Question

Predict the electron pair geometry, the molecular shape, and the bond angle for a phosgene molecule, \(\mathrm{CCl}_{2} \mathrm{O}\), using VSEPR theory.

Short answer

Electron pair geometry: Trigonal planar; Molecular shape: Trigonal planar; Bond angle: 120°.

Step-by-step solution

Determine the Lewis Structure

First, we need to draw the Lewis structure for CCl₂O. Carbon is the central atom, bonded to two chlorine atoms and one oxygen atom. We arrange the atoms as follows: O=C-Cl and assign valence electrons considering oxygen has 6, carbon has 4, and each chlorine has 7, totaling 24 valence electrons. Distribute the electrons to satisfy the octet rule for each of the atoms.

Count the Electron Regions

Count the regions of electron density around the central atom, carbon. In CCl₂O, carbon is bonded to three atoms and does not have any lone pairs of electrons. This gives us a total of three regions of electron density.

Predict the Electron Pair Geometry

Use the VSEPR theory to predict the electron pair geometry based on the number of electron regions. With three regions of electron density (pairs), the geometry is trigonal planar.

Determine the Molecular Shape

Since there are no lone pairs on the central carbon, the molecular shape is the same as the electron pair geometry, trigonal planar.

Determine the Bond Angles

In a trigonal planar geometry, the bond angles are approximately 120 degrees between each pair of bonded atoms around the central atom.

Key concepts

Electron Pair Geometry

Electron pair geometry is an essential concept to grasp, as it forms the foundation for understanding molecular shapes. It describes the spatial arrangement of electron pairs around a central atom. This includes both bonding electron pairs and lone pairs. With VSEPR (Valence Shell Electron Pair Repulsion) theory, electron pairs are assumed to orient in such a way to minimize repulsion, leading to a specific geometric arrangement.
For the phosgene molecule (CCl}_2 O), carbon is the central atom. According to VSEPR theory, if there are three regions of electron density surrounding a central atom, as in the case of phosgene (where carbon is bonded to two chlorine atoms and one oxygen atom), the electron pair geometry is trigonal planar. This arrangement allows for maximum separation of the electron pairs, reducing electron-electron repulsion to its lowest possible level. Understanding electron pair geometry is crucial for predicting the overall shape of molecules and calculating the bond angles between them.

Molecular Shape

Molecular shape refers to the actual 3D structure of a molecule, which arises from the electron pair geometry but only includes the placement of atoms, excluding lone pairs. While the electron pair geometry sets the stage, the shape defines how the molecule interacts with other entities.
In the phosgene molecule (CCl}_2 O), the molecular shape is determined entirely by the bonding pairs of electrons, as there are no lone pairs on the central carbon atom. Thus, the shape is the same as its electron pair geometry, which is trigonal planar.
This tri-cornered configuration allows each atom to be at a maximum distance from one another, promoting stability. Molecular shape is significant because it plays a critical role in the properties and reactivity of the molecule, affecting how it bonds and interacts with other molecules.

Bond Angle

Bond angle is essential for understanding how atoms within a molecule are oriented in space. It is the angle formed between three connected atoms. For the phosgene molecule (CCl}_2 O), where the carbon atom is the center, the bond angles need to correspond with a trigonal planar geometry.
In a trigonal planar configuration, the ideal bond angle is approximately 120 degrees. This angle is a result of electron pairs spreading out as evenly as possible around the central atom to minimize repulsion. The precise bond angle can slightly vary due to the differing electronegativities of the atoms involved (in CCl}_2 O), but it will still be around 120 degrees.
Understanding bond angles is crucial for visualizing the shape of a molecule and predicting how a molecule may interact with others, including how it might fit into a specific receptor site, for example, in biological systems.

Lewis Structure

Drawing a Lewis structure is usually the first step in visualizing a molecule's layout. It represents atoms, bonds, and lone pairs visually with dots and lines, simplifying the view of the molecule's bonding.
For the phosgene molecule (CCl}_2 O), we start by identifying the central atom, which is carbon. Bonded to it are two chlorine atoms and one oxygen atom. We account for valence electrons: 4 from carbon, 6 from oxygen, and 7 from each chlorine, summing up to a total of 24 electrons.
Arrange atoms so that they satisfy the octet rule, ensuring each atom, particularly the central carbon, achieves a full outer shell. This leads to a Lewis structure that accurately represents the molecule's electron distribution. Lewis structures are fundamental for predicting how a molecule will behave in further reactions and can be a tool for refining predictions of molecular shape and bond angles using VSEPR theory.