London Dispersion Force (LDF) is an intermolecular force. It is the weakest of the main attractive interactions that occur between molecules. The force results from the constant, shifting motion of electrons within all atoms and molecules. This fundamental force is crucial for understanding molecular attractions and their physical effects.
What Intermolecular Forces Are
Intermolecular forces (IMFs) are the attractive forces that exist between individual molecules in a substance. They are fundamentally different from intramolecular forces, which are the much stronger chemical bonds that hold atoms together within a single molecule, such as covalent bonds. IMFs are significantly weaker than these chemical bonds but are responsible for holding substances together in their liquid and solid states.
The strength of these attractions dictates the physical properties observed in bulk materials. For instance, a substance with strong IMFs requires more energy to separate its molecules, leading to a higher boiling point and melting point. IMFs also influence properties like solubility, viscosity, and surface tension. Understanding these forces provides context for how substances behave under different temperatures and pressures.
The Temporary Dipole Mechanism
The London Dispersion Force originates from the continuous, random movement of electrons within an atom or molecule, even nonpolar ones. At any given instant, the electrons orbiting the nucleus are not perfectly symmetrical. This momentary, uneven distribution of electron density creates a fleeting charge separation, establishing an instantaneous dipole. This means that for a fraction of a second, one side of the molecule is slightly negative and the other is slightly positive.
This instantaneous dipole affects a neighboring molecule. The transient positive end of the first molecule attracts the electrons in the second molecule, while the negative end repels them. This distortion generates a corresponding charge separation called an induced dipole. The resulting attraction between the instantaneous dipole and the induced dipole is the London Dispersion Force. Though these attractions are extremely short-lived, with dipoles constantly forming and dissolving, the cumulative effect across a vast number of molecules is enough to cause nonpolar substances to condense into liquids and solids.
Molecular Factors That Determine Strength
The strength of the London Dispersion Force depends on two main molecular characteristics. The primary factor is polarizability, which describes the ease with which a molecule’s electron cloud can be distorted by an external electric field or a neighboring dipole. Molecules with high polarizability form stronger temporary dipoles, resulting in a stronger attractive force.
Polarizability is directly related to molecular size and molar mass. Larger atoms and molecules possess a greater number of electrons, and these electrons are typically farther from the positively charged nucleus. Because they are less tightly held, the outer electrons are more easily shifted, leading to a higher polarizability and consequently stronger LDFs. This effect is why the boiling points of the noble gases increase substantially as you move down the group from helium to xenon.
Molecular shape also plays a significant role in determining the overall strength of the dispersion force. Long and linear molecules achieve a greater surface area of contact with neighboring molecules than compact and spherical ones. This increased contact area allows for more points where the instantaneous and induced dipoles can interact. For example, the straight-chain hydrocarbon n-pentane has a higher boiling point than its spherical isomer, neopentane, because the forces between the linear molecules are stronger.
LDF in the Hierarchy of Intermolecular Forces
The London Dispersion Force occupies the lowest position in the hierarchy of intermolecular forces, making it the weakest attraction. Other common forces, such as dipole-dipole interactions and hydrogen bonding, are stronger because they rely on permanent charge separations rather than temporary ones. Dipole-dipole forces occur between polar molecules with permanent partial charges, while hydrogen bonding is a particularly strong form involving hydrogen bonded to nitrogen, oxygen, or fluorine.
LDFs are unique because they are the only attractive force present in nonpolar molecules, such as hydrocarbons and noble gases. London Dispersion Forces are present and active in all molecules and atoms, regardless of whether they are polar or nonpolar. Even when stronger forces like hydrogen bonding are dominant, LDFs still contribute to the overall attraction between particles.