Intermolecular forces are the forces of attraction or repulsion between molecules. Unlike chemical bonds that are responsible for holding atoms together within a molecule, intermolecular forces act between molecules.
These forces are much weaker than chemical bonds, nonetheless, play a crucial role in determining the physical properties of substances. such as boiling and melting points, viscosity, and surface tension.
Types of Intermolecular Forces
There are three primary types of intermolecular forces.
- Dispersion Forces
- Dipole-Dipole Forces
- Hydrogen Bonding
In addition to these primary types of intermolecular forces, there are also ion-dipole forces and ion-induced dipole forces.
Let’s take a closer look at each type.
1. Dispersion Forces (London Forces)
Dispersion forces are the weakest type of intermolecular force, but they are present in all molecules, whether they are polar or nonpolar. These forces occur because of temporary fluctuations in the electron distribution around a molecule.
These fluctuations create instantaneous dipoles (small charges) that attract other molecules. For example, in nonpolar molecules like oxygen (O₂) or nitrogen (N₂), dispersion forces occur even though there is no permanent dipole.
These forces are more pronounced in larger molecules because they have more electrons, which increases the likelihood of fluctuations.
2. Dipole-Dipole Forces
Dipole-dipole forces arise in polar molecules, which have a permanent dipole due to an uneven distribution of charge within the molecule. In these molecules, one part of the molecule is slightly negative, while the other part is slightly positive.
These dipoles attract each other and result in dipole-dipole interactions. For instance, in hydrogen chloride (HCl), the chlorine atom is more electronegative than hydrogen, causing the molecule to have a dipole.
The positive end of one HCl molecule will attract the negative end of another, creating dipole-dipole forces.
3. Hydrogen Bonding
Hydrogen bonding is a special type of dipole-dipole interaction, but they are much stronger. They occur when a hydrogen atom is bonded to highly electronegative atoms like nitrogen (N), oxygen (O), or fluorine (F).
The hydrogen atom, being small, carries a significant partial positive charge. It is strongly attracted to the lone pair of electrons on the electronegative atom of another molecule. Water (H₂O) is a classic example of hydrogen bonding.
The oxygen atom is more electronegative than hydrogen, which gives the water molecules a partial negative charge on oxygen and a partial positive charge on hydrogen.
These positive and negative charges attract, holding the water molecules together and giving water its unique properties, like high boiling and melting points.
4. Ion-Dipole Forces
Ion-dipole forces occur between molecular interactions of an ion (a charged particle) and a polar molecule (a molecule with a dipole). These forces are especially important in solutions where ionic compounds, like salts, dissolve in polar solvents like water.
For example, when table salt (NaCl) dissolves in water, the positively charged sodium ions (Na⁺) are attracted to the partially negative oxygen atoms in water molecules. Whereas, the negatively charged chloride ions (Cl⁻) are attracted to the partially positive hydrogen atoms.
These ion-dipole interactions allow the salt to dissolve, forming a homogeneous solution.
5. Ion-Induced Dipole Forces
Ion-induced dipole forces occur when a charged ion distorts the electron cloud of a nearby nonpolar molecule. This induces a temporary dipole in the molecule.
The induced dipole then interacts with the ion, creating an attractive force. For example, when an iron ion (Fe²⁺) is near a nonpolar oxygen molecule (O₂), the ion’s electric field distorts the electron cloud of the oxygen molecule, inducing a temporary dipole. This leads to a weak attraction between the ion and the nonpolar molecule.
Want to dive deeper into understanding intermolecular forces? Join our tailored chemistry tuition classes to master the lesson.