Covalent bonds basic theory

In this chapter, the basic types of chemical bonds existing in condensed phases are discussed. These interactions include ionic bonds, metallic bonds, covalent bonding (band theory), and intermolecular forces. In Chapter 10, the structures of some inorganic crystalline materials will be presented. 

This discrepancy between experiment and theory (and many others) can be explained in terms of an alternative model of covalent bonding, the molecular orbital (MO) approach. Molecular orbital theory treats bonds in terms of orbitals characteristic of the molecule as a whole. To apply this approach, we carry out three basic operations. 

Covalent bonds can be described with a variety of models, virtually all of which involve symmetry considerations. As a means of illustrating the role of symmetry in bonding theory and laying some foundation for discussions to follow, this section will show the application of symmetry principles in the construction of hybrid orhitals. Since you will have encountered hybridization before now, hut perhaps not in a symmetry context, this provides a ladle introduction to the application of symmetry. You should remember that the basic procedure outlined here (combining appropriate atomic orbitals to make new orbitals) is applicable also to the derivation of molecular orbitals and ligand group orhitals, both of which will be encountered in subsequent chapters. 

To introduce some of the basic ideas of molecular orbital theory, let s look again at orbitals. The concept of an orbital derives from the quantum mechanical wave equation, in which the square of the wave function gives the probability of finding an electron within a given region of space. The kinds of orbitals that we ve been concerned with up to this point are called atomic orbitals because they are characteristic of individual atoms. Atomic orbitals on the same atom can combine to form hybrids, and atomic orbitals on different atoms can overlap to form covalent bonds, but the orbitals and the electrons in them remain localized on specific atoms. 

Oxidic surfaces in particular develop acid or basic properties which are important in catalysis. We will approach this subject first by taking as a starting point the ionic bond model [2]. The lattice is considered to consist of cations and anions held together by electrostatic interactions. Later we will discuss a more balanced theory that also accounts for covalent bonding aspects. 

Determination of macromolecules conformations is one of the basic problems of science about polymers. Simultaneously with development of theory [4-6] the perfection and enrichment of experimental methods of determination of macromolecules conformations in various phase and aggregate states occurs. However the method of neutron scattering was almost the only one method allowing reliable determination of polymer chains conformation in solid amorphous state until now [7], Not long ago they begun to use with this aim also the method based on measurement of rate of electron excitement transfer between molecules of chromophores covalent bonded with polymer chain [8],... 

In all five of these hybrid orbital schemes, the use of hybridisation is only to give an improved directional overlap of orbitals to form two electron pair covalent bonds. Hybridisation does not determine the basic stereochemistry. This must still be determined by VSEPR theory and only then can hybridisation schemes be invoked to describe, more effectively, the covalent bonding present. These hybridisation schemes may equally be applied to cations and anions. The NH4 cation and BF4" anion have already been shown to involve a tetrahedral stereochemistry (Figure 6.4, examples 3 and 4) consequently the bonding in both ions may be described as involving sp hybridisation. 

Bond - Coordinate Bond . Maybe it doesn t exactly roll off the tongue, but it s hard to avoid this adaptation of the personal introduction used by perhaps our best-known and most enduring screen spy to introduce this section - it serves its purpose to remind us of the endurance and strength of bonds between metals and ligands, which at a basic level we can consider as a covalent bond. Moreover, it isn t just any bond, but a specially-constructed coordinate bond - hence the name of this field, coordination chemistry. Unfortunately, the simple covalent bonding concept does not provide valid interpretations for all of the physical properties of coordination complexes, and more sophisticated theories are required. We shall examine a number of bonding models for coordination complexes in this chapter. 

The basic principle of VB theory is that a covalent bond forms when orbitals of two atoms overlap and the overlap region, which is between the nuclei, is occupied by a pair of electrons. ( Orbital overlap is another way of saying that the two wave functions are in phase, so the amplitude increases between the nuclei.) The central themes of VB theory derive from this principle ... 

The chapters on the quantum mechanics of simple systems have been retained with only minor revisions, while the chapter on the covalent bond has been extended to include a description of molecular energy levels. The basic ideas of group theory are introduced here and illustrated by constructing symmetry-adapted molecular orbitals for simple molecules. There is a new chapter on atomic spectroscopy the chapter on molecular spectroscopy has been expanded and reorganized. 

The basic tenet of covalent bonding between atoms in molecules is called the valence bond theory. The basic idea is that covalent bonds are formed by the overlap of atomic orbitals of different atoms. The two electrons are involved in a paired spin that is shared by an atomic orbital of each of the two involved atoms. The more overlap you see, the stronger will be the covalent bond between the atoms. The atomic orbitals can be the same as the original orbitals of each atom however, the bond can t happen if the original orbitals are maintained. The orbitals must come together into a new configuration, called reconfigured orbitals or hybridized orbitals . 

The gap Eq is not the minimum gap Eg which determines the absorption edge and temperature dependence of carrier concentration, but the average gap corresponding to the separation of the bonding and antibonding states in the covalent bond picture. It has fundamental importance for many basic properties of the solid and appears in the theories of the relation between the chemical and optical properties of crystals (Phillips (1970)). 

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