VSEPR theory defined

The VSEPR theory starts with the fundamental idea that electron pairs in the valence shell of an atom repel each other. These include both lone pairs, which are localized on the atom and are not involved in bonding, and bonding pairs, which are shared covalently with other atoms. The electron pairs position themselves as far apart as possible to minimize their repulsions. The molecular geometry, which is defined by the positions of the nuclei, is then traced from the relative locations of the electron pairs. 

The anion can be described as a slightly distorted Archimedean antiprism with Xe—F distances ranging from 1.946 to 2.099 A. The eight-fold xenon coordination provides no clearly defined position for the lone pair, thus seemingly being in violation of the VSEPR theory. 

A provisional Lewis structure may contain the correct bonding framework, but the distribution of the valence electrons may not be the one that gives the maximum stability. The correct stereochemistry is predicted by the valence shell configuration using VSEPR theory, as shown in Chapter 6. A concept called formal charge (FC) can be used to predict which structure of a number of alternative structures is the most reasonable for a Lewis structure. The formal charge (FC) on any atom in a Lewis structure can be defined as ... 

Next we will consider molecules that have both bonded and nonbonded pairs of electrons in the valence shell of the central atom. Water and ammonia have four electron pairs around the central atom. Some of the electron pairs in water and ammonia are bonded to hydrogen atoms, but the central atom also has unshared electron pairs. VSEPR theory describes the distribution of bonded and nonbonded electron pairs. However, molecular structure is defined by the positions of the nuclei. The four pairs of electrons in both water and ammonia are tetrahedrally arranged around the central atom. Water, with only three atoms, is angular, and ammonia, with four atoms, is pyramidal (Figure 1.7). 

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