Intermolecular forces (IMFs) are attractive forces that exist between molecules, rather than within them, and are distinct from the stronger chemical bonds, like covalent or ionic bonds, that hold atoms together within a molecule. IMFs influence a substance’s physical behavior and properties, dictating whether it is a solid, liquid, or gas at a given temperature.
Understanding Intermolecular Forces
While intramolecular forces are considerably stronger, IMFs are essential because they govern how molecules interact with each other. These attractive forces arise from electrostatic interactions due to the distribution of electron density within molecules. Electrons are constantly in motion, and their positions can create temporary or permanent regions of slight positive and negative charge. These partial charges on one molecule can then attract oppositely charged regions on neighboring molecules, leading to intermolecular attraction. This interplay of charges dictates many bulk properties of substances.
Major Types of Intermolecular Forces
There are three primary types of intermolecular forces, each varying in strength and origin. These forces are all electrostatic in nature, stemming from interactions between charged species. They range from the weakest, present in all molecules, to the strongest, which are specific to certain molecular structures.
London Dispersion Forces (LDFs)
London Dispersion Forces are the weakest type of intermolecular force, present in all atoms and molecules. These forces arise from the continuous, random movement of electrons. Electrons might briefly gather on one side of the nucleus, creating a temporary, instantaneous dipole.
This temporary dipole can then induce a similar dipole in a neighboring atom or molecule, leading to a weak, short-lived attraction. The strength of London Dispersion Forces increases with the number of electrons and the size of the molecule because larger electron clouds are more easily distorted, a property known as polarizability. Noble gases like helium and neon, and nonpolar molecules such as methane, exhibit only London Dispersion Forces, which explains their very low boiling points.
Dipole-Dipole Forces
Dipole-dipole forces occur between molecules with a permanent dipole moment. A permanent dipole forms from uneven electron sharing due to differences in electronegativity, creating a molecule with partially positive and negative ends.
The positive end of one polar molecule attracts the negative end of a neighboring polar molecule. These attractions are stronger than London Dispersion Forces for molecules of comparable size and mass because they involve permanent partial charges. Hydrogen chloride (HCl) is a common example, where the partially positive hydrogen of one HCl molecule attracts the partially negative chlorine of another.
Hydrogen Bonding
Hydrogen bonding is a particularly strong type of dipole-dipole interaction. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom (typically nitrogen, oxygen, or fluorine), is attracted to a lone pair of electrons on another electronegative atom in a different molecule. The large difference in electronegativity creates a very polar bond, leaving the hydrogen atom with a significant partial positive charge.
This highly positive hydrogen atom is strongly attracted to the electron-rich electronegative atom of a nearby molecule. Hydrogen bonds are stronger than typical dipole-dipole interactions due to the small size of the hydrogen atom, which allows for a close approach, and the high polarity of the bonds involved. Water is the most well-known example of a substance exhibiting extensive hydrogen bonding. Hydrogen bonding also plays a significant role in biological molecules, such as holding the two strands of DNA together in a double helix.
How Intermolecular Forces Shape Our World
The strength and types of intermolecular forces profoundly influence a substance’s physical properties. These forces dictate how molecules interact, affecting characteristics like melting points, boiling points, and solubility. Understanding these relationships helps explain why different substances behave the way they do in our everyday lives.
Boiling and Melting Points
The strength of intermolecular forces directly correlates with a substance’s boiling and melting points. To change a substance from a solid to a liquid or from a liquid to a gas, enough energy must be supplied to overcome the attractive forces holding the molecules together. Substances with stronger IMFs require more energy to break these attractions, resulting in higher melting and boiling points.
For example, water has a relatively high boiling point of 100°C due to its strong hydrogen bonds. Methane, a nonpolar molecule with only weak London Dispersion Forces, boils at a much lower temperature. This principle allows for predictions of relative boiling points based on the types and strengths of IMFs present.
Solubility
Intermolecular forces also play a significant role in determining solubility. A general rule is “like dissolves like,” meaning substances with similar types and strengths of intermolecular forces tend to dissolve in each other. Polar substances, which exhibit dipole-dipole interactions or hydrogen bonding, typically dissolve well in polar solvents like water.
Conversely, nonpolar substances, primarily interacting through London Dispersion Forces, are more soluble in nonpolar solvents. For instance, oil and water do not mix because water molecules form strong hydrogen bonds with each other, while oil molecules are nonpolar and interact mainly via weaker London Dispersion Forces. The water molecules are more attracted to each other than to the oil molecules, preventing mixing.
Surface Tension and Viscosity
Intermolecular forces contribute to other observable properties of liquids, such as surface tension and viscosity. Surface tension is the energy required to increase a liquid’s surface area, causing the liquid surface to behave like a stretched elastic film. Liquids with stronger IMFs exhibit higher surface tension because molecules at the surface are pulled inward more strongly by their neighbors. Water, with its strong hydrogen bonds, has a high surface tension, allowing some insects to walk on its surface.
Viscosity is a liquid’s resistance to flow. Stronger intermolecular forces lead to higher viscosity because molecules are more strongly attracted to each other, making it more difficult for them to move past one another. Honey is much more viscous than water due to the stronger and more extensive network of IMFs among its molecules.