Intermolecular forces (IMFs) are the attractive forces that exist between separate molecules, dictating how they interact with one another. These attractions are fundamental to chemistry, governing the physical properties of substances, such as whether a compound is a gas, liquid, or solid at room temperature. The energy required to overcome these forces determines a substance’s melting and boiling points. Among these molecular attractions, one is universally present yet distinctly the weakest: the London Dispersion Force. This force is the primary reason some substances have extremely low boiling points and exist as gases.
Defining the Intermolecular Spectrum
Intramolecular forces are the strong chemical bonds—like covalent or ionic bonds—that hold atoms together within a single molecule. Intermolecular forces, in contrast, are the much weaker attractions that occur between neighboring molecules. For instance, it takes significantly more energy to break the bonds within a water molecule (intramolecular) than it does to separate molecules to boil the liquid. Intermolecular forces are generally only about 1% to 10% as strong as chemical bonds. This spectrum of weakness among the IMFs determines the physical state of a substance.
The Mechanism of London Dispersion Forces
The weakest of all intermolecular forces are the London Dispersion Forces (LDF), which are sometimes referred to as induced dipole-induced dipole attractions. These forces are unique because they are the only type of intermolecular force present in all atoms and molecules, whether polar or non-polar. They arise from the continuous and random motion of electrons within the electron cloud.
At any given instant, the electrons might briefly be distributed unevenly, creating a momentary charge imbalance. This fleeting, instantaneous dipole can then influence a neighboring molecule, causing its electron cloud to temporarily distort and create an induced dipole moment.
The resulting weak, short-lived electrostatic attraction between the instantaneous dipole and the induced dipole is the London Dispersion Force. Since this attraction relies on temporary, fluctuating charge imbalances rather than permanent polarity, it is inherently the most feeble type of intermolecular interaction. The strength of LDFs is influenced by “polarizability,” which is the ease with which an electron cloud can be distorted.
Larger molecules and atoms, such as iodine, have more electrons and their electron clouds are further from the nucleus, making them easier to distort and thus more polarizable. This higher polarizability allows them to generate stronger LDFs, which explains why larger, non-polar molecules often have higher boiling points than smaller ones. Conversely, very small molecules like helium have minimal polarizability, resulting in extremely weak LDFs.
Ranking the Strengths of Intermolecular Forces
London Dispersion Forces are positioned at the bottom of the strength hierarchy, serving as the baseline for all molecular attractions. Above LDFs are the Dipole-Dipole Forces (DDF), which occur between molecules that possess a permanent dipole moment. These polar molecules have a permanent separation of charge due to unequal sharing of electrons, causing the positive end of one molecule to attract the negative end of a neighbor.
Because DDFs rely on permanent polarity, they are notably stronger than LDFs. The strongest common intermolecular force is Hydrogen Bonding, a specialized, strong form of dipole-dipole interaction. Hydrogen bonding occurs when a hydrogen atom is directly bonded to a highly electronegative atom (nitrogen, oxygen, or fluorine). This arrangement creates a strong partial positive charge on the hydrogen atom, which is then strongly attracted to a lone pair of electrons on an adjacent electronegative atom. The ranking, from strongest to weakest, is Hydrogen Bonding, Dipole-Dipole Forces, and London Dispersion Forces.
How Weakness Governs Physical States
The inherent weakness of London Dispersion Forces has direct and observable consequences for the physical properties of substances. Substances whose molecules are non-polar, and therefore rely solely on LDFs for attraction, exhibit the lowest melting and boiling points. These weak forces require very little energy to overcome, meaning molecules can easily gain enough thermal energy to separate from one another.
For example, small, non-polar molecules like methane (\(\text{CH}_4\)) or noble gases like neon and helium are gases at room temperature. Methane boils at about -161.5 degrees Celsius, while helium boils at an astonishingly low -268.9 degrees Celsius. This demonstrates that only a minimal amount of energy is needed to break the fleeting LDFs that briefly hold the molecules together in the liquid state. The existence of these substances as gases under everyday conditions is the macroscopic evidence of the weakness of the London Dispersion Force.