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 for the VSEPR (Valence Shell Electron Pair Repulsion) model is the repulsion between negatively charged electron pairs in the valence shell of an atom. To minimize repulsion, electron pairs arrange themselves in specific geometries for different numbers of electron pairs, dictating the overall shape of the molecule. (b) Double or triple bonds are counted as a single electron domain in the VSEPR model because they behave similarly to a single bonded pair in terms of repulsion. The entire charge cloud of the multiple bond acts as a single unit that repels other electron domains, simplifying the model while still predicting molecular shapes accurately.

Step-by-step solution

Part (a): Physical Basis for the VSEPR Model

VSEPR (Valence Shell Electron Pair Repulsion) model is a simple model used to predict the shape of molecules based on the number of electron pairs around a central atom. The physical basis for the VSEPR model can be summarized in the following steps: Step 1: Understand electron pair repulsion Electron pairs in the valence shell of an atom repel each other because they are negatively charged. Since like charges repel, these electron pairs will try to stay as far away from each other as possible within that atom's valence shell to minimize repulsion. Step 2: Geometry of electron pair repulsion To minimize the repulsion, the electron pairs tend to arrange themselves in the most stable orientation. This is where each electron pair is as far apart as possible, resulting in specific geometries for different numbers of electron pairs. Step 3: Shape determination The arrangement of these electron pairs and their interactions with the nuclei of bonded atoms, dictate the overall shape of the molecule. The VSEPR model can predict the molecular geometries based on the number of electron pairs present around the central atom.

Part (b): Justifying Counting Double/Triple Bonds as Single Electron Domains

Step 1: Understand electron domains In the VSEPR model, an electron domain is a region where electrons are most likely to be found. It can be either a lone pair or a bonding pair of electrons. The total number of electron domains around the central atom determines the molecular shape. Step 2: Multiple bond electron domains A double bond is made of two electron pairs shared between two atoms, while a triple bond contains three electron pairs. In terms of electron cloud density, a double bond or triple bond has a higher density compared to a single bond as more electrons are present in the region. Step 3: Justification Even though multiple bonds have more than one electron pair, they are considered as a single electron domain in the VSEPR model because they behave similarly to a single bonded pair in terms of repulsion. The entire charge cloud of the multiple bond, despite containing more electrons, still acts as a single unit that repels other electron domains. This simplification enables the VSEPR model to predict molecular shapes accurately without the need to differentiate between single, double, or triple bonds as separate electron domains.