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, identify the central atom and count the electron regions around it, including single, double, or triple bonds and lone pairs. A bonding electron domain is a region of electron density involved in covalent bonds between atoms (single, double, or triple bonds). In contrast, a nonbonding electron domain is a region of electron density not involved in bonding between atoms, typically as lone pairs or unshared pairs of electrons on the central atom.

Step-by-step solution

(a) Determining the number of electron domains in a molecule or ion

To determine the number of electron domains in a molecule or ion, first, identify the central atom. Then, count the regions around the central atom where electrons are found, keeping in mind that these electrons can be involved in bonding (either single, double, or triple bonds) or present as lone pairs. An electron domain can be: 1. A single bond (e.g., in CH4, the bond between C and each H counts as an electron domain) 2. A double bond (e.g., in CO2, each double bond between C and O counts as a single electron domain) 3. A triple bond (e.g., in N2, the triple bond between the two N atoms counts as a single electron domain) 4. A lone pair (e.g., in NH3, the lone pair on N counts as an electron domain) The total number of such electron domains gives us the electron-domain geometry of the molecule.

(b) Difference between a bonding and nonbonding electron domain

A bonding electron domain is a region of electron density that is involved in covalent bonds between atoms, which can be single, double, or triple bonds. In each case, the bonding electron domain counts as one region of electron density, regardless of the number of bonds involved. A nonbonding electron domain, on the other hand, is a region of electron density that is not involved in bonding between atoms. These are usually in the form of lone pairs or unshared pairs of electrons on the central atom. The presence of nonbonding electron domains can influence the shape of the molecule as they can cause repulsion with the bonding electron domains.