Resonance structure brackets

The compound above has two important resonance structures. Notice that we separate resonance structures with a straight, two-headed arrow, and we place brackets around the structures. The arrow and brackets indicate that they are resonance structures of one molecule. The molecule is not flipping back and forth between the different resonance structures. 

Table 3.37. Leading NRTresonance structures and weightings (with numbers of symmetry-equivalent resonance structures in brackets) for anions of common...

In all the following formulas only one resonance structure is shown. To indicate that a jr-electron system with many possible resonance structures is involved, the total charge of the molecule is given outside a square bracket. 

A comparison of eqns. (5) and (6) reveals that the term in square brackets in eqn. (6) is the ratio of the number of unsynchronized resonance structures per atom to the number of synchronized resonance structures per atom for a hypoelectronic atom. Given the reasonable assumption that the energy corresponding to an unsynchronized resonance structure is the same order of magnitude as that for a synchronized resonance structure, the energy of a crystal composed of hypoelectronic atoms is lowered considerably via unsynchronized resonance. Therefore, one predicts that every element with an extra orbital to serve as the metallic orbital should be a metal. With a single possible exception, namely boron, which will be discussed in a succeeding section, this prediction is borne out. 

Resonance occurs when one or more valid Lewis structures exist for a molecule or polyatomic ion. The structures that represent the substance are called resonance structures. Each resonance structure does not characterize the substance, but the average of all the resonance structures represents the molecule or polyatomic ion. Resonance structures are usually placed in brackets and separated by a double-headed arrow (<- ). 

The double-headed arrow and the brackets are used to indicate that the two structures are resonance structures. Remembering the concept of resonance (that is, the delocalization of electrons), we can see why these two structures are equivalent. Using the arrow convention of moving electrons, we have ... 

Each compound contains both ionic and covalent bonds. Write ionic Lewis structures for each of them, including the covalent structure for the ion in brackets. Write resonance structures if necessary. 

By convention resonance structures are bracketed together with square brackets and linked with a double-headed arrow. 

We use a single double-headed arrow between resonance forms (and often enclose them in brackets) to indicate that the actual structure is a hybrid of the Lewis structures we have drawn. By contrast, an equilibrium is represented by two arrows in opposite directions. Occasionally we use curved arrows (shown in red above) to help us see how we mentally move the electrons between one resonance form and another. The electrons do not actually resonate back and forth they are delocalized over all the resonance forms at the same time. 

Two important conventions involving arrows are introduced in Section 1-5. The first is the use of double-headed arrows between resonance forms. This is a special kind of notation because of the special role resonance forms play in organic chemistry. As shown in this section, many species have structures that cannot be represented by a single Lewis structure. They can only be described as intermediate in nature between two or more contributing forms, each of which by itself is an incomplete picture of the molecule s structure. We represent such a molecule by drawing the resonance forms separated by double-headed arrows and enclosed in brackets. The true structure is called the resonance hybrid. The only difference between the resonance forms is a different location for the electrons from one to the next. The same geometrical arrangement of the atoms is maintained in all the resonance forms. Caution molecules that actually exist as resonance hybrids are still often represented by only one Lewis structure. In cases like this you need to be aware of the fact that this is a shortcut used for convenience purposes only and that the real structure is still the resonance hybrid—the other resonance forms are implied even if they aren t written down. 

Wc use a single double-headed arrow between resonance forms (and often enclose them in brackets) to indicate that the actual structure is a hybrid of the Lewis structures we have drawn. By contrast, an equilibrium is represented by two arrows in opposite directions. 

Fig. 10. The experimentally derived surface states (thick lines) and/or surface resonances (dashed lines) and bulk band critical points (solid dots from Dongqi Li et al. 1994a open squares from Kim et al. 1992 open circles fiom Himpsel and Reihl 1983). The band structure for paramagnetic bulk gadolinium is shown with the hep symmetry labels. Some fee symmetry labels are given in brackets for comparison. (Adapted Irom Dongqi Li et al. 1994a.)...

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