Intermolecular forces are physical forces between molecules. They explain how molecules interact with each other. The strength or weakness of intermolecular forces determine 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 3 intermolecular forces, with examples of each type.
London Dispersion Force
also known as: LDF, London forces, dispersion forces, instantaneous dipole forces, induced dipole forces, induced dipole-induced dipole force
The London dispersion force is the weakest of the intermolecular forces.This is the force between two nonpolar molecules. 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:Â interaction between two methyl (-CH3) groups
example: interaction between nitrogen gas (N2) and oxygen gas (O2) molecules
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: interaction between two sulfur dioxide (SO2) molecules, where the sulfur atom of one molecule is attracted to the oxygen atoms of the other molecule
example:hydrogen bonding is considered a specific example of a dipole-dipole interaction always involving hydrogen. A 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: interaction between a Na+ ion and water (H20) where the sodium ion and oxygen atom are attracted to each other, while the sodium and hydrogen are repelled by each other
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