Chemical bonding simplified models

The previous model can be used on a molecular scale to represent chemical bonds. However, it is necessary to introduce the quantum aspect for species at the atomic level. Because a chemical bond of vibrational frequency v can absorb radiation of identical frequency, the energy will increase by the quantum AE = hv. According to this theory, the simplified relation giving possible energy values Vib is given by ... 

In the next section, we explore how the shell model can be used to explain periodic trends. An even further simplified shell model, known as an electron-dot structure, is then developed in Chapter 6 to assist you in understanding chemical bonding. As you use these models, please keep in mind that electrons are not really confined to the surface of one shell or another. Instead, any... 

Spectroscopy and the electron theory of valence provided valuable support for one another. Together, they took our understanding of the nature of chemical elements to a new level, where chemical behavior and chemical structure could both be interpreted in terms of the number and disposition of electrons in the atoms of any given element. At least, the simplified model of atomic orbitals brilliantly developed by Linus Pauling enabled him to explain and predict a great deal of chemistry, in terms of bonds and structures. 

Theoretical considerations may also help to build up a reaction scheme. The principle of smallest change of structure states that only a few chemical bonds may be broken or formed in a single chemical step. This principle is mainly used in mechanistic modelling, but gives indications about the filiation of constituents. The knowledge of the equilibrium constants allows us to eliminate some reactions. Finally, if a detailed reaction mechanism has been postulated on firmly established experimental and theoretical grounds, it may be simplified to a molecular reaction scheme. 

The simplified model must be effectively representative of the connectivity of the real network. In some cases, however, this step leads to structural descriptions that are rather subjective in the selection of both the nodes and the connections (links) joining the nodes. Alternative rationalizations can be accomplished, that result in motifs of different topology, depending upon the choice of the chemical or/and mechanical bonds and the nodes of the architecture (see Fig. 1.3.1) [8]. These alternative motifs, however, are not something unnatural they reflect the possibility of consideration of a crystal from different points of view or at various levels of structure organization. 

The model above can be approximated to the chemical bond linking two atoms, on condition that the quantum theory governing species of atomic dimensions is employed. Because a bond whose frequency of vibration is v can absorb light radiation of an identical frequency, its energy will increase from the quantum A = hv. According to this theory, the simplified expression 10.8 will yield possible values for vib ... 

The range and sophistication of the scientific models used by chemists to make sense of chemical bonding is one factor which contributes to learners finding this topic difficult. Many of the ideas used to understand chemical bonds would not be accessible at an introductory level. Instead, curricula models need to be used which simplify the topic (Gilbert, 1998). Ideally learners will develop their tool kit of bonding concepts as part of their progression in learning about the subject (Taber, 1995). 

When using simplified Hiickel calculations as a model, we associate the formation of a chemical bond with the formation of a molecular orbital with lower energy than the atomic orbitals containing the valence electrons on the separated atoms. This bonding MO yields particularly high electron density in the space between the two nuclei where the potential energy of the electrons is particularly low. 

In addition to quantum chemistry, which is a theory at a microscopic scale, there is place for macroscopic chemical theories such as chemical thermodynamics. In this context the reformulation of electrochemistry by Hertz [20] also deserves attention. On the other hand, many prequantum theories of the chemical bond had to be abandoned, because they turned out to be inconsistent with quantum chemistry. The theory of me-somerism survived for a while because it was regarded as a mapping to a simplified quantum chemical model, that of the semiempirical valence-bond theory, until both the former and the latter became obsolete. 

The following simplified model of the electronic structure of the transition-metal surface is very useful to discuss the elementary interactions playing a role in the formation of the surface-adsorbate chemical bond. 

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