The Covalent Chemical Bond A Model

Molten NaCI conducts an electric current, indicating the presence of mobile Na+ and CF ions. 

We will avoid these problems by adopting an operational definition of ionic compounds Any compound that conducts an electric current when melted will be classified as ionic. Also, the generic term salt will be used interchangeably with ionic compound in this book. 

Before we develop specific models for covalent chemical bonding, it will be helpful to summarize some of the concepts introduced in this chapter. 

What is a chemical bond Chemical bonds can be viewed as forces that cause a group of atoms to behave as a unit. 

We find it useful to interpret molecular stability in terms of a model called a chemical bond. To help understand why this model was invented, let s continue with methane, which consists of four hydrogen atoms arranged at the corners of a tetrahedron around a carbon atom  

Given this structure, it is natural to envision four individual C—H interactions (we call them bonds). The energy of stabilization of CH4 is divided equally among the four bonds to give an average C—H bond energy per mole of C—H bonds  

consider methyl chloride, which consists of CH3CI molecules having the structure 

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To provide a more quantitative explanation of the magnitudes of the properties of different materials, we must consider several types of intermolecular forces in greater detail than we gave to the Lennard-Jones model potential in Chapter 9. The Lennard-Jones potential describes net repulsive and attractive forces between molecules, but it does not show the origins of these forces. We discuss other intermolecular forces in the following paragraphs and show how they arise from molecular structure. Intermolecular forces are distinguished from intramolecular forces, which lead to the covalent chemical bonds discussed in Chapters 3 and 6. Intramolecular forces between atoms in the covalent bond establish and maintain... 

Empirical Valence Bond Methods. - To examine some important questions relating to enzyme action (e.g. to analyse the causes of catalysis, i.e. why an enzymic reaction proceeds faster than the equivalent, uncatalysed reaction in solution), it is necessary to use a method that not only captures the essential details of the chemical reaction, but also includes the explicit effects of the enzyme and solvent enviroment. One notable method in this area is the empirical valence bond (EVB) model.143 In the empirical valence bond approach, resonance structures (for example ionic and covalent resonance forms)... 

Both the modulus-temperature relationships presented in the preceding sections and the tensile data presented above are strikingly similar to those demonstrated for other rubber-plastic combinations, such as the thermoplastic elastomers (see Chapter 4 and the model system presented in Section 10.13) and the impact-resistant plastics (Chapter 3). The IPN s constitute another example of the simple requirement of needing only a hard or plastic phase sufficiently finely dispersed in an elastomer to yield significant reinforcement. Direct covalent chemical bonds between the phases are few in number in both the model system (Section 10.13) and present IPN materials. Also, as indicated in Chapter 10, finely divided carbon black and silicas greatly toughen elastomers, sometimes without the development of many covalent bonds between the polymer and the filler. 

In contrast to the Lewis model, in which a covalent chemical bond is the sharing of electrons represented by dots, in valence bond theory a chemical bond is the overlap of half-filled atomic orbitals (or in some cases the overlap between a completely filled orbital and an empty one). 

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