Short Answer

London dispersion forces are weak intermolecular forces present in all atoms and molecules. They arise due to temporary fluctuations in the electron cloud, which create momentary dipoles. These temporary dipoles induce dipoles in nearby atoms, resulting in a weak attraction.

Although they are the weakest intermolecular forces, London dispersion forces become stronger as the size and number of electrons in a molecule increase. They play an important role in the boiling points, melting points, and physical states of non-polar substances.

Detailed Explanation :

London Dispersion Forces

London dispersion forces are the weakest type of Van der Waals forces and occur due to temporary changes in electron distribution within atoms or molecules. Even though electrons are always moving, at any instant they may become unevenly distributed. This creates a temporary dipole. That temporary dipole then induces another dipole in a neighbouring atom or molecule, and the two become weakly attracted.

These forces are universal—they exist in everything, including noble gases, non-polar molecules, and large organic compounds. Even substances that appear to have no polarity, like oxygen, nitrogen, and methane, rely on dispersion forces to form liquids and solids at very low temperatures.

1. Origin of London Dispersion Forces

London dispersion forces arise because:

• Electrons move constantly
• Their motion causes momentary uneven distribution
• This creates a temporary dipole
• Nearby atoms respond by forming induced dipoles
• The two dipoles attract each other weakly

This attraction lasts for only a brief moment but occurs continuously, allowing molecules to remain close together.

Example:
A helium atom has no permanent dipole, but temporary dipoles create weak attractions that allow helium to liquefy at extremely low temperatures.

2. Why London Forces Exist in All Molecules

Even non-polar molecules experience dispersion forces because:

• They contain electrons
• Electrons can shift and create temporary dipoles

Examples of non-polar molecules with dispersion forces:

• H₂
• O₂
• N₂
• CH₄
• Noble gases (He, Ne, Ar)

These substances would not condense into liquids or solids without dispersion forces.

3. Strength of London Dispersion Forces

Although weak, their strength depends on several factors:

(a) Number of Electrons

More electrons → stronger dispersion forces.
Example:

• I₂ has stronger dispersion forces than F₂
• Xe has stronger dispersion forces than He

This explains why larger molecules have higher boiling points.

(b) Molecular Size

Larger atoms and molecules have more easily distorted electron clouds.
This increases the likelihood of temporary dipoles.

(c) Molecular Shape

Long, linear molecules have stronger dispersion forces than round or compact molecules.

Example:

• n-pentane (linear) has a higher boiling point than neopentane (compact), even though both have the same formula.

4. Relationship Between London Forces and Physical Properties

(a) Boiling and Melting Points

Increasing dispersion forces leads to higher boiling and melting points.
This is why:

• Larger hydrocarbons (like oils) are liquids
• Small hydrocarbons (like methane) are gases

(b) Solubility

Non-polar molecules dissolve in non-polar solvents because both rely on dispersion forces.

Examples:

• Candle wax dissolves in benzene
• Oils dissolve in non-polar solvents

(c) States of Matter

The phase of non-polar substances (solid, liquid, gas) depends largely on the strength of dispersion forces.

Example:

• I₂ is solid
• Br₂ is liquid
• Cl₂ is gas

This trend corresponds to increasing electron number.

5. Importance of London Dispersion Forces in Chemistry and Biology

London forces are essential for many natural and synthetic processes:

• Stabilizing molecular crystals (e.g., iodine)
• Holding lipid molecules together in cell membranes
• Helping insects walk on water due to molecular attraction on surfaces
• Allowing noble gases to liquefy
• Contributing to the stacking interactions in DNA

Even though they are weak, they become powerful when many molecules interact at once.

6. Difference Between London Forces and Other Intermolecular Forces

Force Type	Cause	Strength
London dispersion	Temporary dipoles	Weakest
Dipole–dipole	Permanent dipoles	Moderate
Hydrogen bonding	Strong dipole with N, O, F	Strongest among Van der Waals

London forces are the only intermolecular forces that act in all substances.

Conclusion

London dispersion forces are weak attractions caused by temporary dipoles that arise from constant electron movement. These forces operate in all atoms and molecules and become stronger with increasing molecular size, number of electrons, and shape. Although weak individually, they are crucial for determining boiling points, melting points, solubility, and the physical state of non-polar substances. They play vital roles in both chemical and biological systems.