Valence electron distribution

Different Lewis structures do not in general make the same contribution to a resonance structure. It is possible to decide which structures are likely to make the major contribution by comparing the number of valence electrons distributed around each atom in a structure with the number of valence electrons on each of the free atoms. The smaller these differences for a structure, the greater is its contribution to a resonance hybrid. 

The complex contains 72 atoms with 244 valence electrons distributed in 226 valence atomic orbitals. In order to reduce the computational effort, and to assess the contribution of the ligand 7r-orbitals to the overall spectrum, we examined a "reduced" model, see Figure 2, in which the benzene rings of the ligands are replaced by -HC=CH- groups. This model compound consists of... 

Figure 2.1. illustrates these differences for the 2s NAOs of Li-, Li°, and Li+, showing the strong decrease in the radius of this valence orbital as the net charge increases. It is evident that an attempt to describe, e.g., the valence electron distribution of Li+ in terms of the fixed 2s AO of Li (or Li-) would incur a large error,... 

The Lewis electron-dot symbol is a way of representing the element and its valence electrons. The chemical symbol is written, which represents the atom s nucleus and all inner-shell electrons. The valence, or outer-shell, electrons are represented as dots surrounding the atom s symbol. Take the valence electrons, distribute them as dots one at a time around the four sides of the symbol and then pair them up until all the valence electrons are distributed Figure 11.1 shows the Lewis symbol for several different elements. 

In inorganic and organometallic solids, the average electron concentration tends to be high. This means that absorption and extinction effects can be severe, and that the use of hard radiation and very small crystals is frequently essential. Needless to say that the advent of synchrotron radiation has been most helpful in this respect. The weaker contribution of valence electrons compared with the scattering of first-row-atom-only solids implies that great care must be taken during data collection in order to obtain reliable information on the valence electron distribution. 

FIG. 11.8 (a) A section of the difference synthesis through the Cr nucleus, parallel to the (110) plane. Contours are drawn at intervals of 0.2 eA 3. (b) Theoretical contour map of valence electron distribution on the (110) plane for chromium metal. Contours are drawn at intervals of 0.5 eA-3. The lobes point towards the nearest neighbors in the body-centered cubic structure. Source Ohba et al. (1982). 

Consider one beautifully symmetrical shape predicted by VSEPR theory the tetrahedron. Four equivalent pairs of electrons in the valence shell of an atom should distribute themselves into such a shape, with equal angles and an equal distance between each pair. But what sort of atom has four equivalent electron pairs in its valence shell Aren t valence electrons distributed between different kinds of orbitals, like s and p orbitals (We introduce these orbitals in Chapter 4.)... 

Just as a is the linear polarizability, the higher order terms p and y (equation 19) are the first and second hvperpolarizabilities. respectively. If the valence electrons are localized and can be assigned to specific bonds, the second-order coefficient, 6, is referred to as the bond (hyper) polarizability. If the valence electron distribution is delocalized, as in organic aromatic or acetylenic molecules, 6 can be described in terms of molecular (hyper)polarizability. Equation 19 describes polarization at the atomic or molecular level where first-order (a), second-order (6), etc., coefficients are defined in terms of atom, bond, or molecular polarizabilities, p is then the net bond or molecular polarization. 

There are a total of 14 valence electrons distributed as shown. Each carbon is surrounded by eight electrons. 

We continue to ignore the core electrons and consider N valence electrons distributed amongst the valence orbitals in configurations of the form... 

The valence electron distribution of a transition metal surface is sketched in Fig. 4.1. A narrow d valence electron band is overlapped by a broad s-p valence... 

Fig. 4.1. The valence electron distribution at a transition metal surface (schematic).

Fig. 4.4. Quantum chemistry of the CO d valence electron interaction (schematic). On the left the d valence electron distribution is sketched, which is typical of a transition metcil.

To take in account the nonspherical shape of the valence electron distribution, the K model has been improved by the addition of multipole parameters [lib]. Then, the pseudoatomic density is written (Molly program),... 

Chemical shifts also exist in X-ray photoelectron (106), X-ray fluorescence (21), and Mossbauer (123) spectroscopy. The common theme in all these phenomena is that inner electronic or nuclear energy levels are measurably affected by chemical changes in the valence electron distribution. 

We note that the correspondence between the quadrupole coupling constant and the number of p holes is based on a measurement of the quadrupole coupling constant for xenon with a 5p electron excited to the 6s state. It is assumed in the analysis of the data for the compounds that the antishielding factor R for the compounds is the same as that for the atom. If this is not so, the derived charges will be in error. We do not, however, see any reason why the antishielding factor should be independent of valence electron distribution. 

Lewis x> wrote in 1916 a paper suggesting that chemical bonds are effectuated by electron-pairs, and that well-behaved elements have 8 outer (valency) electrons distributed in four pairs, either shared with adjacent atoms to form chemical bonds, or remaining on the atom as lone-pairs . With exception of helium, the noble gases have also 8 electrons, but they are on the limit of becoming inner electrons (like in the subsequent alkaline-metals) and lack typical characteristics of lone-pairs, such as proton affinity in solution (however much EH+ and EX+ formed by the halogens can be detected in mass-spectra). 

Symmetry, in one or other of its aspects, is of interest in the arts, mathematics, and the sciences. The chemist is concerned with the symmetry of electron density distributions in atoms and molecules and hence with the symmetry of the molecules themselves. We shall be interested here in certain purely geometrical aspects of symmetry, namely, the symmetry of finite objects such as polyhedra and of repeating patterns. Inasmuch as these objects and patterns represent the arrangements of atoms in molecules or crystals they are an expression of the symmetries of the valence electron distributions of the component atoms. In the restricted sense in which we shall use the term, symmetry is concerned with the relations between the various parts of a body. If there is a particular relation between its parts the object is said to possess certain elements of symmetry. 

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