Question

Describe the characteristic electron-domain geometry of each of the following numbers of electron domains about a central atom: (a) 3 , (b) 4 , (c) 5, (d) 6 .

Short answer

The characteristic electron-domain geometry for each number of electron domains around a central atom are as follows: (a) 3 electron domains result in a trigonal planar geometry with 120-degree bond angles, (b) 4 electron domains lead to a tetrahedral geometry with 109.5-degree bond angles, (c) 5 electron domains create a trigonal bipyramidal geometry with equatorial bond angles of 120 degrees, axial bond angles of 180 degrees, and equatorial-axial bond angles of 90 degrees, and (d) 6 electron domains correspond to an octahedral geometry with 90-degree bond angles between all adjacent domains.

Step-by-step solution

Understanding Electron-Domain Geometry

Electron-domain geometry is all about arranging electron domains (the regions where electron pairs are found) around a central atom to minimize the repulsion between these domains. Several geometries result from different numbers of electron domains, such as linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. Let's examine the characteristic geometry for each given number of electron domains.

Case (a): 3 Electron Domains

In this case, there are 3 electron domains around the central atom, which results in a trigonal planar geometry. This arrangement minimizes the repulsion between the electron domains, and the bond angles are all equal to 120 degrees.

Case (b): 4 Electron Domains

For this case, there are 4 electron domains around the central atom, leading to a tetrahedral geometry. This configuration further minimizes the repulsion between each domain, with bond angles all equal to 109.5 degrees.

Case (c): 5 Electron Domains

When we have 5 electron domains around the central atom, the geometry becomes trigonal bipyramidal. In this arrangement, there are three equatorial domains forming a trigonal plane with 120-degree bond angles, and two axial domains forming a linear configuration with 180-degree bond angles. The equatorial-axial bond angle is 90 degrees.

Case (d): 6 Electron Domains

In this case with 6 electron domains around the central atom, an octahedral geometry is observed. This arrangement has bond angles of 90 degrees between all adjacent electron domains, further minimizing repulsion. In summary, as the number of electron domains increases, the geometry of the molecule changes to minimize repulsion between the electron domains, resulting in different bond angles and shapes. The geometries for 3, 4, 5, and 6 electron domains are trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, respectively.

Key concepts

Trigonal Planar

A trigonal planar geometry arises when a central atom is surrounded by three electron domains. Imagine a flat, triangular shape where each corner represents an atom bonded to the central atom. This arrangement allows for the minimization of electron repulsion.
The bond angles in a trigonal planar geometry are all 120 degrees, which keeps everything evenly spaced.

• Each electron domain can be a bond between atoms or a lone pair of electrons.
• Common in molecules where the central atom has no lone pairs, ensuring a symmetric structure.

Trigonal planar geometry is often seen in molecules like boron trifluoride (BF₃), giving them their distinct shape and angle.

Tetrahedral Geometry

In a tetrahedral geometry, the central atom is surrounded by four electron domains. Picture a pyramid with a triangular base, where the central atom is in the center, and each of the four bonds points to a corner of the tetrahedron.
The bond angles in a tetrahedral molecule are 109.5 degrees, allowing for equal spacing of the electron domains.

• Common in carbon compounds where four single bonds are present, such as in methane (CH₄).
• This geometry ensures that the repulsion between any two electron domains is minimized.

Understanding tetrahedral geometry helps explain the shape and properties of many organic molecules.

Trigonal Bipyramidal

With five electron domains, a central atom forms a trigonal bipyramidal geometry. Visualize two pyramids sharing a triangular base, with the central atom in the middle.
Here, three bonds lie equatorially at 120 degrees, while two lie axially, making a linear shape with a 180-degree angle. The equatorial-axial angle is 90 degrees.

• Found in molecules like phosphorus pentachloride (PCl₅) where different angles help accommodate all electron domains.
• The uneven distribution allows for a mix of bond angles and considerable molecular versatility.

Such geometry provides unique insights into the bonding and reactivity of certain compounds.

Octahedral Geometry

An octahedral geometry occurs when there are six electron domains around the central atom. Picture an octahedron—a shape with eight faces, or imagine two four-sided pyramids base to base.
All bond angles in an octahedral geometry are 90 degrees, ensuring maximum symmetry in three-dimensional space.

• Seen in compounds like sulfur hexafluoride (SF₆), where the arrangement allows for optimal electron spacing.
• This geometry supports various complex bonding scenarios because of its high symmetry.

Octahedral geometry is crucial for understanding how certain chemical structures maintain stability and shape.