Understanding Intermolecular Forces

Intermolecular forces are attractive or repulsive forces that exist between molecules. These forces are much weaker than the intramolecular forces (like covalent or ionic bonds) that hold atoms together within a molecule, but they play a crucial role in determining the physical properties of substances, such as boiling point, melting point, and solubility.

Let's look at the different types of intermolecular forces often discussed in chemistry:

Types of Intermolecular Forces Explained

Here are the common types of intermolecular forces, including the one also known as the London force:

• Dipole-Dipole Forces: These occur between polar molecules. Polar molecules have a permanent dipole because of uneven sharing of electrons, creating partially positive and partially negative ends. The positive end of one polar molecule is attracted to the negative end of another polar molecule.
• Dipole-Induced Dipole Forces: These forces arise when a polar molecule (with a permanent dipole) comes near a nonpolar molecule. The permanent dipole of the polar molecule can distort the electron cloud of the nonpolar molecule, inducing a temporary dipole in it. An attractive force then exists between the permanent dipole and the induced dipole.
• Hydrogen Bond Forces: This is a special, particularly strong type of dipole-dipole interaction. It occurs when a hydrogen atom is bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine) and is attracted to a lone pair of electrons on a nearby electronegative atom in another molecule or part of the same molecule.
• Dispersion Forces: Also known as London Dispersion Forces or simply London Forces, these are the weakest type of intermolecular force but are present between all atoms and molecules, whether polar or nonpolar. They arise from temporary fluctuations in electron distribution around an atom or molecule, creating instantaneous, temporary dipoles. These temporary dipoles can induce temporary dipoles in neighboring atoms or molecules, leading to weak, short-lived attractions.

Identifying the London Force

Based on the descriptions above, the intermolecular force that is also called the London force is the dispersion force.

These forces were first explained by the physicist Fritz London in 1930, which is why they are often named after him. While they are the weakest individually, their collective strength can be significant, especially in large molecules with many electrons.

Therefore, the intermolecular force also called as London force is Dispersion Forces.

Revision Table: Intermolecular Forces Summary

Additional Information: Factors Affecting Dispersion Forces

The strength of dispersion forces is influenced by:

• Number of electrons: More electrons lead to a larger electron cloud which is more easily distorted, resulting in stronger temporary dipoles and thus stronger dispersion forces. This is why larger molecules generally have higher boiling points.
• Molecular shape: Long, skinny molecules can make more contact with neighboring molecules compared to compact, spherical ones of similar size, leading to stronger dispersion forces.

Even though dispersion forces are weak individually, they are the only intermolecular forces present in nonpolar substances (like noble gases or hydrocarbons) and are therefore solely responsible for holding these substances together in liquid and solid states.