Covalent bonds resonance structures

Define the terms covalent bond, resonance structures, and arrow pushing. 

Figure 8.1. Energetics of the proton transfer between an acid (A) and a base (B) by valence bond resonance structures. The reaction is described in terms of a covalent resonance structure (A-H B) and an ionic resonance structure (A H-B+). The energies of the valence bond structures are as given in the text and depend on the coordinates of the reacting atoms, R, and the coordinates of the rest of...

In order to account for the existence of hydrogen bonds we must consider the possible contributions of covalent bonding, resonance and electrostatic attraction and attempt to assess the importance of each. In short, we must ask how much each of the possible canonical structures (5-VI), (5-VII) and... 

The more covalent bonds a structure has, the more stable it is. Consider the resonance structures for formaldehyde below. (Formaldehyde is a chemical used to preserve biological specimens.) Structure A has more covalent bonds, and therefore makes a larger contribution to the hybrid, hi other words, the hybrid is more like stmc-tnre A than stmctnre B. 

In Chapter 7, we used valence bond theory to explain bonding in molecules. It accounts, at least qualitatively, for the stability of the covalent bond in terms of the overlap of atomic orbitals. By invoking hybridization, valence bond theory can account for the molecular geometries predicted by electron-pair repulsion. Where Lewis structures are inadequate, as in S02, the concept of resonance allows us to explain the observed properties. 

Reactive trajectories, 43-44,45, 88,90-92,215 downhill trajectories, 90,91 velocity of, 90 Relaxation processes, 122 Relaxation times, 122 Reorganization energy, 92,227 Resonance integral, 10 Resonance structures, 58,143 for amide hydrolysis, 174,175 covalent bonding arrangement for, 84 for Cys-His proton transfer in papain, 141 for general acid catalysis, 160,161 for phosphodiester hydrolysis, 191-195,... 

In recent years it has become clear that the structure of metals and alloys may be described in terms of covalent bonds that resonate among the alternative interatomic positions in the metals, and that this resonance is of greater importance for metals than for substances of any Other class, including the aromatic hydrocarbons. Moreover, the phenomenon of metallic resonance of the valency bonds must be given explicit consideration in the discussion of metallic valency it is necessary in deducing the metallic valency from the number of available electrons and bond orbitals to assign to one orbital a special r le in the metallic resonance. 

A theory of resonating covalent bonds in metals, developed over the period 1938-1953 (1-3), was recently refined by the formulation of a statistical treatment for hypoelectronic metals (4). We have now extended the statistical treatment to include hyperelectronic metals. This extension has resulted not only in the evaluation of the number of resonance structures for these metals but also in the determination for them of the values of the metallic valence, which have been somewhat uncertain. 

The individual structural features of the high-tem-perature superconductors are found in other substances. A substance with alternating metal-salt layers is Ag2F, with sequence FAgAgFAgAgF . Resonance between a covalent bond and a no-bond is found in B (l =3,Z. 6) and in metals and organometallic clusters. Hyperelectronic-hypoelectronic electron transfer occurs... 

The first represents a hypothetical HC1 molecule with a purely covalent bond in which the two bonding electrons are equally shared between the two atoms, and the second a hypothetical molecule with a purely ionic bond in which both the bonding electrons have been transferred to the chlorine atom. In this case the two resonance structures do not necessarily... 

The SiF bond in SiF4 has a length of 155.5 pm compared to the value of 169 pm calculated using the Schomaker-Stevenson equation and the value of 177 pm calculated from the sum of the covalent radii (Table 2.6). So Pauling wrote resonance structures such as the following ... 

It is difficult to give a localized orbital description of the bonding in a period 3 hypervalent molecule that is based only on the central atom 3s and 3p orbitals and the ligand orbitals, that is, a description that is consistent with the octet rule. One attempt to do this postulated a new type of bond called a three-center, four-electron (3c,4e) bond. We discuss this type of bond in Box 9.2, where we show that it is not a particularly useful concept. Pauling introduced another way to describe the bonding in these molecules, namely, in terms of resonance structures such as 3 and 4 in which there are only four covalent bonds. The implication of this description is that since there are only four cova-... 

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