Question

Ethylene ( \(\mathrm{C}_{2} \mathrm{H}_{4}\) ) has two central atoms. Determine the geometry around each central atom and the shape of the overall molecule.

Short answer

Each carbon in \(\mathrm{C}_2\mathrm{H}_4\) is trigonal planar, and the molecule is planar overall.

Step-by-step solution

Identify Central Atoms

In the ethylene molecule (\(\mathrm{C}_2\mathrm{H}_4\)), the central atoms are the two carbon atoms. Each carbon atom is bonded to two hydrogen atoms and one carbon atom.

Determine Electron Geometry Around Carbon

Each carbon atom in ethylene (\(\mathrm{C}_2\mathrm{H}_4\)) has three regions of electron density: two bonds with hydrogen atoms and one double bond with the other carbon atom. According to the VSEPR theory, three regions of electron density result in a trigonal planar geometry.

Identify Molecular Geometry Around Carbon

For a carbon atom with a trigonal planar electron geometry and no lone pairs, the molecular geometry is also trigonal planar. Thus, each carbon atom in ethylene exhibits a trigonal planar shape around it.

Determine Overall Molecular Shape

The overall shape of the ethylene molecule is determined by the combination of the two trigonal planar structures formed around each carbon. As the double bond restricts rotation, the ethylene molecule is flat with a planar geometry.

Key concepts

Ethylene Molecule

The ethylene molecule, known chemically as \( \text{C}_2\text{H}_4 \), is an intriguing compound that offers a great example of how molecular geometry works. Ethylene is composed of two carbon atoms and four hydrogen atoms. This basic setup allows us to explore how atomic arrangements influence the shape of a molecule.
In ethylene, the two carbon atoms act as the central atoms. Each carbon atom forms two single bonds with hydrogen atoms and a double bond with the other carbon atom. This configuration is an essential factor dictating how the molecule shapes itself. The double bond is particularly significant because it introduces rigidity, preventing rotation around the bond axis and maintaining a specific planar structure.
By examining the structure of ethylene, students gain an understanding of how these bonding connections result in particular geometric outcomes, reinforcing the importance of molecular bonding and shape in chemistry.

Trigonal Planar Geometry

The trigonal planar geometry is a fundamental concept in molecular shape determination, particularly relevant to organic compounds like ethylene. Understanding this geometry starts with recognizing the influence of electron pairs around a central atom.
According to the VSEPR (Valence Shell Electron Pair Repulsion) theory, electron pairs, including bonding and non-bonding pairs, repel each other and arrange themselves as far apart as possible. In the case of each carbon atom in the ethylene molecule, there are three bonding regions: two single bonds with hydrogen atoms and a shared pair in the form of a double bond with another carbon atom.
This results in a planar configuration where the carbon atom and its bonded partners – the two hydrogens and one other carbon atom – align themselves in a 120-degree angle. The three points form a triangle, giving rise to the name 'trigonal planar.' By visualizing and understanding this formation, students can appreciate how molecular geometries are not just fanciful diagrams but representations of physical space and electron behavior.

Molecular Shape Determination

Molecular shape determination involves understanding the spatial arrangement of atoms in a molecule and is crucial for predicting the behavior and properties of compounds. The VSEPR theory is often employed to predict the molecular geometries of compounds like ethylene.
In the ethylene molecule, each carbon atom is surrounded by three groups of electrons. These groups repel each other equally, resulting in a trigonal planar shape for the regions of electron density around each carbon. Since there are no lone pairs of electrons around the carbon atoms, the molecular shape mirrors the electron geometry.
For ethylene, determining its molecular shape implies analyzing the interconnections around its carbon atoms, understanding the impact of the double bond, and recognizing how these elements result in a flat, planar molecular structure. With this flatness, ethylene demonstrates how even simple molecules can have precisely determined shapes that influence their chemical nature. This type of molecular analysis allows chemists to predict many properties of molecules, including reactivity, polarity, and phase state.