Question

(a) How does one determine the number of electron domains in a molecule or ion? (b) What is the difference between a bonding electron domain and a nonbonding electron domain?

Short answer

To determine the number of electron domains in a molecule or ion, first draw its Lewis structure and identify the central atom. Count the single, double, or triple bonds, lone pairs, and single electrons around the central atom, considering each as one electron domain. The difference between bonding and nonbonding electron domains is that bonding electron domains participate in the formation of covalent bonds (single, double, or triple bonds), while nonbonding electron domains consist of lone pairs or single electrons that do not participate in bonding. Both types of electron domains affect the molecular geometry due to repulsion forces, influencing the shape and bond angles within the molecule.

Step-by-step solution

Part (a): Determining the Number of Electron Domains

To determine the number of electron domains in a molecule or ion, follow these steps: 1. Write down the Lewis structure or electron-dot structure of the molecule or ion. This structure shows the arrangement of valence electrons around the central atom. 2. Identify the central atom in the molecule or ion. 3. Count the number of electron domains around the central atom, which includes: - Single, double, or triple bonds (each bond is considered one electron domain) - Lone pairs (each lone pair is considered one electron domain) - Single electrons (each individual unpaired electron is considered one electron domain) The total number of electron domains is the sum of bonds, lone pairs, and single electrons surrounding the central atom.

Part (b): Difference between Bonding and Nonbonding Electron Domains

Bonding electron domains and nonbonding electron domains differ in their involvement in the formation of chemical bonds: 1. Bonding electron domains: These include single, double, or triple bonds. Each bond is considered one electron domain, regardless of its multiplicity. Bonding electron domains involve the sharing of electrons between two atoms to form covalent bonds. 2. Nonbonding electron domains: These comprise of lone pairs and single electrons that are not involved in the formation of chemical bonds. Nonbonding electron domains affect the molecular geometry by repelling against other electron domains, including bonding electron domains, causing deviation from the ideal bond angles predicted by VSEPR (Valence Shell Electron Pair Repulsion) theory. In summary, bonding electron domains are involved in the formation of chemical bonds, while nonbonding electron domains consist of lone pairs or single electrons that are not participating in bonding. Both bonding and nonbonding electron domains contribute to the overall molecular geometry and influence the shape and bond angles within a molecule.

Key concepts

Lewis Structure

Understanding the Lewis structure is foundational in grasping molecular geometry and electron interaction. The Lewis structure, also known as the electron-dot structure, is a diagram that represents the valent electrons of atoms within a molecule.

To create a Lewis structure, you begin by arranging the symbols for the atoms to show which atoms are connected to which, and then you distribute dots around each atom to represent its valence electrons. It's essential to follow the 'octet rule' for main-group elements, aiming for each atom (except hydrogen) to be surrounded by eight electrons. Here's how you can sketch a proper Lewis structure:

• Count the total number of valence electrons in the molecule or ion.
• Place electrons around the outer atoms to fulfill their octets before completing the octet of the central atom.
• Use the remaining electrons to complete the octet of the central atom, creating double or triple bonds if necessary.

A correct Lewis structure will have the least electronegative atom in the center with other atoms surrounding it, unless it's hydrogen, which always goes on the outside. Keep in mind that atoms can share electrons, leading to multiple bonds until the octet rule is satisfied.

Bonding Versus Nonbonding Electron Domains

While exploring electron domains in molecules, distinguishing between bonding and nonbonding electron domains is critical. Bonding electron domains are shared between atoms and are responsible for holding the molecule together. They can be single, double, or triple bonds, with each type considered a single domain despite the number of shared electron pairs.

On the other hand, nonbonding electron domains are localized on a single atom, typically as lone pairs or, less commonly, as a single unpaired electron (in radicals). Although nonbonding electron domains do not contribute to holding atoms together, they significantly influence molecular shape by repelling other electron domains.

This distinction is paramount in predicting the three-dimensional structure of molecules since nonbonding domains exert a greater repulsive force on adjacent electron domains, resulting in adjusted bond angles. For example, in water (H2O), the two lone pairs on the oxygen force the bonding hydrogen atoms closer together, leading to a bent molecular shape rather than a linear one.

Valence Shell Electron Pair Repulsion (VSEPR) Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory offers a simple way of understanding the three-dimensional arrangement of atoms in a molecule. It is based on the principle that electron domains around a central atom will arrange themselves as far apart as possible to minimize repulsion between these negatively charged areas.

For instance, a molecule with two electron domains (bonding or nonbonding) around the central atom will adopt a linear shape for maximum separation. As you increase the number of domains, the shapes adjust accordingly: three domains form a trigonal planar shape, four form a tetrahedral shape, and so on. Nonbonding electron domains play a role too, so for example, ammonia (NH3) with its three bonding and one nonbonding domain has a trigonal pyramidal shape.

VSEPR theory not only helps in predicting linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral structures among others but also assists in understanding deviations from ideal bond angles due to the varying repulsive strengths of bonding and nonbonding electron domains.