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 (Valence Shell Electron Pair Repulsion) Model is the electrostatic repulsion between negatively charged electron pairs in the valence shell of the central atom. By minimizing these repulsions, the electron pairs, and atoms arrange themselves to form a stable molecular geometry. (b) Double or triple bonds are counted as single electron domains in the VSEPR model because their electron pairs from pi bonds are more delocalized and have less repulsion compared to sigma bonds or lone pairs. This simplification allows for more accurate prediction of molecular geometries by focusing on the most significant repulsions.

Step-by-step solution

(a) Physical Basis of VSEPR Model

Valence Shell Electron Pair Repulsion (VSEPR) Model is based on the idea that electron groups (lone pairs, single, double, and triple bonds) surrounding a central atom in a molecule repel one another. This repulsion causes these electron groups to arrange themselves as far apart as possible to minimize the repulsion energy. The physical basis of VSEPR is electrostatic repulsion between negatively charged electron pairs in the valence shell of the central atom. By minimizing these repulsions, the arrangement of electron pairs and atoms around the central atom can result in a stable molecular geometry.

(b) Justification for Counting Double or Triple Bonds as Single Electron Domains

The main reason for treating double or triple bonds as single electron domains in the VSEPR model is the concept of electron domain localization. In a double bond, there are two electron pairs shared between two atoms – one of them is a sigma bond and the other is a pi bond. Similarly, in a triple bond, there are three electron pairs shared - one sigma bond and two pi bonds. Although double and triple bonds consist of multiple bonding pairs, these electron pairs belonging to pi bonds are more delocalized and spread-out over the bonding region, compared to sigma bonds. As a result, their repulsion with other electron groups is relatively less compared to sigma bonds or lone pairs. Considering a double or triple bond as a single electron domain simplifies the VSEPR model and allows for more accurate prediction of molecular geometries by focusing on the most significant repulsions: those between sigma bonds and lone pairs. This approach has been found to be quite effective in predicting the molecular structures in most cases, making the counting of double or triple bonds as single electron domains a justified simplification in the VSEPR model.