(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?

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.

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.

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.