3 Types of Intermolecular Forces

Intermolecular forces or IMFs are physical forces between molecules. In contrast, intramolecular forces are forces between atoms within a single molecule. Intermolecular forces are weaker than intramolecular forces.

How Molecules Interact

The interaction between intermolecular forces may be used to describe how molecules interact with one another. The strength or weakness of intermolecular forces determines the state of matter of a substance (e.g., solid, liquid, gas) and some of the chemical properties (e.g., melting point, structure).

There are three major types of intermolecular forces: London dispersion force, dipole-dipole interaction, and ion-dipole interaction. Here's a closer look at these three intermolecular forces, with examples of each type.

London Dispersion Force

The London dispersion force is also known as LDF, London forces, dispersion forces, instantaneous dipole forces, induced dipole forces, or the induced dipole-induced dipole force

The London dispersion force, the force between two nonpolar molecules, is the weakest of the intermolecular forces. The electrons of one molecule are attracted to the nucleus of the other molecule, while repelled by the other molecule's electrons. A dipole is induced when the electron clouds of the molecules are distorted by the attractive and repulsive electrostatic forces.

Example: An example of London dispersion force is the interaction between two methyl (-CH₃) groups.

Example: A second example of London dispersion force is the interaction between nitrogen gas (N₂) and oxygen gas (O₂) molecules. The electrons of the atoms are not only attracted to their own atomic nucleus, but also to the protons in the nucleus of the other atoms.

Dipole-Dipole Interaction

Dipole-dipole interaction occurs whenever two polar molecules get near each other. The positively charged portion of one molecule is attracted to the negatively charged portion of another molecule. Since many molecules are polar, this is a common intermolecular force.

Example: An example of dipole-dipole interaction is the interaction between two sulfur dioxide (SO₂) molecules, in which the sulfur atom of one molecule is attracted to the oxygen atoms of the other molecule.

Example: H​ydrogen bonding is considered a specific example of a dipole-dipole interaction always involving hydrogen. A hydrogen atom of one molecule is attracted to an electronegative atom of another molecule, such as an oxygen atom in water.

Ion-Dipole Interaction

Ion-dipole interaction occurs when an ion encounters a polar molecule. In this case, the charge of the ion determines which part of the molecule attracts and which repels. A cation or positive ion would be attracted to the negative part of a molecule and repelled by the positive part. An anion or negative ion would be attracted to the positive part of a molecule and repelled by the negative part.

Example: An example of the ion-dipole interaction is the interaction between a Na⁺ ion and water (H₂O) where the sodium ion and oxygen atom are attracted to each other, while the sodium and hydrogen are repelled by each other.

Van der Waals Forces

Van der Waals forces are the interaction between uncharged atoms or molecules. The forces are used to explain the universal attraction between bodies, the physical adsorption of gases, and the cohesion of condensed phases. The van der Waals forces encompass intermolecular forces as well as some intramolecular forces including Keesom interaction, the Debye force, and the London dispersion force.

Sources

• Ege, Seyhan (2003). Organic Chemistry: Structure and Reactivity. Houghton Mifflin College. ISBN 0618318097. pp. 30–33, 67.
• Majer, V. and Svoboda, V. (1985). Enthalpies of Vaporization of Organic Compounds. Blackwell Scientific Publications. Oxford. ISBN 0632015292.
• Margenau, H. and Kestner, N. (1969). Theory of Inter-molecular Forces. International Series of Monographs in Natural Philosophy. Pergamon Press, ISBN 1483119289.