Problem 75 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

(a) The physical basis for the VSEPR model is the repulsion between negatively charged valence shell electron pairs, which arrange themselves to maintain a maximum distance between them, determining the three-dimensional shape of molecules. (b) Treating double and triple bonds as a single electron domain in the VSEPR model is justified because they have similar spatial constraints and repulsive force effects upon molecular geometry as a single bond does, despite having additional electron pairs.

Step 1: (a) Explanation of VSEPR Model's Physical Basis

The physical basis for the VSEPR model lies in the fact that electron pairs in the valence shell of an atom will repel each other due to their negative charges. Electrons being negatively charged, they will repel each other in order to minimize electrostatic force or repulsion. As a result, these valence shell electron pairs will arrange themselves in such a way to maintain a maximum distance between them, leading to distinct molecular geometries. Thus, the VSEPR model is built upon the concept of repulsion between valence shell electron pairs, which determines the three-dimensional shape of molecules.

Step 2: (b) Justification for Treating Double and Triple Bonds as a Single Electron Domain

Although double and triple bonds involve more than one electron pair, they are still considered a single electron domain in the VSEPR model. There are two main reasons for this: 1. Molecular geometry is primarily influenced by repulsions between electron pairs, and since the electron pairs involved in bonding are usually closer to each other than to other nearby electron pairs, the repulsive force their interaction creates will tend to be more significant, so the actual repulsion due to other electron pairs in double or triple bonds becomes less critical. 2. In double or triple bonds, the additional electron pairs are part of the same bonding structure and are shared between the same two atoms, so they tend to be concentrated in the same region of space. This means that they effectively act as a single electron domain for the purposes of considering repulsions and molecular geometry determination. In summary, counting double or triple bonds as a single electron domain in the VSEPR model is justified because they have similar spatial constraints and repulsive force effects upon molecular geometry as a single bond does, despite having additional electron pairs.