Electron pairs arrangement

Table 4.2 Electron Pair Arrangements and the Geometry of AX Em Molecules3...

Pairs of electrons in the valence shell repel each other and therefore stay as far apart as possible. For example, if four pairs of electrons are arranged around a central atom so that they are as far apart as possible, they will be located at the corners of a tetrahedron.The geometries resulting from other numbers of electron pairs, arranged as far apart as possible, are given in Figure l.l I. 

Electron Pairs Electron-Pair Arrangement Geometry Bond Angles... 

The shapes of most molecules can be predicted using the six models for electron pair arrangements. What you need to do for each one is ... 

Determine the number of electron pairs on the central atom—this determines the electron pair arrangements. 

In a molecule in which the central atom exhibits sp3d hybrid orbitals, the electron pair arrangements form the shape of a(n)... 

There are five main structures on which almost all molecules are based. These are related to the five geometries associated with electron pair arrangements, which are linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral. 

This simple method of deducing the structure of molecules is called Valence Shell Electron Pair Repulsion Theory (VSEPRT). It says that all electron pairs, both bonding and nonbonding, in the outer or valence shell of an atom repel each other. This simple approach predicts (more or less) the correct structures for methane, ammonia, and water with four electron pairs arranged Lctrahedrally in each case. 

In what directions can five electron pairs arrange themselves in space so as to minimize their mutual repulsions In the cases of coordination numbers 2, 3, 4, and 6, we could imagine that the electron pairs distributed themselves as far apart as possible on the surface of a sphere the resulting shapes correspond to the regular polyhedron whose number of vertices is equal to the coordination number. 

Now that we have obtained the electron-pair arrangement that gives the least repulsions, we can determine the positions of the atoms and thus the molecular structure of CH4. In methane each of the four electron pairs is shared between the carbon atom and a hydrogen atom. Thus the hydrogen atoms are placed as shown in Fig. 13.14, giving the molecule a tetrahedral structure with the carbon atom at the center. 

Possible electron-pair arrangement for XeF4. Since arrangement (a) has lone pairs 90 degrees apart, it is less favorable than anangement (b), where the lone pairs are 180 degrees apart. 

The electron pairs around the central atom of the molecule arrange themselves to minimize electronic repulsion. This means that the electron pairs arrange themselves so that they can be as far as possible from each other. We may use this fact to predict molecular shape. This approach is termed the nalence shell electron pair repulsion (VSEPR) theory. 

An sp d hybridization indicates that the electron-pair arrangement about iodine is trigonal bipyramidal. If four fluorines are placed around iodine, the total number of valence electrons is 35. Only 34 electrons are required to complete a trigonal bipyramidal electron-pair arrangement with four bonds and one lone pair of electrons. Taking one valence electron away gives the cation, IF4. ... 

Since there are three electron pairs around N, the electron-pair arrangement that minimizes electron-pair repulsion is trigonal planar. 

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