Short Answer

A pi bond is a type of covalent bond formed by the sideways overlap of two parallel p orbitals. This overlap occurs above and below the internuclear axis rather than directly between the nuclei. A pi bond is weaker than a sigma bond because the overlap is less effective.

Pi bonds are present only in multiple bonds. A double bond contains one pi bond, while a triple bond contains two pi bonds. Pi bonds restrict the rotation of atoms around the bond, making the molecule more rigid in structure.

Detailed Explanation :

Pi Bond

A pi bond is a covalent bond formed by the lateral (side-by-side) overlap of unhybridized p orbitals on adjacent atoms. Unlike a sigma bond, which forms through direct head-on overlap along the internuclear axis, a pi bond forms when p orbitals overlap sideways above and below the axis. This type of overlap produces regions of electron density on opposite sides of the bond, resulting in a weaker but important bond in chemical structures.

Pi bonds are essential for creating double and triple bonds, contributing to the strength, shape, and rigidity of molecules. They prevent rotation around the bond axis and play a major role in determining molecular geometry and reactivity.

1. How a Pi Bond Is Formed

Pi bonds form when:

• Two unhybridized p orbitals on adjacent atoms lie parallel to each other.
• These orbitals overlap sideways rather than directly.
• The electron density forms two lobes—one above and one below the internuclear axis.

This creates a characteristic shape where the electrons are shared in two regions instead of one.

Pi bonds can be formed by:

• p–p sideways overlap
• Overlaps of hybridized orbitals with unhybridized p orbitals (in some cases)

However, pi bonds cannot form from s orbitals because s orbitals do not have directional lobes.

2. Where Pi Bonds Occur in Molecules

Pi bonds occur only in multiple bonds:

(a) Double Bond (σ + π)

A double bond contains:

• One sigma bond
• One pi bond

Example:

• C=C in ethene
• O=C in carbon dioxide

(b) Triple Bond (σ + 2π)

A triple bond contains:

• One sigma bond
• Two pi bonds

Example:

• C≡C in ethyne
• N≡N in nitrogen gas

Thus, the first bond is always a sigma bond, and the additional bonds are pi bonds.

3. Characteristics of Pi Bonds

Pi bonds have several important features:

(a) Weaker Than Sigma Bonds

Sideways overlap is less effective than head-on overlap, making pi bonds weaker.

(b) Always Form After Sigma Bond

A pi bond cannot form unless a sigma bond already exists.

(c) Restrict Rotation

Pi bonds create electron density above and below the axis.
Trying to rotate the bond would break the overlap, so the bond becomes rigid.

Example:

• Ethene (C₂H₄) cannot rotate around the double bond.

(d) Important for Molecular Shape

Pi bonds affect geometry because they prevent free rotation and create planar structures.

(e) Higher Energy Than Sigma Bonds

Electrons in pi bonds are less tightly held, making them more reactive.

4. Pi Bonds and Hybridization

Hybridization explains how pi bonds occur:

• In sp² hybridization, one p orbital remains unhybridized → used to form a pi bond
• In sp hybridization, two p orbitals remain → used to form two pi bonds

Examples:

• Ethene (C₂H₄): sp² → 1 pi bond
• Ethyne (C₂H₂): sp → 2 pi bonds
• Benzene (C₆H₆): overlapping p orbitals → delocalised pi bonds

Thus, pi bonds play a major role in aromaticity and resonance.

5. Pi Bonds and Reactivity

Pi bonds are more reactive than sigma bonds because:

• Their electrons are exposed in regions above and below the molecule.
• Electrophiles (electron-loving species) attack these pi electrons easily.

This is why:

• Alkenes (with pi bonds) undergo addition reactions.
• Aromatic compounds react through electrophilic substitution.

Pi bonds also influence:

• Strength of materials
• Stability of polyatomic molecules
• Behaviour of resonance structures

6. Differences Between Sigma and Pi Bonds (Summary)

Sigma Bond	Pi Bond
Head-on overlap	Sideways overlap
Strongest covalent bond	Weaker bond
Electron density between nuclei	Electron density above/below axis
Allows rotation	Prevents rotation
Present in all covalent bonds	Only in multiple bonds

This comparison helps show why both types of bonds are equally important in chemistry.

Conclusion

A pi bond is a covalent bond formed by the sideways overlap of parallel p orbitals, creating electron density above and below the internuclear axis. It occurs only in multiple bonds—one pi bond in double bonds and two in triple bonds. Pi bonds are weaker than sigma bonds, restrict rotation, and influence molecular geometry and reactivity. Understanding pi bonds is essential for studying organic molecules, double and triple bond behaviour, and resonance.