Lewis structure representation

Table 3.1 compares key bonding and ionicity descriptors for the NBO Lewis-structure representations of the five AF diatomic fluorides in this series. From this table it can be seen that the single-term natural Lewis-structure description is practically exact for all species (with %p 99.9% for A = F, Cl, Br, H, and only a... 

The NBO analysis reveals the strong electronic similarities between C2B9H112-and C5H5-. The optimal NBO Lewis structure representations, (3.268a)-(3.268c), all have two-center single bonds connecting the B7, Bg, B9, C10, and Cn rim atoms, with the familiar motif of a two-center B—B bond flanked by four... 

Solution From the standard Lewis structure representation... 

The strong non-Lewis contributions (particularly in the a manifold) suggest that no single Lewis structure representation is adequate to describe the vinoxy electronic distribution. A more accurate resonance-theoretic description will therefore be required to describe the delocalization patterns of each spin manifold. 

Table 7.5 exhibits details of a and fi Lewis-type NBOs for vinoxy A A, corresponding to the Lewis structure representations 8a and 8b. 

Lewis structure Representation using dots to show valence electrons. 

Fig. 5 Case studies of (a) bridging hydride, (b) bridging carbonyl with negligible -backbonding, (c) bridging thiolate, (d) bridging carbonyl with strong it-backbonding, (e) bridging carbonyl with its o-donation averaged over two metals, and (f) an alternative Lewis structural representation of a bridging CO with the addition of a metal-metal bond, an addition that is often invoked to satisfy the 18-e rule...

When more than one acceptable Lewis structure for a given species is possible, the one with the smallest formal charges is generally the best representation of the actual electron distribution in the species. Therefore, we can conclude that the best Lewis structure representation of CO2 is structure (1)... 

FIGURE 6.9 Lewis structure representations of oxygen species involved in oxidative stress processes. Although triplet (ground state) molecular oxygen molecules are sometimes depicted by the simplified Lewis structural formulas shown, a complete description of the electronic behavior of these molecules requires use of molecular orbital theory, which is beyond the scope of this work. 

Pauling (and Slater) envisioned the extension of the Heitler-London Ansatz to each localized gAB electron-pair bond of a Lewis structural representation by replacing hydro-genic orbitals with directed hybrids hA, hs. 

Lewis structure (Section 1.5) A representation of a molecule showing valence electrons as dots. 

The Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

Which of these options is the best Lewis structure Actually, no single Lewis structure by itself is an accurate representation of NO3. Any single structure of the anion shows nitrate with one NDO double bond and two N— O single bonds. In Section 9 1, we show that single and double bonds between the same types of atoms have different lengths and different energies. In contrast, experiments show that the three nitrate N—O bonds are identical. To show that the nitrate N—O bonds are all alike, we use a composite of the three equivalent Lewis structures. These are traditionally called resonance structures. Resonance stmctures are connected by double-headed arrows to emphasize that a complete depiction requires all of them. 

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. 

The best Lewis-type representation of the bonding in OCF3 would therefore appear to be as in 4, even though the carbon atom does not obey the octet rule. This molecule can be considered to be a hypervalent molecule of carbon just like the hypervalent molecules of the period 3 elements, such as SFfi. We introduced the atom hypervalent in Chapter 2 and we discuss it in more detail in Chapter 9. But it is important to emphasize that the bonds are very polar. In short, OCF3 has one very polar CO double bond and three very polar CF single bonds. A serious limitation of Lewis structures is that they do not give any indication of the polarity of the bonds, and much of the discussion about the nature of the bonding in this molecule has resulted from a lack of appreciation of this limitation. 

A third, more extreme, conceptual model, based on a completely ionic picture of hypervalent bonding, can also be invoked to remove perceived conflicts with Lewis-structural principles. In PF5, for example, the completely ionic ( oxidation number ) P5+(F )5 representation... 

Another type of hypervalency is encountered in textbook descriptions of the oxyanions of common laboratory acids. Generations of chemistry students have been taught that the correct representations of these species are in terms of resonance-delocalized hypervalent Lewis-structure diagrams, such as sulfate (S042-),... 

The specific realization (3.259) of the favored 0.10.3.0 styx pattern is only one of many equivalent localized representations. The central 1—4—5 triangle of Fig. 3.110 could equivalently be chosen as any of the 20 possible triangular facets in Fig. 3.111, leading to 20 equivalent three-center Lewis structures that contribute to the overall resonance delocalization of Bi2Hi22. The non-Lewis density of any such structure is found to be... 

As we learn about the distribution of electrons within a covalent bond, we start with a popular representation known as a Lewis structure. Figure 2.11 depicts the... 

Representations of a covalent bond in a molecule of hydrogen (A) Lewis structure (B) Overlap of two Is orbitals... 

Representation of the Molecule in LISP. We have used the chemist s sketch, or its Lewis structure equivalent, as the model of a data structure in LISP ( ). This language has the flexibility needed to express an essentially non-numerical object, in terms of lists. LISP will permit us to organize molecular structure information in a way that mimics the human expert s knowledge. To accomplish this representation, we must develop a clear idea how the chemist assimilates the information provided directly and explicitly by the sketch, and how the properties of the molecule are recalled to the chemist s awareness. 

Lewis formula (for an ionic compound) A representation of the structure of an ionic compound showing the formula unit of ions in terms of their Lewis symbols. Lewis structure A diagram showing how electron pairs are shared between atoms in a molecule. Examples H-C1 0=C=0. 

Whenever there are two alternative Lewi.s structures, one alone will be an inaccurate representation of the molecular itructure. A more accurate picture will be obtained by the superposition of. the two structures into a new model, which lor benzene indicated by 3. The. superposition of two or more Lewis structures into a composite picture is called resonance. 

Almost simultaneous with the publication of Kossel s paper there appeared a rival electronic theory. The American chemist Lewis introduced the idea of the covalent electron-pair bond. Like Kossel, he was impressed by the apparent stability of the noble gas configuration. He was also impressed by the fact that, apart from many compounds of the transition elements, most compounds when rendered as molecules have even numbers of electrons, suggesting that electrons are usually found in pairs. Lewis devised the familiar representations of molecules and polyatomic ions (Lewis structures, or Lewis diagrams) in which electrons are shown as dots (or as noughts and crosses) to show how atoms can attain noble gas configurations by the sharing of electrons in pairs, as opposed to complete transfer as in Kossel s theory. It was soon apparent from the earliest X-ray studies that Kossel s theory was more appropriate... 

Sometimes more than one satisfactory Lewis structure can be written and there is no reason to select one over another. In such cases a single structural formula is inadequate for a correct representation, and we say that the true structure is a resonance hybrid of the several structures. Common examples of species requiring resonance structures are ozone, 03, carbonate ion, CO " and benzene, C6H6. These... 

Lewis structure A representation of a molecule that depicts covalent bonds and nonbonding valence electrons. 

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