Problem 24 - Chemistry The Central Science

Question

An \(\mathrm{AB}_{3}\) molecule is described as having a trigonal-bipyramidal electron-domain geometry. How many nonbonding domains are on atom A? Explain.

Step-by-Step Solution

Answer

In an AB3 molecule with a trigonal-bipyramidal electron-domain geometry, the central atom A has 5 electron domains, 3 of which are bonding domains with surrounding B atoms. Therefore, there are \(2\) nonbonding domains on atom A.

Step 1: Identify the Electron Domains on Atom A

AB3 molecules have the general composition A central atom surrounded by three B atoms. In a trigonal-bipyramidal electron-domain geometry, there are 5 electron domains surrounding the central atom.

Step 2: Determine the Number of Bonding and Nonbonding Domains

With three surrounding B atoms, we can conclude that atom A forms three bonding domains with these B atoms. Since there are 5 electron domains in a trigonal-bipyramidal geometry, we can calculate the number of nonbonding domains by subtracting the bonding domains from the total domains.

Step 3: Calculate the Number of Nonbonding Domains

We know the total number of electron domains (5) and the number of bonding domains (3). Let's use this information to find the number of nonbonding domains: Nonbonding Domains = Total Domains - Bonding Domains = 5 - 3 = 2

Step 4: Conclusion

In an AB3 molecule with a trigonal-bipyramidal electron-domain geometry, the central atom A has 2 nonbonding domains.

Key Concepts

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. It's a crucial concept in chemistry that affects how molecules interact with each other and their chemical properties. The geometry is determined by the number of electron domains surrounding the central atom, which includes both bonding and nonbonding electron pairs. For example, when we consider an \textbf{AB}\(_3\) molecule with a trigonal-bipyramidal geometry, the five electron domains are arranged such that three are in equatorial positions forming a triangle in a single plane and two occupy axial positions, above and below the plane. This arrangement minimizes electron repulsion, following the Valence Shell Electron Pair Repulsion (VSEPR) theory.

Molecules with different numbers of bonding and nonbonding electron pairs will have distinct molecular geometries, such as linear, bent, tetrahedral, trigonal-planar and octahedral, among others. Understanding an atom's molecular geometry helps in predicting the molecule's polarity, reactivity, phase of matter, color, magnetism, biological activity, and many other chemical properties.

Nonbonding Electron Pairs

Nonbonding electron pairs, also known as lone pairs, are pairs of valence electrons that are not shared with another atom and do not participate in bonding. These electron domains exert a greater repulsive force than bonding domains because they are localized closer to the central atom. This repulsion affects the molecular geometry of the atom, often reducing the bond angles between the bonding electron pairs.

Let's use the \textbf{AB}\(_3\) molecule as an example. Given that it has five electron domains and only three bonds with surrounding atoms, we can infer that there are two nonbonding electron pairs associated with the central atom. These nonbonding domains can be envisioned as invisible partners that occupy space and can push the bonding domains closer together, altering the ideal bond angles implied by the basic electron-domain geometry.

Trigonal-Bipyramidal Structure

The trigonal-bipyramidal structure is one of the electron-domain geometries explained by the VSEPR theory. It's characterized by a central atom surrounded by five electron domains. These are arranged such that three form an equatorial triangle around the central atom, and two occupy positions above and below this plane, termed the axial positions.

In this geometry, there's angular distinction due to the different positioning of the electron domains. The equatorial-equtorial bond angles are \textbf{120}\(^\text{o}\) and the equatorial-axial bond angles are \textbf{90}\(^\text{o}\). However, if nonbonding electron pairs are present, they will typically occupy the equatorial positions where they are most effective at minimizing electron repulsion, as these positions allow for greater distance between electron pairs. This configuration leads to a less symmetric molecule and can have significant effects on the molecule's physical and chemical properties.