Molecular Geometry The Valence Shell Electron Pair Repulsion Model

16 Molecular Geometry The Valence Shell Electron Pair Repulsion Model  

We can predict the arrangement of atoms in molecules and ions on the basis of a relatively simple idea called the valence shell electron pair repulsion (VSEPR) model. 

We consider molecules (or ions) in which the central atom is covalently bonded to two or more atoms or groups. 

We consider all of the valence electron pairs of the central atom—both those that are shared in covalent bonds, called bonding pairs, and those that are unshared, called nonbonding pairs or unshared pairs or lone pairs. 

Because electron pairs repel each other, the electron pairs of the valence shell tend to stay as far apart as possible. The repulsion between nonbonding pairs is generally greater than that between bonding pairs. 

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HOWTO PREDICT MOLECULAR GEOMETRY THE VALENCE SHELL ELECTRON PAIR REPULSION MODEL... 

Molecular geometry and the valence-shell electron pair repulsion model... 

Once a Lewis structure is drawn, you can determine the molecular geometry, or shape, of the molecule. The model used to determine the molecular shape is referred to as the Valence Shell Electron Pair Repulsion model, or VSEPR model. This model is based on an arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom. 

Like Charges Repel It is the repulsion of the electrons in covalent bonds of the valence shell of a molecule that is central to the valence shell electron pair repulsion model for explaining molecular geometry. And, although it is not so obvious, this same factor underlies the explanations of molecular geometry that come from orbital hybridization because these repulsions are taken into account in calculating the orientations of the hybrid orbitals. 

VSEPR The valence shell electron pair repulsion model, originally introduced by Nyholm and Gillespie (with antecedents from Sidgwick and Powell), which assumes that molecular geometry associated with a central atom is determined by the number of groups (single bonds, double bonds, triple bonds, or lone pairs) surrounding that atom. 

The major features of molecular geometry can be predicted on the basis of a quite simple principle—electron-pair repulsion. This principle is the essence of the valence-shell electron-pair repulsion (VSEPR) model, first suggested by N. V. Sidgwick and H. M. Powell in 1940. It was developed and expanded later by R. J. Gillespie and R. S. Nyholm. According to the VSEPR model, the valence electron pairs surrounding an atom repel one another. Consequently, the orbitals containing those electron pairs are oriented to be as far apart as possible. 

The molecular geometry of a complex depends on the coordination number, which is the number of ligand atoms bonded to the metal. The most common coordination number is 6, and almost all metal complexes with coordination number 6 adopt octahedral geometry. This preferred geometry can be traced to the valence shell electron pair repulsion (VSEPR) model Introduced In Chapter 9. The ligands space themselves around the metal as far apart as possible, to minimize electron-electron repulsion. 

This chapter reviews molecular geometry and the two main theories of bonding. The model used to determine molecular geometry is the VSEPR (Valence Shell Electron Pair Repulsion) model. There are two theories of bonding the valence bond theory, which is based on VSEPR theory, and molecular orbital theory. A much greater amount of the chapter is based on valence bond theory, which uses hybridized orbitals, since this is the primary model addressed on the AP test. 

We have previously (Chapter 8) discussed the valence-shell electron pair repulsion method as a predictive model of molecular geometry which. 

THE VSEPR MODEL We see how molecular geometries can be predicted using the valence-shell electron-pair repulsion, or VSEPR, model, which is based on Lewis structures and the repulsions between regions of high electron density. 

THE VSEPR MODEL (SECTION 9.2) The valence-shell electron-pair repulsion (VSEPR) model rationalizes molecular geometries based on the repulsions between electron domains, which are regions about a central atom in which electrons are likely to be found. Bonding pairs of electrons, which are those involved in making bonds, and nonbonding pairs of electrons, also called lone pairs, both create electron domains aroimd an atom. According to the VSEPR model, electron domains orient themselves to minimize electrostatic repulsions that is, they remain as far apart as possible. 

Molecular geometry is the general shape of a molecule as determined by the relative positions of the various atomic nuclei. A number of physical properties such as melting point, boiling point, density and a number of chemical properties are based on the molecular geometry. A very useful model to predict the general shape of a molecule was developed by Gillespie and Nyholm in 1957. The theory called the Valence Shell Electron Pair Repulsion (VSEPR pronounced as vesper) theory is an... 

In all of the above cases, the feometry is in afreement with the expectations of the valence shell electron pair repulsion (VSEPR) model of Gillespie and Nyholm [77]. The VSEPR model accounts for the molecular geometry in nearly all cases for main group elements in free molecules. Deviations are observed in solids because of the contribution from the lattice energy. For example, in SnO and red PbO, the coordination MO4E is square pyramidal, instead of seesaw expected by the VSEPR model [78,79]. 

Molecular Geometry Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. For relatively small molecules, in which the central atom contains two to six bonds, geometries can be rehably predicted by the valence-shell electron-pair repulsion (VSEPR) model. This model is based on the assumption that chemical bonds and lone pairs tend to remain as far apart as possible to minimize repulsion. 

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