Chemical bonds representations

Figure 1. Connectivities and principle bonding properties of carbon. From top to bottom connectivity, chemical bonding representation, distribution of n electrons, hybridization symbol, bond length, orientation of the n bonds relative to the carbon skeleton. The spectra represent polarization-dependent carbon 1 s XAS data for sp2 and sp3 carbons. The angles denote the orientation of the E vector of the incident light relative to the surface normal of the oriented sample. The assignment of the spectral regions is given and was deduced from the angular dependence of the intensities of each feature. The graphite impurity in the CVD diamond film is less than 0.1 monolayers.

Boranes are typical species with electron-deficient bonds, where a chemical bond has more centers than electrons. The smallest molecule showing this property is diborane. Each of the two B-H-B bonds (shown in Figure 2-60a) contains only two electrons, while the molecular orbital extends over three atoms. A correct representation has to represent the delocalization of the two electrons over three atom centers as shown in Figure 2-60b. Figure 2-60c shows another type of electron-deficient bond. In boron cage compounds, boron-boron bonds share their electron pair with the unoccupied atom orbital of a third boron atom [86]. These types of bonds cannot be accommodated in a single VB model of two-electron/ two-centered bonds. 

With better hardware and software, more exact methods can be used for the representation of chemical structures and reactions. More and more quantum mechanical calculations can be utilized for chemoinformatics tasks. The representation of chemical structures will have to correspond more and more to our insight into theoretical chemistry, chemical bonding, and energetics. On the other hand, chemoinformatics methods should be used in theoretical chemistry. Why do we not yet have databases storing the results of quantum mechanical calculations. We are certain that the analysis of the results of quantum mechanical calculations by chemoinformatics methods could vastly increase our chemical insight and knowledge. 

Figure 16-3D shows the simplified representation of the interaction of two helium atoms. This time each helium atom is crosshatched before the two atoms approach. This is to indicate there are already two electrons in the Is orbital. Our rule of orbital occupancy tells us that the Is orbital can contain only two electrons. Consequently, when the second helium atom approaches, its valence orbitals cannot overlap significantly. The helium atom valence electrons fill its valence orbitals, preventing it from approaching a second atom close enough to share electrons. The helium atom forms no chemical bonds. ... 

We propose, then, that chemical bonds can form if valence electrons can be shared by two atoms using partially filled orbitals. We need a shorthand notation which aids in the use of this rule. Such a shorthand notation is called a representation of the bonding. 

We shall use both orbital and electron dot representations to show chemical bonding. 

OH molecules, reaction between, 282 Oil-drop experiment, 241 Oil of wintergreen, 346 Oleomargarine, 407 Open hearth furnace, 404 Operational definition, 195 Orbital representation of chemical bonding, 278 Orbitals atomic, 262, 263 dand/, 262... 

In chemistry, perhaps because of the significance in visualizing molecular strac-ture, there has been a focus on how students perceive three-dimensional objects from a two-dimensional representation and how students mentally manipulate rotated, reflected and inverted objects (Stieff, 2007 Tuckey Selvaratnam, 1993). Although these visualization skills are very important in chemistry, it is evident that they are not the only ones needed in school chemistry (Mathewson, 1999). For example, conceptual understanding of nature of different types of chemical bonding, atomic theory in terms of the Democritus particle model and the Bohr model, and... 

It is essential to realize that electrons In the nitrate anion do not flip back and forth among the three bonds, as implied by separate structures. The true character of the anion is a blend of the three, In which all three nitrogen-oxygen bonds are equivalent. The need to show several equivalent structures for such species reflects the fact that Lewis structures are approximate representations. They reveal much about how electrons are distributed in a molecule or ion, but they are imperfect instruments that cannot describe the entire story of chemical bonding, hi Chapter 10, we show how to interpret these structures from a more detailed bonding perspective. 

Figure 3.10 Representations of the electron density ip2 of the Is orbital and the 2p orbital of the hydrogen atom. (b,e) Contour maps for the xe plane. (c,f) Surfaces of constant electron density. (a,d) Dot density diagrams the density of dots is proportional to the electron density. (Reproduced with permission from the Journal of Chemical Education 40, 256, 1963 and M. J. Winter, Chemical Bonding, 1994, Oxford University Press, Fig. 1.10 and Fig. 1.11.)...

The NRT formalism will be used to describe the interacting species along the entire reaction coordinate. Such a continuous representation allows the TS complex to be related both to asymptotic reactant and product species and to other equilibrium bonding motifs (e.g., 3c/4e hypervalent bonding Section 3.5). A TS complex can thereby be visualized as intermediate between two distinct chemical bonding arrangements, emphasizing the relationship between supramolecular complexation and partial chemical reaction. 

Chemical bonds are classified into two groups transfer of electrons creates an ionic bond while the sharing of electrons leads to a covalent bond. Before studying chemical bonds we need to become familiar with their representation. Chemical bonds may be represented in several ways. We are going to study orbital representation, electron dot representation and line representation. Let s examine these three types using the example of the fluorine molecule, F2. 

At Harvard, Theodore William Richards, like Noyes, inherited a course in theoretical chemistry. He renamed it physical chemistry. However, he cautioned students that the molecular kinetic hypotheses might prove ephemeral, and, to the young Lewis s consternation, Richards showed contempt for the notion of chemical bonds. "Twaddle about bonds A very crude method of representing certain known facts about chemical reactions. A mode of representation] not an explanation. "68 It was not so much that Richards sided with energeticists against kinetic and mechanical representations, but he did have a distrust of mathematical formulations too far removed from the laboratory. When J. Robert Oppenheimer enrolled in Richards s course in physical chemistry in 1925, he pronounced it "a great disappointment,. .. a very meager hick course.. . . Richards was afraid of even rudimentary mathematics."69 Thus, physical chemistry by no means necessarily meant mathematical chemistry. 

A simpler model for ethane recognizes what we already know for methane that each carbon atom is bonded to four other atoms. Given that knowledge, we can now write simply, CH3—CH3, showing only the carbon-carbon bond. Since each carbon atom forms four bonds and since only one is shown (the carbon-carbon bond), it follows that each carbon atom must make three bonds to hydrogen atoms. Even simpler is the model CH3CH3, in which none of the chemical bonds is shown directly. Once we have gained more experience, it will be clear that this simple representation contains all the information that the more detailed one does. Here are two other models for ethane ... 

Graphical representations of the electron density will be provided in Chapter 4, and connections drawn between electron density and both chemical bonding and overall molecular size and shape. 

Theoretical aspects of the bond valence model have been discussed by Jansen and Block (1991), Jansen et al. (1992), Burdett and Hawthorne (1993), and Urusov (1995). Recently Preiser et al. (1999) have shown that the rules of the bond valence model can be derived theoretically using the same assumptions as those made for the ionic model. The Coulomb field of an ionic crystal naturally partitions itself into localized chemical bonds whose valence is equal to the flux linking the cation to the anion (Chapter 2). The bond valence model is thus an alternative representation of the ionic model, one based on the electrostatic field rather than energy. The two descriptions are thus equivalent and complementary but, as shown in Section 2.3 and discussed further in Section 14.1.1, both apply equally well to all types of acid-base bonds, covalent as well as ionic. 

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