What Are London Dispersion Forces Definition Mechanism and Examples
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
FAQs on London Dispersion Forces in Intermolecular Interactions
1. What are London dispersion forces?
London dispersion forces are weak intermolecular forces caused by temporary, instantaneous dipoles formed due to fluctuations in electron distribution.
- They occur when electrons in an atom or molecule momentarily become unevenly distributed.
- This creates a temporary dipole that induces a dipole in a neighboring particle.
- They are present in all atoms and molecules, including nonpolar molecules like O2 and CH4.
- They are a type of van der Waals force.
2. Why are London dispersion forces called instantaneous dipole–induced dipole forces?
London dispersion forces are called instantaneous dipole–induced dipole forces because they arise from a temporary dipole that induces a dipole in a nearby particle.
- An instantaneous dipole forms due to random electron movement.
- This temporary charge separation induces a dipole in a neighboring atom or molecule.
- The attraction between these temporary dipoles creates the intermolecular force.
3. Do London dispersion forces occur in polar molecules?
Yes, London dispersion forces occur in all molecules, including both polar and nonpolar molecules.
- In polar molecules, they exist alongside dipole–dipole interactions and sometimes hydrogen bonding.
- In nonpolar molecules, they are the only intermolecular force present.
- For example, nonpolar Cl2 is held together in the liquid state only by London dispersion forces.
4. What factors affect the strength of London dispersion forces?
The strength of London dispersion forces increases with greater molar mass, more electrons, and larger surface area.
- Number of electrons: More electrons increase polarizability.
- Molar mass: Heavier atoms or molecules have stronger dispersion forces.
- Surface area/shape: Long, flat molecules experience stronger attractions than compact ones.
5. How do London dispersion forces affect boiling points?
Stronger London dispersion forces lead to higher boiling points because more energy is required to separate the molecules.
- As molar mass increases, boiling point generally increases.
- For example, boiling points increase down Group 18: He < Ne < Ar < Kr < Xe.
- This trend occurs because larger atoms have stronger dispersion forces.
6. What is the difference between London dispersion forces and dipole–dipole forces?
The key difference is that London dispersion forces arise from temporary dipoles, while dipole–dipole forces occur between permanent dipoles in polar molecules.
- Dispersion forces: present in all molecules, especially nonpolar ones.
- Dipole–dipole forces: occur only in polar molecules with permanent charge separation.
- Dipole–dipole interactions are generally stronger than dispersion forces of similar-sized molecules.
7. Why are London dispersion forces stronger in larger atoms?
London dispersion forces are stronger in larger atoms because their electron clouds are more easily distorted, increasing polarizability.
- Larger atoms have more electrons and greater electron cloud size.
- This makes temporary dipoles stronger and easier to induce.
- For example, I2 has stronger dispersion forces than F2.
8. Are London dispersion forces the weakest intermolecular forces?
Yes, London dispersion forces are generally the weakest type of intermolecular force, but they can become significant in large molecules.
- They are weaker than hydrogen bonding and dipole–dipole interactions for small molecules.
- In large molecules like hydrocarbons, they can be strong enough to dominate physical properties.
- They are still much weaker than covalent or ionic bonds.
9. Can you give an example of London dispersion forces in real life?
A common example of London dispersion forces is the liquefaction of noble gases such as Ar and Xe.
- Noble gases are nonpolar atoms.
- The only force holding their atoms together in the liquid state is dispersion forces.
- Similarly, nonpolar hydrocarbons like C5H12 are liquids due to these forces.
10. How do London dispersion forces relate to van der Waals forces?
London dispersion forces are a type of van der Waals force, which is a general term for weak intermolecular attractions.
- Van der Waals forces include:
- London dispersion forces
- Dipole–dipole interactions
- Dipole–induced dipole forces
