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Understanding Hybridisation of C₂H₄: Ethene

Understanding Hybridisation of C₂H₄: Ethene

Ethene (C₂H₄), also known as ethylene, is a hydrocarbon with a trigonal planar structure and a double bond between its two carbon atoms. It is a classic example of sp² hybridisation in organic chemistry.

Let us understand how hybridisation occurs in C₂H₄ and how it determines bonding and molecular shape.

What is the Hybridisation of C₂H₄?

Ethene consists of two carbon atoms and four hydrogen atoms. Each carbon atom forms two bonds with hydrogen atoms and one bond with the other carbon atom. To satisfy the octet rule, each carbon undergoes sp² hybridisation.

Breakdown of C₂H₄ Hybridisation

Ethene is the simplest alkene. It is an unsaturated hydrocarbon containing:

  • 2 carbon atoms
  • 4 hydrogen atoms
  • 1 C=C double bond and 4 C–H single bonds

Electronic Configuration of Carbon

The atomic number of carbon is 6.

Ground state electronic configuration:

1s² 2s² 2p²

Only two unpaired electrons → insufficient to form four bonds.

Excited state electronic configuration:

1s² 2s¹ 2pₓ¹ 2pᵧ¹ 2p_z¹

Four unpaired electrons → sufficient to form four bonds.

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Alt text: Ground state vs excited state orbital diagram

Formation of Hybrid Orbitals

sp² hybridisation occurs when:

  • 1 s orbital mixes with 2 p orbitals
  • 3 sp² hybrid orbitals are formed per carbon atom
  • 1 unhybridised p orbital (2p_z) remains available for π bonding

Bond Formation in Ethene

Each carbon atom uses:

  • 2 sp² orbitals to form σ bonds with hydrogen atoms
  • 1 sp² orbital to form a σ bond with the other carbon atom
  • The unhybridised p orbitals overlap sideways to form 1 π bond

Result:

  • 1 σ bond and 1 π bond between the two carbon atoms
  • 4 C–H σ bonds
  • Hybridisation type: sp²
  • Bond angle ≈ 120°
  • Geometry: Trigonal planar

 

Hybridisation and Geometry of Ethene (C₂H₄)

Alt text: Geometry and bonding of ethene

Details at a Glance

Property Details
Molecule Ethene (C₂H₄)
Hybridisation sp²
Geometry Trigonal planar
Bond angle ~120°
Bonding 5 σ bonds (4 C–H, 1 C–C) and 1 π bond (C=C)
Unhybridised orbitals One p orbital on each carbon for π bonding
Carbon valency satisfied? Yes

Formal Charge in C₂H₄

Formal charge is calculated using the formula:

Formal charge = Valence electrons − (Lone pair electrons + ½ × Bonding electrons)

Carbon (C)

  • Valence electrons = 4
  • Lone pair electrons = 0
  • Bonding electrons = 8

Formal charge = 4 − (0 + ½ × 8) = 4 − 4 = 0

Hydrogen (H)

  • Valence electrons = 1
  • Lone pair electrons = 0
  • Bonding electrons = 2

Formal charge = 1 − (0 + ½ × 2) = 1 − 1 = 0

Thus, all atoms in C₂H₄ carry zero formal charge, confirming the stability of its Lewis structure.

Summing Up

Each carbon atom in C₂H₄ forms four bonds: two C–H σ bonds, one C–C σ bond, and one C–C π bond. sp² hybridisation leads to trigonal planar geometry with a bond angle of approximately 120°. The π bond is responsible for the C=C double bond.

Frequently Asked Questions

Q1. Why does carbon undergo hybridisation in C₂H₄?

To form three coplanar σ bonds and one π bond, carbon promotes an electron and mixes orbitals to form sp² hybrid orbitals.

Q2. How many σ and π bonds are present in C₂H₄?

There are 5 σ bonds and 1 π bond in ethene.

Q3. What is the shape of ethene?

Ethene has a trigonal planar shape due to sp² hybridisation on each carbon atom.

Q4. Is C₂H₄ polar or non-polar?

Ethene is non-polar because the bond dipoles cancel out due to its symmetric structure.

Q5. What are some uses of ethene?

Ethene is widely used in polyethylene and plastic production. It also plays a key role in the ripening of fruits.

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