Describing Covalent Bonds

When atoms of hydrogen are adsorbed on a metal surface, the changes in potential energy may be schematically represented by a single curve as shown in Fig. 8. Our present knowledge of the forces responsible for the formation of the covalent bond described in Sec. V,8,b does not enable... 

The valence-bond model for covalent bonds, described below, has its limitations, but it is still extremely useful. For example, you will see in Chapter 14 that it helps us understand the attractions between molecules and predict relative melting points and boiling points of substances. The model is also extremely useful in describing the mechanisms of chemical changes. Therefore, even though it strays a bit from what scientists think is the most accurate description of real molecules, the valence-bond model is the most popular model for explaining covalent bonding. 

Covalent bonding describes well the bonding in small molecules. Explain how covalent bonding leads naturally to the concept of definite molecular geometry. [Note answer is not provided at the end of this book.]... 

There are two distinct types of coordination compounds separated from one another by their reactivity, which is due to the nature of the bonding from the metal to the ligand. Complex compounds are bound by coordinate covalent bonds described by valence bond theory (see Chapter 6). Addition salts or double salts, however, are bound according to electrostatic interactions, or ionic bonds. (Turn to Chapter 8 for details on ionic bonds and salts.)... 

What is the covalent bond Describe with reference to the molecules H2 and CH4. 

Ionic bonding and covalent bonding are treated and described separately, but in practice there are many examples of substances with bonding that has both ionic and covalent character. The simple and extreme models of ionic and covalent bonding described in this chapter ate very useful ill describing the structures and properties of compounds. 

In valence bond theory a covalent bond is described m terms of m phase overlap of a half filled orbital of one atom with a half filled orbital of another When applied to bonding m H2 the orbitals involved are the Is orbitals of two hydrogen atoms and the bond is a ct bond... 

The absence of an electron from a covalent bond leaves a hole and the neighboring valence electron can vacate its covalent bond to fill the hole, thereby creating a hole in a new location. The new hole can, in turn, be filled by a valence electron from another covalent bond, and so on. Hence, a mechanism is estabUshed for electrical conduction that involves the motion of valence electrons but not free electrons. Although a hole is a conceptual artifact, it can be described as a concrete physical entity to keep track of the motion of the valence electrons. Because holes and electrons move in opposite directions under the influence of an electric field, a hole has the same magnitude of charge as an electron but is opposite in sign. 

The efficiency of inactivation by covalent bond formation vs release of the reactive species into solution has been described by its partition ratio. The most efficient inactivators have catalytic partition ratios of 0, in which case each inhibitor molecule leads to inactivation of the enzyme. To this date, many of these inhibitors have been designed, and alternative names like suicide substrate, Trojan Horse inactivator, enzyme induced inactivator, inhibitor, and latent inactivator have been used for this class of inhibitors. A number of comprehensive reviews are available (26—32). 

The simplest example of covalent bonding is the hydrogen molecule. The proximity of the two nuclei creates a new electron orbital, shared by the two atoms, into which the two electrons go (Fig. 4.5). This sharing of electrons leads to a reduction in energy, and a stable bond, as Fig. 4.6 shows. The energy of a covalent bond is well described by the empirical equation... 

The metallic bond, as the name says, is the dominant (though not the only) bond in metals and their alloys. In a solid (or, for that matter, a liquid) metal, the highest energy electrons tend to leave the parent atoms (which become ions) and combine to form a sea of freely wandering electrons, not attached to any ion in particular (Fig. 4.8). This gives an energy curve that is very similar to that for covalent bonding it is well described by eqn. (4.4) and has a shape like that of Fig. 4.6. 

We will first examine the simple structures given by ionic and covalent bonding, and then return to describe the microstructures of ceramics. 

The unequal distribution of electron density in covalent bonds produces a bond dipole, the magnitude of which is expressed by the dipole moment, having the units of charge times distance. Bonds with significant bond dipoles are described as being polar. The bond and group dipole moments of some typical substituents are shown in Table 1.7. 

The stereochemistry of the most fundamental reaction types such as addition, substitution, and elimination are described by terms which specify the stereochemical relationship between the reactants and products. Addition and elimination reactions are classified as syn or anti, depending on whether the covalent bonds which are made or broken are on the same face or opposite faces of the plane of the double bond. 

Acid-base interactions in the most general Lewis sense occur whenever an electron pair from one of the participants is shared in the formation of a complex, or an adduct . They include hydrogen bonding as one type of such a bond. The bond may vary from an ionic interaction in one extreme to a covalent bond in the other. Acid-base interactions and their importance in interfacial phenomena have been reviewed extensively elsewhere [35,78] and will be described only briefly here. 

Catalysis occurs because the catalyst in some manner increases the probability of reaction. This may result from the reactants being brought closer together [catalysis by approximation, or the propinquity effect ], or somehow assisted to achieve the necessary relative orientation for reaction. Noncovalent interactions may be responsible for the effect. Covalent bond changes may also take place in catalysis. In a formal way, the manner in which catalysis occurs can be described by schemes such as Schemes I and II. 

How does electron sharing lead to bonding between atoms Two models have been developed to describe covalent bonding valence bond theory and molecular orbital theory. Each model has its strengths and weaknesses, and chemists tend... 

The valence-bond concept of orbital hybridization described in the previous four sections is not limited to carbon compounds. Covalent bonds formed by-other elements can also be described using hybrid orbitals. Look, for instance, at the nitrogen atom in methylamine, CH3NH2, an organic derivative of ammonia (NH3) and the substance responsible for the odor of rotting fish. 

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