Short Answer

sp² hybridization is a type of hybridization where one s orbital and two p orbitals mix to form three identical hybrid orbitals. These hybrid orbitals lie in a plane and are spaced 120° apart, giving the molecule a trigonal planar shape. The remaining unhybridized p orbital is used to form a pi (π) bond.

Atoms showing sp² hybridization usually form molecules with one double bond, such as ethene (C₂H₄) and boron trifluoride (BF₃). This hybridization explains the bond angles, geometry, and strength of sigma bonds in such molecules.

Detailed Explanation :

sp² Hybridization

sp² hybridization is an important concept in chemical bonding that explains the structure and shape of molecules containing double bonds. In this type of hybridization, one s orbital and two p orbitals from the same atom combine to form three equivalent hybrid orbitals. These hybrid orbitals are arranged in a flat plane (planar arrangement) and point toward the corners of an equilateral triangle, maintaining a bond angle of 120°.

This hybridization commonly occurs in atoms that form double bonds or have regions of electron density arranged in a trigonal planar shape. It is especially important in organic chemistry, where carbon often uses sp² hybrid orbitals when forming compounds such as alkenes.

How sp² Hybridization Occurs

sp² hybridization occurs in the following steps:

1. Mixing of Orbitals
   • One s orbital mixes with two of the three p orbitals.
   • This forms three sp² hybrid orbitals of equal energy.
2. Arrangement of Hybrid Orbitals
   • The three hybrid orbitals spread out in a plane.
   • They arrange themselves 120° apart to reduce electron repulsion.
3. Remaining Unhybridized p Orbital
   • One p orbital remains unhybridized.
   • This orbital is used to form pi (π) bonds in molecules with double bonds.

This hybridization allows atoms to form strong sigma bonds and additional pi bonds for double bonding.

Key Characteristics of sp² Hybridization

1. Trigonal Planar Geometry

The three sp² orbitals form a triangle around the atom.
Bond angle = 120°
Shape = Trigonal planar

Example:
BF₃ and CH₂O (formaldehyde)

2. Formation of Sigma and Pi Bonds

• Each sp² hybrid orbital forms a sigma (σ) bond.
• The unhybridized p orbital forms a pi (π) bond.

Thus, sp² hybridization is always associated with double bonds.

Example:
In ethene (C₂H₄), each carbon forms:

• Three sigma bonds (using sp² orbitals)
• One pi bond (using unhybridized p orbitals)

3. Bond Strength and Shorter Bond Lengths

sp² hybridization creates bonds that are:

• Stronger than sp³ bonds
• Shorter due to more s-character (33%)

More s-character means electrons stay closer to the nucleus, increasing bond strength.

4. Planar Structure

Molecules with sp² hybridization are flat (two-dimensional).
This shape affects:

• Reactivity
• Rotation
• Chemical properties

Double bonds cannot rotate freely because the pi bond restricts movement.

Examples of sp² Hybridization

1. Ethene (C₂H₄)

Each carbon atom is sp² hybridized.

• Three sigma bonds are formed using sp² orbitals.
• One pi bond forms from the unhybridized p orbitals.
• The molecule is planar with 120° bond angles.

2. Boron Trifluoride (BF₃)

Boron forms three sp² sigma bonds with fluorine atoms.

• No double bond, but still trigonal planar.
• Bond angles are 120°.

3. Carbonyl Compounds (C=O Group)

In aldehydes and ketones:

• Carbon is sp² hybridized.
• One pi bond forms between carbon and oxygen.

The planar structure influences reactivity in organic reactions.

4. Benzene (C₆H₆)

All six carbon atoms in benzene are sp² hybridized.

• Each carbon forms three sigma bonds.
• Unhybridized p orbitals overlap to form a ring of delocalised pi electrons.

This explains benzene’s stability and aromatic behaviour.

Importance of sp² Hybridization

sp² hybridization is important because it explains:

1. Presence of Double Bonds

Any molecule with a C=C bond or other double bonds uses sp² hybridization.

2. Planar Molecule Shapes

Many biological molecules and organic compounds depend on planar geometry.

3. Restricted Rotation

Because of the pi bond, molecules with sp² hybridization cannot rotate freely, which affects stereochemistry.

4. Bond Strength and Reactivity

sp² bonds are shorter and stronger than sp³ bonds, influencing chemical behaviour.

5. Aromaticity

Aromatic compounds like benzene rely on sp² hybridization to allow delocalisation of electrons.

Conclusion

sp² hybridization occurs when one s orbital and two p orbitals mix to form three identical hybrid orbitals arranged in a trigonal planar geometry with 120° bond angles. It plays a key role in forming double bonds, creating strong sigma bonds, enabling pi bonding, and giving molecules their planar shapes. This hybridization is essential in understanding the bonding and structure of many important organic and inorganic compounds.