(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?

The Valence Shell Electron Pair Repulsion (VSEPR) model is based on the idea that electron pairs in the valence shell of an atom repel one another, thereby arranging themselves in space so that their repulsion is minimized. This is because electron pairs are negatively charged, and thus they experience repulsive forces when in close proximity to each other. To predict the geometry of molecules, the VSEPR model considers the arrangement of valence electrons around the central atom in a molecule. The regions where electrons are most likely found are referred to as electron domains, which can be bonding electron pairs (shared between atoms) or lone pairs (unshared electrons). The electron domains are arranged in space to minimize the repulsive forces between them, determining the molecular geometry.

In the VSEPR model, we treat double and triple bonds as single electron domains because they occupy roughly the same space as single bonds. This is due to the nature of the overlapping orbitals in double and triple bonds. In a single bond, one sigma (σ) bond is formed by the end-to-end overlap of orbitals. In a double bond, there is one σ bond and one pi (π) bond, which is formed by the side-by-side overlap of parallel orbitals. Similarly, in a triple bond, there is one σ bond and two π bonds. The important aspect to consider for the VSEPR model is the spatial arrangement of electron domains. In double and triple bonds, the additional π bonds are formed by parallel orbitals, which don't significantly affect the shape or size of the electron domain compared to a single bond. Therefore, treating double and triple bonds as single electron domains in the VSEPR model is justified because it simplifies geometry predictions without sacrificing accuracy.