12 A Textbook of Inorganic Chemistry – Volume I The basic proposal of Sidgwick and Powell was that all the electron-clouds in the outermost shell of an atom (valence shell) must be taken into consideration before any geometry profiling is carried out. In other words, all electron pairs of the valence shell, whether they participate in bonding or not (lone pair as well as bond pairs), have their space requirement; and therefore govern the bonding profile around the central atom.
The initial version of the VESPR model also postulated that electron pairs in a Lewis description of a molecule can be represented by points which are arranged on the surface of a hypothetical sphere as far apart as possible. However, later it was thought that the more realistic representation of valence shell electron-pair is a negatively charged cloud which is comprised of two opposite-spin electrons; and this cloud is trying to occupy as much space as possible while eliminating its other counterparts from this space.
Therefore, in addition to the “points on the sphere” model; an alternative model can also be given which is based on the different numbers of circles of equal radii arranged in such a way that they occupy maximum possible surface of a sphere without any mutual overlap; though both of these models lead to the same geometrical profile. Figure 2. The points-on-the-sphere model of molecular geometries. A third version, the tangent-sphere model, was also developed by Kimball and Bent that considers all electron-pairs as the spherical entities of the same size. These spherical domains are expected to get packed around the central atom as efficiently as possible.
Sidgwick and Powell, in 1940, proposed these most primitive coordination profiles of two to six electron pair domains which are actually fundamental to the VSEPR model and they set the stage for the prediction of the molecular geometries. Now although these predictions explained a wide range of molecular geometries, the distortion from these ideal structures was still a challenge to solve. The most decisive step towards the development of modern VSEPR theory was made Gillespie and Nyholm in 1957 when they published their revolutionary paper entitled “Inorganic Stereochemistry”. They treated bond pair and lone pair distinctly and incorporated the necessary allowances.
They not only coined the term “VSPER theory”, but also worked a lot to popularize the same. Buy the complete book with TOC navigation, high resolution images and no watermark. Copyright © Mandeep Dalal