Question

(a) What is the physical basis for the VSEPR model? (b) When applying the VSEPR model, we count a double or triple bond as a single electron domain. Why is this justified?

Short answer

(a) The physical basis of the VSEPR model is the repulsion between electron pairs (both bonding pairs and lone pairs) around a central atom in a molecule due to their negative charges. The model predicts the geometric shape of a molecule by minimizing this repulsion. (b) Double or triple bonds are counted as a single electron domain in VSEPR because, despite the presence of additional pi bonds, the electron density remains localized and concentrated in the same region between the two bonded atoms. This electron density effectively causes repulsion in that specific region and does not increase the overall repulsive effect in other directions.

Step-by-step solution

(a) Physical Basis for VSEPR Model

The VSEPR (Valence Shell Electron Pair Repulsion) model is based on the idea that the electron pairs (both bonding pairs and lone pairs) around a central atom in a molecule will repel each other and try to move as far apart as possible. This repulsion is a result of the electron pairs' negative charges. The VSEPR model aims to predict the geometric shape of the molecule by minimizing this repulsion between electron pairs.

(b1) Understanding Single and Multiple Bonds

A single bond consists of one sigma bond formed by the head-on overlap of atomic orbitals between two atoms. A double bond consists of one sigma bond and one pi bond formed by the sideways overlap of atomic orbitals, while a triple bond is made up of one sigma bond and two pi bonds. It's important to understand that while multiple bonds involve more than one bonding interaction, they still occur between just two atoms.

(b2) Justifying Counting Multiple Bonds as a Single Domain

In VSEPR model, the electron domain represents a region of electron density. In both single and multiple bonds, this electron density lies between the two atoms involved in bonding. Despite the presence of additional pi bonds in double or triple bonds, the electron density remains localized and concentrated in the same region between the two bonded atoms. We can consider this electron density as a single domain because it effectively causes repulsion only in that particular region and does not increase the overall repulsive effect in other directions. Hence, counting a double or triple bond as a single electron domain is justified in the VSEPR model.