Some Theoretical Atomic Models

The earliest concept of atomic structure dates back to Greece in the fifth century bce, when Leucippus and Democritus postulated that tiny particles of matter, which they called atomos, were indivisible. 

Over the centuries, many other concepts were proposed to explain the nature of matter— many of them extensions of the Greek concept of an ultimately indivisible and indestructible elementary bit of matter. But it was not until J. J. Thomson proposed his model of the atom, consisting of a sphere with an agglomeration of particles with negative electric charges somehow positioned randomly inside a very small ball of matter, that the modern structure of the atom began to take shape. 

The Bohr model of the atom took shape in 1913. Niels Bohr (1885-1962), a Danish physicist, started with the classic Rutherford model and applied a new theory of quantum mechanics to develop a new model that is still in use, but with many enhancements. His assumptions are based on several aspects of quantum theory. One assumption is that light is emitted in tiny bunches (packets) of energy call photons (quanta of light energy). 

the orbital or quantum theory of matter assumes that the electron is not a particle, as we normally think of particles. Orbital theory considers the electron as a three-dimensional wave that can exist at several energy levels (orbitals), but not at the same time. 

Second, the electrons are in constant motion around the nucleus, even though it is not easy to determine the position of a particular electron in its shell at any particular moment. 

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The first sections of this reference book set the stage for the presentation of the elements. First is the section How to Use This Book followed by a short introduction. Next is A Short History of Chemistry, the narrative of which progresses from prehistoric times to the Age of Alchemy and then to the Age of Modern Chemistry. Next is the section titled Atomic Structure, which traces the history of our knowledge of the structure of the atom some theoretical models, including quantum mechanics the discovery of subatomic (nuclear) particles... 

Values of the Taft steric constants are collected in Table S-2 for some substituents. A Model of the Frontier Steric Effect is a theoretical approach recently proposed to estimate the Taft steric constant of substituents on the basis of the fundamental characteristics of the constituent atoms [Cherkasov and Jonsson, 1998b] ... 

Eustathopoulos has also summarized some theoretical calculations of a for binary alloys, which are based on lattice models in which the interface layers are treated in the Bragg-Williams approximation, assuming complete atomic disorder in the interface and in the crystal. The calculated surface free energies are related to the surface free energies of the pure components and to the activities in the bulk phases. 

The Lewis model has become so famous and has for years been used so mechanically that some of the considerations underlying it are here presented. They demonstrate how deeply Lewis was concerned with the underlying physical cause of valence rather than with a simple rule of thumb. He further points up an operational difficulty in Bohr s atomic model if it gives no information as to the electron s movement within an orbit, we have no business postulating its movement. Furthermore, electrons orbiting around a nucleus could not possibly explain directed valences. Lewis, therefore, proposed a static model, confident that theoretical chemistry would somehow, some day, confirm it ... 

The polarizability of some neutral atoms (H, Li, K, Cs) has been recently determined by a method similar to the Stern-Gerlach experiment ( 7, p. 166), namely, by measuring the deflexion of a beam of atoms in an inhomogeneous electric field (Stark, 1936). The results do not agree very well with theoretical computations from atomic models. 

Na(3 i/) + CO. Extensive experimental data have been reported for the quenching of excited Na atoms by various gases, most of it dealing with the overall quenching cross-sections. In order to understand the mechanism by which the electronic energy is released in the course of a collision, it would be extremely helpful to have detailed microscopic data. Theoretical developments have reached the stage where detailed dynamical properties can now be predicted or correlated. By means of CO laser absorption, the vibrational population distribution of the excited CO produced in the reaction Na(3 / ,/2,3/2) + CO(u = 0)- Na(3, S) + CO(u) has been measured and compared with some theoretical models. ... 

In this section we report some theoretical work on the molecular spectroscopy in rare gas matrices. Due to the difficulty in modelling the environment, theoretical studies for a long time compared the results obtained from gas phase calculations with experimental data in matrices. However, it has been demonstrated that the effect of the matrix is not negligible, and may strongly depend on the rare gas, as discussed recently by Li et al [186]. Even if relativistic pseudopotentials lead to a substantial reduction of computational costs, one still needs to treat explicitly a certain number of valence electrons, for instance eight electrons for each rare gas (Rg) atom. A further simplification is to consider a rare gas atom as a zero-electron system with all its electrons in the core represented by the so-called zero-electron pseudopotential or e-Rg pseudopotential (for further details see Grofi and Spiegelmaim [187,188] and references therein). As... 

We will now attempt to extract qualitative models for the chemical interpretation of NMR chemical shifts from the admittedly sometimes quite complicated theoretical formalisms outlined above. And we will provide examples of explicit quantum chemical calculations that corroborate the proposed models. The most widely studied field, in which much insight has been obtained recently, are SO effects of atoms neighboring some heavy-atom SO center . A significantly revised view of heavy -atom effects on chemical shifts has arisen from the theoretical studies. 

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