them more easily distorted or “polarized.”
The ease with which an electron cloud can be distorted to form a temporary dipole is known as polarizability. Molecules with higher polarizability develop stronger temporary dipoles, leading to more substantial London dispersion forces. For example, as you move down a group in the periodic table, atoms become larger and have more electrons, leading to increased polarizability and stronger dispersion forces.
Molecular shape also plays a role in determining the strength of these interactions. Molecules with larger surface areas or more linear structures allow for greater contact between molecules, which can enhance the cumulative effect of the temporary dipoles. For instance, n-pentane, a straight-chain molecule, has stronger London dispersion forces than its branched isomer, neopentane, despite having the same molecular weight. This is because n-pentane’s linear shape allows for more extensive surface contact.
London Dispersion in Action
London dispersion forces are observable in various real-world phenomena, especially regarding the physical properties of nonpolar substances. Their influence on boiling points is a clear example. For nonpolar substances like noble gases or hydrocarbons, London dispersion forces are the dominant intermolecular forces. Overcoming these forces requires energy, which dictates their boiling points; substances with stronger dispersion forces will have higher boiling points.
Consider the halogens: fluorine and chlorine are gases at room temperature, while bromine is a liquid, and iodine is a solid. This progression reflects the increasing strength of London dispersion forces as molecular size and polarizability increase down the group. The liquefaction of gases like oxygen and nitrogen at low temperatures is also largely due to the attractive influence of these forces.
London dispersion forces also contribute to phenomena like adhesion. For instance, geckos can cling to surfaces due to billions of tiny London dispersion interactions between their feet and the surface. In the principle of “like dissolves like,” London dispersion forces are the primary reason why nonpolar substances dissolve in nonpolar solvents; the attractive forces between solute and solvent molecules are similar in strength to those within the pure substances, allowing for mixing.