Question

compare a bonding domain to a non-bonding domain. How are these domains related to each other in 3D? How does adding either a bonding domain or a lone pair affect the position of existing atoms, lone pairs, and bond angles?

Answer 1

A bonding domain consists of shared electrons between the two atoms to form a bond, in order to complete their octet. For example, H has 1 electron; so in order to complete its octet it would require 1 more electron which can be gained from another H having the same requirement. Therefore, the sharing of 1 electron each to form a bond of 2 electron is what is known as a bonding domain.

An example of 4 bonding domains can be seen in the case of CH₄.

Now, at times the atom might not share all the electrons while completing its octet. In NH₃, for N to complete its octet it would require 3 more electrons as it already has 5. These 3 electrons are made available through each Hyrdrogen, which itself require 1 electron each. Leaving us with 3 bonding domains and one non-bonding domains, i.e. pair of electrons which are not involved in any type of bonding, also called as lone-pair of electrons:

Bonding and Non-bonding domains are very well connected to each other, as there presence or absence of any decides the geometry and shape of the molecule. We can also say that these two are responsible for any variation, if observed, in the positioning of already existing atoms or lone pairs, as well as bond angles. This is because these two domains repel each other very strongly amongst and within itself.

The trend can be seen:

Since lone pairs cause the greatest amount of repulsion, any addition of these pushes the bond pairs away from it, as a result they become closer to each other and the bond angle decreases between the bonding domains and increases between bonding and non-bonding domain:

Similarly, if a bonding domain is added into the molecule it also causes repulsion (lot lesser than lone pairs). As a result they show similar changes in the positioning of domains and bond angles:

In fact, it also leads to the change in the geometry of the molecule. This variation in bond angles and geometry occurs in order to attain stability.

For example:

2 bonding domains with 0 non-bonding domain has geometry as LINEAR and a bond anlge of 180

Whereas on addition of a non-bonding domain, the geometry changes to BENT and bond angle decrease from 180 to 120

On the other hand, on addition of an extra boning domain, the geometry becomes Trigonal Planar and Bond angle, as in the previous case, 120.