Question

Give the electron-domain and molecular geometries of a molecule that has the following electron domains on its central atom: (a) four bonding domains and no nonbonding domains, (b) three bonding domains and two nonbonding domains, (c) five bonding domains and one nonbonding domain, (d) four bonding domains and two nonbonding domains.

Short answer

The electron-domain and molecular geometries for the given molecules are as follows: a) Tetrahedral electron-domain geometry and tetrahedral molecular geometry. b) Trigonal bipyramidal electron-domain geometry and T-shaped molecular geometry. c) Octahedral electron-domain geometry and square pyramidal molecular geometry. d) Octahedral electron-domain geometry and square planar molecular geometry.

Step-by-step solution

Identify the electron-domain geometry.

With four bonding domains and no nonbonding domains, the central atom has a total of four electron domains. According to the VSEPR model, the electron-domain geometry will be tetrahedral.

Determine the molecular geometry.

Since all electron domains are bonding domains, the molecular geometry will be the same as the electron-domain geometry. Hence, the molecular geometry is also tetrahedral. #b) Three bonding domains and two nonbonding domains.#

Identify the electron-domain geometry.

With three bonding domains and two nonbonding domains, the central atom has a total of five electron domains. According to the VSEPR model, the electron-domain geometry will be trigonal bipyramidal.

Determine the molecular geometry.

As there are two nonbonding electron domains, the molecular geometry will differ from the electron-domain geometry. The nonbonding electron domains will occupy the equatorial positions, while the three bonding domains occupy the remaining positions forming a "T" shape. Hence, the molecular geometry is T-shaped. #c) Five bonding domains and one nonbonding domain.#

Identify the electron-domain geometry.

With five bonding domains and one nonbonding domain, the central atom has a total of six electron domains. According to the VSEPR model, the electron-domain geometry will be octahedral.

Determine the molecular geometry.

As there is one nonbonding electron domain, the molecular geometry will differ from the electron-domain geometry. The nonbonding electron domain will occupy one position, while the five bonding domains occupy the remaining positions sharing vertices with the nonbonding domain. The shape formed by the five bonding domains is a square pyramid. Hence, the molecular geometry is square pyramidal. #d) Four bonding domains and two nonbonding domains.#

Identify the electron-domain geometry.

With four bonding domains and two nonbonding domains, the central atom has a total of six electron domains. According to the VSEPR model, the electron-domain geometry will be octahedral.

Determine the molecular geometry.

As there are two nonbonding electron domains, the molecular geometry will differ from the electron-domain geometry. These nonbonding electron domains will occupy two positions, while the four bonding domains occupy the remaining positions, forming a square plane. Hence, the molecular geometry is square planar.

Key concepts

Electron-Domain Geometry

Electron-domain geometry refers to the spatial arrangement of all electron domains (bonding or nonbonding) around the central atom. According to the Valence Shell Electron Pair Repulsion (VSEPR) model, these arrangements help us understand the shape of a molecule. It's based on the principle that electron pairs will repel each other and the molecule will arrange itself so as to minimize these repulsions.

When you count the total number of electron domains around the central atom, you can predict the electron-domain geometry using the VSEPR model. For example, with four total electron domains, the geometry will typically be tetrahedral. Here are some typical geometries with their corresponding domain numbers:

• Two domains result in a linear geometry.
• Three domains lead to trigonal planar geometry.
• Four domains result in a tetrahedral geometry.
• Five domains form a trigonal bipyramidal geometry.
• Six domains form an octahedral geometry.

Understanding the general electron-domain geometry is crucial to predict how molecules will interact.

Molecular Geometry

While electron-domain geometry considers all regions of electron density, molecular geometry focuses more specifically on only the arrangement of the atoms (bonding domains) in the molecule, ignoring the nonbonding electron pairs. This distinction is important because nonbonding pairs tend to take up more space, influencing the overall shape of the molecule.

For instance, in a molecule with four bonding domains and no nonbonding domains, both the electron-domain geometry and the molecular geometry will be tetrahedral. However, if you introduce nonbonding domains, the molecular geometry will shift. For example:

• With three bonding domains and two nonbonding domains, the electron-domain geometry is trigonal bipyramidal, but the molecular geometry becomes T-shaped due to the influence of the nonbonding pairs.
• In a case with five bonding domains and one nonbonding domain, the electron-domain geometry is octahedral, yet the molecular geometry is square pyramidal.

Explanation of molecular geometry is essential for understanding the actual shape and behavior of molecules.

Bonding Domains

Bonding domains are specific electron domains where electrons are shared between atoms, forming chemical bonds. The number of bonding domains a molecule has directly affects its shape and properties. In the VSEPR model, bonding domains can be single, double, or even triple bonds, but they are all treated as one single domain when determining shape.

In a molecule, bonding domains alongside nonbonding domains can provide insights into the overall 3D shape. For example:

• If a central atom has three bonding domains and no nonbonding domains, the molecule and electron-domain geometries will both be trigonal planar.
• In contrast, if there are three bonding domains and two nonbonding domains, the general electron-domain geometry is trigonal bipyramidal, but due to the presence of nonbonding domains, the molecular shape evolves into a T-shape.

Understanding bonding domains allow us to predict and explain the behavior and interaction of molecules, impacting everything from physical properties like boiling and melting points, to the chemical nature and reactivity of substances.