Intermolecular forces or IMFs are attractive and repulsive electromagnetic forces between molecules. These forces determine most of a substance’s physical properties and state of matter.
- Intermolecular forces are attractive and repulsive forces between atoms, groups of atoms, or ions in separate molecules.
- The three main types of intermolecular forces are hydrogen bonding (dipole-dipole forces), ion-dipole forces (and ion-induced dipole forces), and Van der Waals forces (Debye force, London dispersion force, Keesom force).
- Ion-dipole forces are the strongest intermolecular forces, followed by hydrogen bonding, other dipole-dipole forces, and dispersion forces. Van der Waals forces are the weakest intermolecular forces.
Intramolecular vs Intermolecular Forces
Intermolecular forces act between molecules. In contrast, intramolecular forces are the attractive and repulsive forces within molecules that are responsible for chemical bonds and molecular structure. In both cases, forces act between atoms or groups of atoms. Intermolecular forces are weaker than intramolecular forces, but both types of forces play important roles in the shapes of molecules, their properties, and their interactions with one another. Intermolecular forces are dotted lines in diagrams, while intramolecular forces (bonds) are solid lines.
Types of Intermolecular Forces
Intermolecular forces can either attract (opposite electrical charges) or repel (like charges), but the main classes of intermolecular forces deal with attraction. The three types of intermolecular forces are:
- Dipole-dipole forces (including hydrogen bonding)
- Ion-dipole forces and ion-induced dipole forces
- Van der Waals forces (Debye force, London dispersion force, Keesom force)
So, although there are three broad categories of intermolecular forces, you can expand them from their categories to get five or six types of forces. Some sources also include ion-ion forces, for example, between aqueous ions like Na+ and Cl–.
A hydrogen bond is a type of dipole-dipole bond where a hydrogen atom feels attraction to a more electronegative atom (usually oxygen, fluorine, or nitrogen) that already shares a bond with another atom. Hydrogen bonding is directional. It is similar to a covalent bond. Hydrogen bonds are stronger than Van der Waals forces, but weaker than ion-dipole or ion-induced dipole forces.
A good example of hydrogen bonding is the attraction between water molecules. Hydrogen atoms on one molecule form hydrogen bonds with oxygen atoms of neighboring water molecules. A consequence of hydrogen bonding is the high boiling point of water compared to similar molecules. Hydrogen bonding also stabilizes nucleic acids, proteins, and other polymers.
More generally, dipole-dipole forces occur between all polar molecules. The positive part of a molecule aligns with the negative portion of its neighbor.
Ion-Dipole and Ion-Induced Dipole Forces
Ion-dipole and ion-induced dipole forces are intermolecular forces involving ions instead of polar or nonpolar molecules.
An ion-dipole force arises when an ion interacts with a polar molecule. The positive portion of one group aligns with the negative portion of the other. An example of ion-dipole interaction is the hydration of metal ions in water, where the metal cations align with the oxygen atoms in neighboring water molecules. The strength of ion-dipole interactions depends on the magnitude of the dipole moment, the size and charge of the ion, and the size of the polar molecule.
An ion-induced dipole force occurs when an ion and a nonpolar molecule interact. The charge of the ion distorts the electron cloud surrounding the nonpolar molecule.
Van der Waals Forces
Van der Waals forces are the relatively weak attraction between uncharged atoms or molecules, such that all molecules feel some attraction to one another. There are multiple components to Van der Waals forces, include the Keesom force, Debye force, and London dispersion force.
- Keesom force (permanent dipole – permanent dipole): The Keesom force is a temperature-dependent interaction between rotating permanent dipoles. This force only occurs between two polar molecules (or other molecules with permanent dipole moments). The Keesom force is very weak.
- Debye force (permanent dipole – induced dipole): The Debye force is a polarization from the interactions between rotating permanent dipoles and the induced dipoles formed by polarizable atoms and molecules. Here, a molecule with a permanent dipole induces a dipole in another molecule, repelling its electrons. An example if the interaction between Ar and HCl, where the argon electrons are attracted to the H side of the molecule and repelled by the Cl side.
- London dispersion force (fluctuating dipole – induced dipole): This force arises from the non-zero instantaneous dipole moments of all atoms and molecules due to random fluctuations in electron density. Atoms with more electrons experience a larger London dispersion force than atoms with fewer electrons.
Which Type of Intermolecular Force Is the Strongest?
The nature of the chemical species involved in intermolecular forces matters, so there is no hard-and-fast ranking of strongest to weakest intermolecular forces. But, ion-dipole interactions tend to be the strongest, followed by hydrogen bonding, other types of dipole-dipole bonding, and London dispersion forces.
|Type of Intermolecular Force||Description/Strength||Example|
|Ion-Dipole||Occurs between ions and polar molecules; strongest||Na+ and Cl– ions interacting with H2O|
|Hydrogen Bond||Hydrogen atom is attracted to nitrogen, fluorine, or oxygen from another molecule; strong||NH3 molecules interacting with each other|
|Dipole-Dipole||Polar molecules attract each other; strength increases with increasing polarity||CH3CN molecules interacting with each other|
|London Dispersion||Occurs between all molecules; weakest but increases with increasing molecular weight||CH4 with itself, Br2 with itself|
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