Valence-bond formulas

The structure of a molecule depends essentially on the covalent bond forces acting between its atoms. In the first place, they determine the constitution of the molecule, that is, the sequence of the linkage of the atoms. The constitution can be expressed in a simple way by means of the valence bond formula. For a given constitution the atoms arrange themselves in space according to certain principles. These include atoms not bonded directly with one another may not come too close (repulsion of interpenetrating electron shells) and the valence electron pairs of an atom keep as far apart as possible from each other. 

In order to apply the theory, one first draws a valence bond formula with the correct constitution, including all lone electron pairs. This formula shows how many valence electron pairs are to be considered at an atom. Every electron pair is taken as one unit (orbital). The electron pairs are being attracted by the corresponding atomic nucleus, but they exercise a mutual repulsion. A function proportional to 1 /rn can be used to approximate the... 

Despite the given formula, the calculation of the bond order is not always clear in the case of the occupation of orbitals having only a minor bonding or minor antibonding effect should they be counted or not Nevertheless, the bond order is a simple and useful concept. In valence-bond formulas it corresponds to the number of bonding lines. 

The one-dimensional chain of hydrogen atoms is merely a model. Flowever, compounds do exist to which the same kind of considerations are applicable and have been confirmed experimentally. These include polyene chains such as poly acetylene. The p orbitals of the C atoms take the place of the lx functions of the H atoms they form one bonding and one antibonding n band. Due to the Peierls distortion the polyacetylene chain is only stable with alternate short and long C-C bonds, that is, in the sense of the valence bond formula with alternate single and double bonds ... 

Draw valence bond formulas for the following ZiNTL anions. 

Vi corresponds to a bonding orbital that essentially involves Are interaction of the C atom with the first H atom its charge density i/zf is concentrated in the region between these two atoms. Tlrls matches the idea of a localized C-H bond Tire electron pair of this orbital is assigned to a bond between these two atoms and symbolized by a dash in the valence bond formula. 

Values of Bond Energies for Multiple Bonds.—In Section 3-5 there is given a table of values of bond energies for single bonds. In the construction of this table care was taken to make use of data for only those molecules to each of which an unambiguous assignment of a valence-bond formula could be made. This consideration of bond energies is extended in Table 6-1, which contains values for some multiple bonds, obtained by methods similar to those described in Section 3-5. 

Carbon Dioxide and Related Molecules.—It is not surprising that so unconventional a molecule as carbon monoxide should have a resonating structure but recognition of the faci, that the carbon dioxide molecule, for which the valence-bond formula 0—C---0 has been written ever since the development of valence theory, is not well represented by this structure alone and that other valence-bond structures also make important contributions must have come as a surprise to everyone. 

The older conventional valence-bond formulas for an ion such as the sulfate ion,... 

The crucial question is Which bonds in the molecule are to be regarded as topologically significant 73, 87 Different authors differ in their answers to this question.72 The root of the difficulty lies in the historical sanction of localized valence bond formulas, which, for all their virtues of convenience, imply a sharpness of definition that is physically unsound For the sake of simplicity,... 

Structure of fullerene-Cy,o (a) molecular shape, (b) valence-bond formula, and (c) planar formula with carbon atom-numbering scheme. 

Fig. 9.1. Representative phosphonium, sulfonium, and sulfoxonium ylides— formation reactions and valence bond formulas.

In 1770 the British scientist Henry Cavendish showed that water is a compound of hydrogen and oxygen. In 1804 another British scientist, John Dalton, assigned to the water molecule the formula shown in the drawing he assumed the molecule to be made of one atom of hydrogen and one atom of oxygen. By 1860 most chemists had accepted H20 as the formula of water, and the valence-bond formula H—O—H was being used. 

In 1858 the Scottish chemist A. S. Couper invented valence-bond formulas for chemical compounds and wrote the formula... 

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