The Nature of Covalence

The atoms of most molecules are held tightly together by a very important sort of bond, the shared-electron-pair bond or covalent bond.This bond is so important, so nearly universally present in substances that Professor Gilbert Newton Lewis of the University of California (1875-1946), who discovered its electronic structure, called it the chemical bond. 

It is the covalent bond that is represented by a dash in the valence-bond 

Modern chemistry has been greatly simplified through the development of the theory of the covalent bond. It is now easier to understand and to remember chemical facts —by connecting them with our knowledge of the nature of the chemical bond and the electronic structure of molecules — than was possible fifty years ago. It is accordingly wise for the student of chemistry to study this chapter carefully, and to get a clear picture of the covalent bond. 

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Topological analysis of the total density has a considerable advantage over the use of the deformation densities in that it is reference-density independent. There is no need to define hybridized atoms to analyze the nature of covalent bonding, and the ambiguity when using the standard deformation density, noted above in the discussion on propellanes, does not occur. 

Much progress has been made in the development of a detailed understanding of the nature of covalent bonds through the consideration of the atomic orbitals (bond orbitals) that can be used as the basis of the quantum-mechanical treatment of the bonds. 

However, beyond this overt similarity, there are differences. For example, Covalon by the nature of covalency would have to operate under a much more stringent correlation than that existing in the Frohlich s model between one paired n-electron and all other such pairs along the chain. This is a natural consequence of distortion in the alternating single double bonds. This treatment also differs from that of self-consistent field treatment [19] of a linear chain and that of Little [20] in our inclusion of bond vibration. Covalon also differs from polaron treatments [21] in the consideration of the movement of spin-paired correlated electrons in a covalent bond, instead of movement of spin-uncorrelated electrons in the zeroth order. 

For his work on coordination chemistry and stereochemistry, Werner became the fourteenth Nobel Prize winner in chemistry and the first Swiss chemist to be so honored. Werner s work is even more remarkable when one realizes that his ideas preceded any real understanding of the nature of covalent bonds by many years. ... 

Chemists have discovered that part of the reason the small difference in structure leads to large differences in properties lies in the nature of covalent bonds and the arrangement of those bonds in space. This chapter provides a model for explaining how covalent bonds form, teaches you how to describe the resulting molecules with Lewis structures, and shows how Lewis structures can be used to predict the three-dimensional geometric arrangement of atoms in molecules. 

We will first present the EDA results for the dihydrogen molecule which is the standard molecule in curricula and textbooks for discussing the nature of covalent bonding. Then we compare the results for H2 with heavier diatomic molecules N2 and isoelectronic CO... 

In the previous chapter, we have discussed different ways of thinking about the nature of covalent bonding in molecules. In most of the examples we have encountered so far, the molecules existed in the gas phase, so we only needed to focus our attention on the interactions of atoms in molecules. On the other hand, most metals exist in the solid state at room temperature. In the ensuing discussion of metallic bonding, it Is therefore essential that we consider the arrangement of the metal atoms with respect to one another within the crystalline lattice. 

What can be said about the nature of covalent bonds between (a) 2 atoms with almost identical electronegativity values and (b) 2 atoms with substantially different electronegativity values ... 

Our first goal in this chapter is to learn the relationship between two-dimensional Lewis structures and three-dimensional molecular shapes. Armed with this knowledge, we can then examine more closely the nature of covalent bonds. The lines that are used to depict bonds in Lewis structures provide important clues about the orbitals that molecules use in bonding. By examining these orbitals, we can gain a greater understanding of the behavior of molecules. You will find that the material in this chapter will help you in later discussions of the physical and chemical properties of substances. 

Double bonds, as we have seen, severely restrict the freedom of motion of the polymer chain. In actuality the nature of covalent... 

Techniques such as Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) are used in determining information about the chemical structure of the monomers and the nature of covalent bonds between them. Molecular mass and molecular mass distribution, as well as chemical nature of side groups, determine the interaction between polymer chains. Where interactions between chains lead to ordered regions, crystalline phases are observed, whilst other less ordered regions are said to be amorphous. X-ray diffraction is often used to assess structural information, such as the degree of crystallinity and specific crystal structures while microscopy techniques, such as SEM or TEM, are used to determine morphology. 

The physical forces described above aptly account for most molecular interactions in the gas phase. We now direct our discussion toward the condensed phases. Sohds and liquids form when the net attractive intermolecular forces are stronger than the thermal energy in the system and, consequently, hold the molecules together. While the force of attraction can sometimes be attributed to the electrostatic and van der Waals interactions described above, chemical forces also frequently play a role in condensed phases. Chemical forces are based on the nature of covalent electrons, the concept of the chemical bond, and the formation of new chemical species. The main difference between chemical and physical forces is that chemical forces saturate whereas physical forces do not, since chemical interactions are specific to the electronic wavefunc-tions of the chemical species involved. Indeed, a complete quantitative description of chemical interactions involves solution of the Schrodinger equation to describe the overlap of the molecular orbitals involved. We will consider chemical interactions only qualitatively. The goal of this discussion is to realize that there may be other important forces that govern the behavior of solids and liquids and to get a flavor of what these forces might be. 

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