Question

The shape of ethene is (A) planar (B) tetrahedral (C) linear (D) pyramidal

Short answer

The shape of ethene (C2H4) is determined by the hybridization of the carbon atoms, which is sp2, resulting in a trigonal planar electron pair geometry around each carbon atom. Since the two carbon atoms are in the same plane, the overall shape of ethene is planar. Therefore, the correct answer is (A) planar.

Step-by-step solution

Determine the Hybridization of Carbon Atoms

In ethene, there are two carbon atoms bonded to each other, with each carbon atom bonded to two hydrogen atoms. Each carbon atom has three sigma (σ) bonds, so the hybridization will be sp2.

Identify the Shape Around the Carbon Atoms

With sp2 hybridization, the electron pair geometry around each carbon atom is trigonal planar. The carbon-carbon double bond in ethene consists of one σ-bond and one pi (π) bond. The σ-bond is formed by the overlap of sp2 orbitals of two carbon atoms, and the π-bond is formed by the overlap of unhybridized p orbitals of the two carbon atoms. The two other sp2 orbitals in the hybridized carbon atom are occupied by the two single C-H σ-bonds. Therefore, the shape around each carbon atom is a planar trigonal with bond angles of approximately 120°.

Identify the Overall Shape of Ethene

As the shape around each carbon atom is trigonal planar, and the two carbon atoms are in the same plane, the overall shape of ethene is planar.

Choose the Correct Answer

Based on the analysis above, the correct option for the shape of ethene is (A) planar.

Key concepts

sp2 Hybridization

Understanding the concept of sp2 hybridization is fundamental in predicting and visualizing the shape and bonding of certain molecular structures, such as ethene (C2H4). Hybridization occurs when atomic orbitals mix to form new hybrid orbitals that are better suited for the bonding with other atoms. In the case of ethene, each carbon atom undergoes sp2 hybridization, where one 's' orbital and two 'p' orbitals combine to form three equivalent hybrid orbitals.

These hybrid orbitals have a trigonal planar arrangement, which means they spread out evenly in the horizontal plane, making an angle of 120 degrees with each other. The unhybridized 'p' orbital remains perpendicular to this plane. The hybridized orbitals form stronger, more stable sigma (σ) bonds with hydrogen atoms and each other, while the unhybridized 'p' orbitals overlap sideways to create a weaker, but very important, pi (π) bond.

Trigonal Planar Geometry

The trigonal planar geometry of a molecule such as ethene is a result of the sp2 hybridization process. Each carbon atom in ethene is surrounded by three regions of electron density, which consist of two C-H bonds and one C-C bond. Due to the repulsion between these regions of electron density, the shape that minimizes this repulsion is a flat, triangular arrangement known as trigonal planar.

This leads to uniform bond angles of approximately 120 degrees around each carbon atom. The trigonal planar geometry ensures that the molecules' electron clouds are distributed evenly around the central atoms, minimizing the repulsive forces between them. This geometric consideration is crucial when determining the shape of molecules and contributes significantly to the stability and chemical properties of the molecule.

Sigma and Pi Bonds

Ethene prominently features two types of covalent bonds: sigma (σ) and pi (π) bonds. These bonds have different spatial orientations and originate from different types of orbital overlaps. A sigma bond is the strongest type of covalent chemical bond, formed by the head-on or axial overlap of atomic orbitals, such as the overlap between the hybridized sp2 orbitals of each carbon atom in ethene.

The pi bond is formed by the side-on overlap of two unhybridized 'p' orbitals, one from each carbon atom. The pi bond is responsible for the double bond feature alongside the sigma bond, making the overall C=C bond in ethene. The existence of pi bonds affects the molecule's rigidity and is why certain rotations around the double bond are not possible without breaking the pi bond. Hence, the pi bond plays a vital role in maintaining the planar shape of ethene and contributes to its chemical reactivity.