Question

$$ \mathrm{C}_{2} \mathrm{Cl}_{2} \text { , which is connected } \mathrm{Cl}-\mathrm{C}-\mathrm{C}-\mathrm{Cl} . $$

Short answer

The molecular geometry of C2Cl2 (dichloroethylene) is planar, with the Cl-C-C-Cl atoms all in the same plane. The carbon atoms have a trigonal planar geometry, and the chlorine atoms have a trigonal pyramidal geometry.

Step-by-step solution

Determine the number of electron domains for each atom

To start with, we need to calculate the number of electron domains for each carbon and chlorine atom. An electron domain is an area where electrons are likely to be found, which includes lone electron pairs, bonding electron pairs, and multiple bonds (double and triple bonds). For the carbon atoms in C2Cl2, there are three electron domains each: one from the double bond between the carbon atoms and one from each single bond with the chlorine atoms. For the chlorine atoms in C2Cl2, there are a total of four electron domains: one from the single bond with the carbon atom and three from the lone pairs of electrons on the chlorine.

Determine the geometries of each atom using VSEPR theory

VSEPR (Valence Shell Electron Pair Repulsion) theory states that electron domains around an atom will arrange themselves in a way that minimizes repulsion, dictating the shape of the molecule. Using VSEPR theory, we'll determine the electron geometry and the molecular geometry of the carbon and chlorine atoms in the C2Cl2 molecule. For the carbon atoms with three electron domains (one double bond and two single bonds), the electron geometry will be trigonal planar, and the molecular geometry will be also trigonal planar, as there are no lone pairs on the carbon atoms. For the chlorine atoms with four electron domains (one single bond and three lone pairs), the electron geometry will be tetrahedral, and the molecular geometry will be trigonal pyramidal, as there is one bonding and three lone electron pairs.

Combine the individual atom geometries to find the overall molecular geometry

Finally, we will combine the molecular geometries of the individual atoms (carbon and chlorine) to find the overall molecular geometry of the C2Cl2 molecule. Since both carbon atoms have a trigonal planar geometry, and the chlorine atoms have a trigonal pyramidal geometry, the C2Cl2 molecule will also have a planar geometry. The Cl-C-C-Cl atoms are all in the same plane, with bond angles close to 120° for the carbon atoms and slightly less than 109.5° for the chlorine atoms due to the presence of the lone electron pairs. The overall molecular geometry of the C2Cl2 molecule is planar with the Cl-C-C-Cl atoms are all in the same plane.

Key concepts

Electron Domains

In chemistry, particularly in the realm of molecular shapes, electron domains are crucial. An electron domain includes any region where electrons are concentrated. These regions can be single, double, or triple bonds, as well as lone pairs of electrons that are not involved in bonding.
Each domain consists of a pair of electrons, and understanding this helps us predict the shape of a molecule.

• Bonding pairs: These are electron pairs found within bonds (like single, double, or triple bonds).
• Lone pairs: Electrons that belong exclusively to one atom, not shared between atoms.

VSEPR (Valence Shell Electron Pair Repulsion) theory utilizes these electron domains to forecast molecular shapes. By considering the number and types of electron domains, one can predict the geometric arrangement of atoms around a central atom.

Molecular Geometry

Molecular geometry is a term used to describe the three-dimensional arrangement of atoms within a molecule. Understanding molecular geometry is vital as it influences a molecule's physical properties, reactivity, and more.
Molecular geometry refers to the spatial positions of atoms only, not the position of electron pairs. VSEPR theory can help ascertain the molecular geometry by noting the repulsions between electron pairs and how these dictate the positions of atoms.
The relationship between electron domains and molecular geometry determines the visual shape of the molecule and can be predicted through systematic study.

Trigonal Planar Geometry

A trigon planar geometry refers to a symmetrical arrangement of atoms around a central atom, like an umbrella with three spokes forming an angle of 120°.
In a trigonal planar shape, three electron single bonds, or domains, are oriented as far apart as possible, typically in a plane around the central atom. This occurs when a central atom is bonded to three surrounding atoms with no lone pairs, as seen with carbon atoms in the C2Cl2 molecule.
In such cases, VSEPR theory informs us that the electron domains will adjust themselves to minimize electronic repulsion, solidifying the trigonal planar arrangement.

Lone Pairs

Lone pairs are pairs of valence electrons that are not shared between atoms and are not involved in bonding. They belong to a single atom. Since they do not participate in bonding, they significantly affect the shape and geometry of molecules.

• Electron Repulsion: Lone pairs occupy more space around the nucleus than shared bonding pairs.
• Influence: They can alter bond angles by exerting greater repulsive forces, pushing bonding pairs closer together.

In C2Cl2, chlorine atoms have three lone pairs each, impacting the local geometry more than carbon atoms, which have no lone pairs. Despite fewer direct interactions, they push bonding domains, adjusting bond angles and contributing to the final molecular geometry.