Valence Bond VB Theory and Orbital Hybridization

What is a covalent bond, and what characteristic gives it strength And how can we explain molecular shapes based on the interactions of atomic orbitals The most useful approach for answering these questions is valence bond (VB) theory. 

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Valence Bond (VB) Theory and Orbital Hybridization 329 The Central Themes of VB Theory 329... 

Predict whether a molecule is polar or nonpolar Describe the basic ideas of the valence bond (VB) theory Describe the hybrid orbitals used in bonding in polyatomic molecules and ions... 

Using Valence Bond (VB) theory, the central atoms of the molecules with formulas AB2U2 and AB3U should undergo sp3 hybridized with predicted bond angles of 109.5°. If no hybridization occurs, bonds would be formed by the use of p orbitals. Since the p orbitals are oriented at 90° from each other, the bond angles would be 90°. Note that hybridization is only invoked if the actual molecular geometry data indicate that it is necessary. 

Theory is a term that is very widely used by chemists. To take the area of chemical bonding as an example, chemists widely refer to molecular orbital (hereafter MO) theory, valence bond (VB) theory, hybridization theory, valence shell electron pair repulsion (VSEPR) theory, and ligand field theory. And even those probably do not exhaust the list. 

Two theories go hand in hand in a discussion of covalent bonding. The valence shell electron pair repulsion (VSEPR) theory helps us to understand and predict the spatial arrangement of atoms in a polyatomic molecule or ion. It does not, however, explain hoav bonding occurs, ] ist where it occurs and where unshared pairs of valence shell electrons are directed. The valence bond (VB) theory describes how the bonding takes place, in terms of overlapping atomic orbitals. In this theory, the atomic orbitals discussed in Chapter 5 are often mixed, or hybridized, to form new orbitals with different spatial orientations. Used together, these two simple ideas enable us to understand the bonding, molecular shapes, and properties of a wide variety of polyatomic molecules and ions. 

In addition to the absolutely small size of the 2p-shell, the relative similarity of its radial extent to that of the 2s-orbital (see earlier text) is crucial for understanding the differences between the 2p-elements and their heavier congeners. Covalent bonding is often discussed using the tools of valence-bond (VB) theory (see Chapter 5 in Volume 1), and hybridization is a main requirement of VB models. Kutzelnigg, in... 

Molecular orbital (MO) theory is based on the idea that electrons are standing waves that add either constructively (to form bonding orbitals) or destructively (to form anti-bonding orbitals). Valence bond (VB) theory is an orbital-based model of bonding that includes the concept of hybridization and focuses on the bonds formed by the valence electrons of different atoms. 

Despite the quantitative victory of molecular orbital (MO) theory, much of our qualitative understanding of electronic structure is still couched in terms of local bonds and lone pairs, that are key conceptual elements of the valence bond (VB) picture. VB theory is essentially the quantum chemical formulation of the Lewis concept of the chemical bond [1,2]. Thus, a chemical bond involves spin-pairing of electrons which occupy valence atomic orbitals or hybrids of adjacent atoms that are bonded in the Lewis structure. In this manner, each term of a VB wave function corresponds to a specific chemical structure, and the isomorphism of the theoretical elements with the chemical elements creates an intimate relationship between the abstract theory and the nature of the... 

Hybridization means the mixing of atom orbitals of different types of the given atom in one molecular (or atom) orbital. Hybridization principles are well-developed in accordance with the experimental data in the frames of general theories of valence bond (VB) and molecular orbitals (MO). 

These simple ideas were formulated before the advent of wave mechanics. Quantum theory not only justifies their use but enables us to refine and extend them. In attempting quantitative quantum-mechanical treatment of chemical bonds, approximations must be made. Traditionally, there have been two broad groups of approximations, called the valence bond (VB) and the molecular orbital (MO) treatments. The former is essentially a direct attempt to invest the qualitative ideas just outlined with quantum-mechanical validity, and it is therefore logical to continue the discussion with a summary of the valence-bond formalism, including such concepts as resonance, valence states and hybridization that arise within this framework. The molecular-orbital formalism will be presented in a following Section. 

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