London Dispersion Forces – Examples And Formula - Vedantu

London dispersion forces are essential in chemistry and help students understand various practical and theoretical applications related to this topic.

What is London Dispersion Force in Chemistry?

A London dispersion force refers to the weakest type of intermolecular attraction formed by temporary fluctuations in the electron clouds of atoms and molecules, resulting in instantaneous dipoles.

This concept appears in chapters related to intermolecular forces, Van der Waals interactions, and the study of physical properties, making it a foundational part of your chemistry syllabus.

London Dispersion Forces – Simple Definition

London dispersion forces (LDFs) are weak intermolecular forces that occur due to temporary, momentary shifts of electrons, creating short-lived dipoles in atoms or molecules. These forces exist in all substances but are most important in nonpolar molecules.

Origin & Mechanism of London Dispersion Forces

• London dispersion forces arise when the electron cloud around an atom or molecule suddenly becomes uneven.
• This momentary imbalance creates a temporary dipole, which can induce a similar effect in neighboring particles, resulting in an attraction.
• The strength of these forces depends on the size of the electron cloud (polarizability).
• Larger atoms and molecules with more electrons have stronger dispersion forces as their electron clouds are more easily distorted.

For example, in a container of helium gas, even though the atoms are nonpolar, random movement of electrons causes temporary dipoles that attract other helium atoms. This attraction explains why noble gases can become liquids at very low temperatures.

Differences: London Dispersion vs. Dipole-Dipole vs. Van der Waals

Property	London Dispersion	Dipole–Dipole	Hydrogen Bonding
Present In	All atoms/molecules (mainly nonpolar)	Permanent dipoles (polar molecules)	H attached to N, O, or F
Strength	Weakest	Moderate	Strongest (of the three)
Example	He, Ne, I2	HCl, SO2	H2O, NH3
Nature	Temporary, fluctuating	Permanent, fixed	Hydrogen + small, electronegative atom

Examples of London Dispersion Forces in Chemistry

• Noble gases (He, Ne, Ar): Liquidify at low temperatures due to dispersion forces alone.
• Halogens (Cl2, Br2, I2): Trends in boiling point reflect strength of London dispersion forces as molecule size increases.
• Hydrocarbons (pentane, hexane): More carbons mean stronger dispersion forces and higher boiling points.

Factors Affecting Strength of London Dispersion Forces

• Larger atomic or molecular size (more electrons) increases polarizability and London forces.
• Shape of molecules: Straight chains allow better contact, enhancing dispersion forces.
• Molecular weight: Heavier molecules usually exhibit stronger London forces.

Uses of London Dispersion Forces in Real Life

• London dispersion forces play a crucial role in determining boiling and melting points of substances.
• For example, bromine is a liquid at room temperature due to strong dispersion forces, while chlorine is a gas.
• They're also important for explaining the condensation of nonpolar gases like argon, and how plastic wrap sticks to surfaces (via dispersion forces).

Try This Yourself

• Arrange: Ne, Ar, Kr, Xe in order of increasing boiling point and explain why.
• Identify if CH4 molecules attract each other via dipole or dispersion forces.
• Give an everyday product whose function relies on London dispersion forces.

Relation with Other Chemistry Concepts

London dispersion forces are closely related to Van der Waals forces and are a specific subset. Understanding them helps you explain boiling point trends, intermolecular interactions, and are foundational for the study of states of matter and noble gases.

Frequent Related Errors

• Confusing London dispersion forces with dipole-dipole or hydrogen bonds.
• Assuming nonpolar molecules have “no” attractive forces at all.
• Forgetting that all molecules/atoms (even noble gases) experience London dispersion forces.

Step-by-Step Reaction Example

1. Consider two Cl2 molecules approaching each other. 2. Electrons in one molecule shift momentarily, creating a temporary dipole. 3. This temporary dipole induces a dipole in a nearby Cl2 molecule. 4. Weak attraction forms between the induced dipoles—this is the London dispersion force. 5. When the temperature drops, these forces become stronger, leading to condensation into liquid or solid.

Final Wrap-Up

We explored London dispersion forces—their mechanism, why they matter, and how they affect real substances. These weak, yet universal, forces help explain boiling points and phase changes in everyday life. For more explanations and exam tips, explore the resources and live classes at Vedantu.

Related Topics: Van der Waals Forces | Boiling Point | States of Matter | Noble Gases: Physical and Chemical Properties