Valence bond theory Bonding polyatomic molecules

SECTION 9.5 To extend the ideas of valence-bond theory to polyatomic molecules, we must envision mixing s, p, and sometimes d orbitals to form hybrid orbitals. The process of hybridization leads to hybrid atomic orbitals that have a large lobe directed to overlap with orbitals on another atom to make a bond. Hybrid orbitals can also accommodate nonbonding pairs. A particular mode of hybridization can be associated with each of three common electron-domain geometries (linear = sp trigonal planar = sp -, tetrahedral = sp ). 

Valence Bond Theory for Polyatomic Molecules Requires the Use of Hybrid Orbitals 240... 

In the valence bond theory of polyatomic molecules, hybridized atomic orbitals are formed by the combination and rearrangement of orbitals from the same atom The hybridized orbitals are all of equal energy and electron density, and the number of hybridized orbitals is equal to the number of pure atomic orbitals that combine. 

Although the idea of orbital overlap allows us to understand the formation of covalent bonds, it is not always easy to extend these ideas to polyatomic molecules. When we apply valence-bond theory to polyatomic molecules, we must explain both the formation of electron-pair bonds and the observed geometries of the molecules. 

Due to the simplicity and the ability to explain the spectroscopic and excited state properties, the MO theory in addition to easy adaptability for modern computers has gained tremendous popularity among chemists. The concept of directed valence, based on the principle of maximum overlap and valence shell electron pair repulsion theory (VSEPR), has successfully explained the molecular geometries and bonding in polyatomic molecules. 

Valence-bond theory, as described so far, cannot account for bonding in polyatomic molecules like methane, CH4, nor for their bond angles. For example, if we tried to apply the theory to methane, we would note that... 

Polyatomic Molecules 7.15 Combining Valence Bond Theory... 

How does valence bond theory describe the electronic structure of a polyatomic molecule, and how does it account for molecular shape Let s look, for example, at a simple tetrahedral molecule such as methane, CH4. There are several problems to be dealt with. 

If we prefer to describe the bonding of a polyatomic molecule using localized two-center, two-electron (2c-2e) bonds, we can turn to the hybridization theory, which is an integral part of the valence bond method. In this model, for AX systems, we linearly combine the atomic orbitals on atom A in such a way that the resultant combinations (called hybrid orbitals) point toward the X atoms. For our BeH2 molecule in hand, two equivalent, colinear hybrid orbitals are constructed from the 2s and 2pz orbitals on Be, which can overlap with the two Is hydrogen orbitals to form two Be-H single bonds. (The 2p and 2py... 

Two basic methods, the valence-bond (VB) and the molecular orbital (MO) method, have been developed for the determination of approximate state functions. In practice, the MO method constitutes the simplest and most efficient approach for the treatment of polyatomic molecules. And, in fact, all the calculations for the systems under consideration have been carried out within the framework of the MO theory. 

The electrons in the outer shell, or valence shell, of an atom are the electrons involved in bonding. In most of our discussion of covalent bonding, we will focus attention on these electrons. Valence shell electrons are those that were not present in the preceding noble gas, ignoring filled sets of d and / orbitals. Lewis formulas show the number of valence shell electrons in a polyatomic molecule or ion (Sections 7-4 through 7-7). We will write Lewis formulas for each molecule or polyatomic ion we discuss. The theories introduced in this chapter apply equally well to polyatomic molecules and to ions. 

This shows no unpaired electrons, so it predicts that O2 is diamagnetic. Experiments show, however, that O2 is paramagnetic therefore, it has unpaired electrons. Thus, the valence bond structure is inconsistent with experiment and cannot be accepted as a description of the bonding. Molecular orbital theory accounts for the fact that O2 has two unpaired electrons. This ability of MO theory to explain the paramagnetism of O2 gave it credibility as a major theory of bonding. We shall develop some of the ideas of MO theory and apply them to some molecules and polyatomic ions. 

Since the valence bond theory is insufficient to explain the structure and behavior of polyatomic molecules, the molecular orbital theory was developed. In this theory, it is accepted that electrons in a polyatomic molecule should not be regarded as belonging to particular bonds but should be treated as spreading throughout the entire molecule every electron contributes to the strength of every bond. A molecular orbital is considered to be a linear combination of all the atomic orbitals of all the atoms in the molecule. Quantum... 

The molecular orbital theory is one of the two approximate theories which have been used to investigate the electronic structures of atoms and molecules. This theory, like its counterpart, the valence bond theory, was advanced very soon after the advent of wave mechanics twenty-five years ago, but it is only in recent years that the molecular orbital theory has come into general use for describing not only the ground states, but also the excited states of polyatomic molecules.7 ... 

Covalent bonds in polyatomic molecules and ions are formed by the overlap of hybrid orbitals, or of hybrid orbitals with unhybridized ones. Therefore, the hybridization bonding scheme is still within the framework of valence bond theory electrons in a molecule are assumed to occupy hybrid orbitals of the individual atoms. 

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