Intermolecular forces (IMFs) are the weak electrical attractions that exist between neutral molecules, contrasting sharply with the much stronger forces that hold atoms together within a molecule, such as covalent bonds. These attractions are responsible for determining the physical properties of a substance, including its boiling point, melting point, and state of matter at a given temperature. Understanding these forces is necessary to explain why some substances exist as gases while others are liquids or solids under the same conditions.
The Universal Presence of Dispersion Forces
All molecules, regardless of their electrical polarity, experience dispersion forces, which are also known as London dispersion forces (LDFs). These forces are the most universal type of intermolecular attraction because they arise from a phenomenon present in all matter. Dispersion forces are the sole type of intermolecular force acting between nonpolar molecules, such as oxygen gas (O2) or methane (CH4). Without these attractions, nonpolar substances would never condense into liquids or freeze into solids, even at extremely low temperatures.
Dispersion forces always contribute to the total attractive force between molecules, even those with stronger interactions. For nonpolar molecules, the strength of this force alone dictates the substance’s physical properties.
How Temporary Dipoles Create Dispersion Forces
Dispersion forces originate from the constant, random motion of electrons within a molecule’s electron cloud. Although the electron distribution is generally symmetrical over time, at any given instant, the electrons might be momentarily clustered on one side of the nucleus. This fleeting, uneven distribution of negative charge creates a transient, instantaneous dipole moment within the atom or molecule.
The instantaneous dipole in one molecule can then affect the electron distribution in a neighboring molecule. The charge separation in the first molecule causes the second molecule’s electron cloud to distort. This distortion creates a corresponding temporary charge separation, known as an induced dipole, in the second molecule. The instantaneous dipole and the induced dipole then attract each other, creating the weak, short-lived dispersion force.
Factors Influencing Dispersion Force Strength
While dispersion forces are always present, their strength varies significantly depending on the characteristics of the molecule. The primary factor determining LDF strength is a molecule’s polarizability, which is the ease with which its electron cloud can be temporarily distorted to form an instantaneous dipole. Larger atoms and molecules possess a greater number of electrons, and these electrons are typically further from the controlling pull of the nucleus. This makes the larger, more diffuse electron clouds more easily perturbed, resulting in higher polarizability and consequently stronger dispersion forces.
Molecular shape also plays a significant role, particularly in molecules with similar mass and electron counts. Long, linear molecules, such as n-pentane, have a greater surface area that can come into close contact with neighboring molecules. This extensive contact area allows for more numerous and stronger instantaneous and induced dipole interactions across the surface. Conversely, compact, spherical molecules, like neopentane, have a smaller contact area and cannot pack together as tightly. This reduction in surface interaction leads to weaker overall dispersion forces, which is why n-pentane has a higher boiling point than its spherical isomer.
Distinguishing Dispersion Forces from Other Intermolecular Attractions
Dispersion forces belong to a broader category of intermolecular attractions that also includes dipole-dipole interactions and hydrogen bonding. Dipole-dipole interactions occur only in polar molecules that possess a permanent, rather than temporary, separation of charge. These molecules align themselves so that the partially positive end of one molecule attracts the partially negative end of its neighbor. This permanent attraction makes dipole-dipole forces generally stronger than LDFs in molecules of comparable size.
Hydrogen bonding is an especially strong type of dipole-dipole interaction. This force arises specifically when a hydrogen atom is covalently attached to one of three highly electronegative atoms: nitrogen, oxygen, or fluorine. The resulting large, permanent charge separation creates a powerful intermolecular attraction that is significantly stronger than both regular dipole-dipole forces and dispersion forces.