Short Answer

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It shows how strongly charges are pulled apart due to unequal sharing of electrons in a polar bond. The greater the difference in electronegativity between two atoms, the larger the dipole moment.

Dipole moment helps us understand molecular polarity. Molecules with a dipole moment are polar, while those with zero dipole moment are non-polar. It influences solubility, boiling point, and the behaviour of molecules in electric fields.

Detailed Explanation :

Dipole Moment

A dipole moment is a fundamental property that describes how electrons are distributed within a molecule when bonds are polar. When two atoms share electrons unequally, one atom becomes slightly negative (δ–), and the other becomes slightly positive (δ+). This separation of charge creates an electric dipole. Dipole moment is the quantitative measure of this charge separation.

Dipole moment is expressed in Debye units (D) and is a vector quantity, meaning it has both magnitude and direction. A molecule with a higher dipole moment has a greater imbalance of charge and is more polar. The dipole moment helps determine molecular shape, physical properties, strength of intermolecular forces, and solubility.

1. Origin of Dipole Moment

A dipole moment arises due to unequal sharing of electrons between atoms with different electronegativities.

How it forms:

• More electronegative atom pulls electrons closer → becomes δ–
• Less electronegative atom loses electron density → becomes δ+

This creates a charge separation, forming a dipole.

Example:
In H–Cl,

• Cl is more electronegative → δ–
• H becomes δ+
  Thus, HCl has a dipole moment.

The greater the electronegativity difference, the larger the dipole moment.

2. Formula of Dipole Moment

Dipole moment (μ) is calculated using:

Where:

• μ = dipole moment
• q = magnitude of charge separation
• d = distance between the charges

This shows that dipole moment increases when either charge difference or distance increases.

3. Dipole Moment as a Vector Quantity

Dipole moment has both:

• Magnitude → strength of charge separation
• Direction → from δ+ to δ–

In molecules with multiple bonds, dipole moments combine like vectors. The overall dipole moment depends on how individual dipole vectors add up.

Examples:

• CO₂: Dipoles cancel → μ = 0 → non-polar
• H₂O: Dipoles do not cancel → μ > 0 → polar

Thus, vector addition determines molecular polarity.

4. Dipole Moment and Molecular Polarity

Dipole moment is directly linked to whether a molecule is polar or non-polar.

• Non-polar molecules → zero dipole moment (μ = 0)
• Polar molecules → non-zero dipole moment (μ ≠ 0)

Examples:

• CH₄ → symmetrical → no net dipole → non-polar
• NH₃ → lone pair creates asymmetry → polar
• HF → strong polarity → large dipole moment

Thus, dipole moment helps classify molecules.

5. Factors Affecting Dipole Moment

(a) Electronegativity Difference

Higher electronegativity difference → larger dipole moment.

(b) Bond Length

Longer bond → greater charge separation → increases μ.

(c) Molecular Geometry

• Symmetrical shape → dipoles cancel → μ = 0
• Asymmetrical shape → dipoles add → μ ≠ 0

Geometry plays a major role in determining dipole behaviour.

(d) Multiple Bonds

Double and triple bonds increase electron density and may increase dipole moment.

6. Dipole Moment and Intermolecular Forces

Dipole moment affects the strength of intermolecular forces:

• Higher dipole moment → stronger dipole–dipole interactions
• Stronger interactions → higher boiling and melting points

Example:

• Water has a high dipole moment → strong hydrogen bonding → high boiling point

This shows how dipole moment influences physical properties.

7. Dipole Moment in Resonance Structures

Dipole moment may change when resonance increases electron delocalization.

Example:
In carbonate ion, resonance spreads charge over three oxygens, reducing the dipole effect on any single bond.

Thus, resonance can lower or redistribute dipole strength.

8. Applications of Dipole Moment

Dipole moment helps in:

• Predicting molecular shape
• Identifying if a molecule is symmetrical
• Understanding solubility (“polar dissolves polar”)
• Studying reaction mechanisms
• Designing pharmaceuticals and polymers
• Analysing behaviour of molecules in electric fields

Therefore, dipole moment is widely used in chemical, biological, and industrial research.

Conclusion

A dipole moment measures the separation of charges in a molecule caused by unequal sharing of electrons. It depends on electronegativity difference, bond length, and molecular geometry. Molecules with non-zero dipole moments are polar, while those with zero dipole moment are non-polar. Dipole moment influences physical properties, intermolecular forces, and chemical reactivity, making it an essential concept for understanding molecular behaviour.