Stereochemical arrangement has an issue whenever an atom is bonded to two or more others. Thus triatomic substance AB2 can be bent or linear. It is observed that when a central atom has no nonbonding electrons, the surrounding atoms are generally arranged in a regular way that spaces them as far apart as possible. When nonbonding electron pairs are present in the valence structure, however, less regular arrangements of bonds are found. The valence shell electron pair repulsion (VSEPR) model is based on the idea that both nonbonding and bonding electron pairs in the valence shell of an atom 'repel' each other. This idea is useful but can be misleading if taken literally. Detailed calculations give that the shape of a molecule is determined by a combination of factors, of which the electrostatic repulsion between electrons is not the most important. Furthermore, the real electron distribution in a molecule is much more evenly spread out than the localized pictures used in VSEPR (1, 2, ...) suggest. It is best to think of 'repulsion' as coming primarily from the exclusion principle, which forces electron pairs to contain orbitals in different regions of space.
The basic principles of the model are shown as.
(i) Valence electron pairs round an atom (whether bonding or nonbonding) acquire a geometry that maximizes the distance between them. The basic geometries generally observed with 2-7 pairs are shown in Fig.
(ii) Nonbonding electron pairs are closer to the central atom than bonding pairs and have bigger repulsions: in fact, the order of interactions is
(iii) If double (or triple) bonds are present the four (or six) electrons involved behave as if they were a single pair, although they exert more repulsion than do the two electrons of a single bond
(iv) As the terminal atoms convert more electronegative relative to the central one, bonding electron pairs are drawn away from the central atom and so repel less.