Bonds and Valence Electrons

The octet rule for chemical bonding was mentioned briefly in Chapter 3. At this point, it will be useful to examine this rule in more detafl. Although there are many exceptions to it, the octet rule remains a valuable concept for an introduction to chemical bonding. 

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Table 3-11 Bonding and Valence-Electron Hybridization for Iron-Oxygen Compounds...

In the case of organic molecules chemists consider a logical choice to express the potential in terms of valence bonds and valence electrons [20] while physicists often proceed with an unbiassed cartesian space which is sometimes transformed in that of a suitable model aimed at a specific physical property. Often the potentials used in treating the vibrations of ionic crystals or metals are transferred and constrained to the case of organic molecules. However the chemical reality of molecular covalent bonds and of their interactions is different from any type of "ionic" system. For example the popular "shell model" has not been as successful as the classical "valence force field" in the study of the lattice dinamics of "covalent crys-... 

Bond-eltctron matrix describes connections, bond orders, and valence electrons of the atoms cannot be represented by bits... 

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

Based on its structure and valence electron count, draw a Lewis structure or series of Lewis structures for diborane Examine the bond density surface. Does it substantiate 01 refute your speculation ... 

What valence orbital and valence electron conditions must exist if a chemical bond is to form between two approaching atoms ... 

The constitution of molecules is given by lists of atoms and bonds (connectivity lists topological representation )6 . In addition, the number of free electrons for each atom is also carried in a separate vector. This is necessary as some reaction generators may transform free electrons into bonds, or vice versa. Thus, by working with free electrons and the electrons involved in bonds, all valence electrons of a molecule are explicitly specified. 

In Sections 9-3 and 9-4, we will show you two types of chemical bonds ionic and covalent. It is important to be able to represent compounds in terms of the atoms and valence electrons that make up the chemical species (compounds or polyatomic ions). One of the best ways is to use Lewis symbols and structures. 

REMARKS ON THE CHEMICAL BOND FACTOR AND VALENCE-ELECTRON COUNTING RULES... 

In this formula, which can only be applied if all bonds are two-electron bonds and additional electrons remain inactive in non-bonding orbitals (or, in other words, if the compound is semiconductor and has non-metallic properties), ecc is the average number of valence electrons per cation which remain with the cation either in nonbonding orbitals or (in polycationic valence compounds) in cation—cation bonds similarly cAA can be assumed to be the average number of anion—anion electron-pair bonds per anion (in polyanionic valence compounds). 

The concept of the molecular connectivity index (originally called branching index) was introduced by Randic [266]. The information used in the calculation of molecular connectivity indices is the number and type of atoms and bonds as well as the numbers of total and valence electrons [176,178,181,267,268]. These data are readily available for all compounds, synthetic or hypothetical, from their structural formulas. All molecular connectivity indices are calculated only for the non-hydrogen part of the molecule [269-271]. Each non-hydrogen atom is described by its atomic 6 value, which is equal to the number of adjacent nonhydrogen atoms. For example, the first-order Oy) molecular connectivity index is calculated from the atomic S values using Eq. (38) ... 

Figure 4 depicts the different forms of chemisorption for a Na atom by means of the symbolic valence signs. In weak bonding the valence electron of the Na atom remains unpaired (see Fig. 2a), and in this sense the free valence of the Na atom may be considered unsaturated. This form of bond thus represents the radical form of chemisorption, which is symbolically depicted in Fig. 4a. Upon transition to strong n- or p-bonding a free electron or, respectively, a free hole of the lattice becomes involved in the bond the electron becomes localized and coupled to the valence electron of the Na atom (see Fig. 2b) or, respectively, the free hole recombines with the valence electron of the Na atom (see Fig. 2c).

We shall now examine recent theoretical results concerning those ground state properties of metals and compounds which in the previous section, we have called bond and valence indicators. In this section, those properties are not taken as starting points for correlations aiming at the composition of the bond. They are instead the final point of electronic structure theories in which the different contributions to cohesion are analysed. 

According to this concept, the metallic properties are based on the possession by some or all of the atoms in a given metal of a free orbital (the metallic orbital ), in addition to the orbitals required for bonding and nonbonding electrons, thus permitting uninhibited resonance of valence bonds. For the case of tin, the following scheme illustrates these relationships for three electronic structures (A, B, and C) of this metal ... 

Electrons in the outermost occupied shell of any atom may play a significant role in that atoms chemical properties, including its ability to form chemical bonds. To indicate their importance, these electrons are called valence electrons (from the Latin valentia, strength ), and the shell they occupy is called the valence shell. Valence electrons can be conveniently represented as a series of dots surrounding an atomic symbol. This notation is called an electron-dot structure or, sometimes, a Lewis dot symbol, in honor of the American chemist G. N. Lewis, who first proposed the concepts of shells and valence electrons. Figure 6.2 shows the electron-dot structures for the atoms important in our discussions of ionic and covalent bonds. (Atoms of elements in groups 3 through 12 form metallic bonds, which we ll study in Chapter 18.)... 

Exercise 2-1 Draw the Lewis electron-pair structure of 2-propanone (acetone) clearly showing the bonding and nonbonding electron pairs in the valence shell of each atom. Draw structural formulas for other compounds having the composition C3H60 that possess... 

As second-series transition metals, Ag, Mo, and Y have 5s and 4d valence electrons. All the valence electrons will occupy a composite s-d band, which can accommodate 12 electrons per metal atom. To identify each metal, count the number of 5s and 4d valence electrons and compare that number with the population of bands in pictures (1), (2), and (3) above. The melting point and hardness of a metal is expected to increase as the difference between the number of bonding and antibonding electrons increases. 

Core and valence electrons can be divided based on a criterion of whether their distributions and populations are changed or not owing to bond formation. The electron population analysis conventionally uses spherical form factors, f, which are given by the spherically averaged Fourier transform of electron-density distributions. 

Both valence bond (VB) and molecular orbital (MO) theories have been used to explain the observed shapes of molecules. What we wish to know here is the shape of a transition state containing m atoms and n electrons. Fortunately, the preferred shapes of the simple species are known or can be guessed from the numbers and kinds of bonding and nonbonding electron pairs (Gillespie, 1967). Therefore, we must examine the preferred shape of clusters of three, four or more atoms. For, to envision the topology of a transition state is tantamount to a description of the stereochemical result of an elementary process. 

The distribution of bonding, or valence, electrons is the largest contributor to the electric field gradient. In a molecular frame of reference, we can define three orthogonal components of the electric field gradient, Vxx, Vyy, and Vzz, where Vxx + Vyy + Vzz = 0 and, by convention, VXX < V l < VZZ. From these electric field gradient components, we define two parameters ... 

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