Shell structure, elementary

Mayer, M. Goeppert, and J. H. D. Jensen, Elementary Theory of Nuclear Shell Structure (New York and London John Wiley and Sons, Inc., 1955). 

M. Mayer, Elementary Theory of Nuclear Shell Structure, Wiley, New York... 

Feenberg, E. The shell structure of the nucleus. Princeton The University Press 1954. — Mayer, M. G., and J. H. Jensen Elementary theory of nuclear shell structure. New York John Wiley 1955. 

Fig. 2 Atomic shell structure as it emerges from electron-density optimization on a golden spiral. The variable convergence angle of An/ In — 1) manifests in the appearance of 2n — 1 additional cycles s, p, d, /) in each interval between Bohr levels n and n — I, shown here as elementary ripples. In contrast to the Bohr-Schrodinger (BS) model, closed shells in the Ford-circle simulation (FC) invariably coincide with noble-gas configurations...

In Chap. 3 the elementary structure of the atom was introduced. The facts that protons, neutrons, and electrons are present in the atom and that electrons are arranged in shells allowed us to explain isotopes (Chap. 3), the octet rule for main group elements (Chap. 5), ionic and covalent bonding (Chap. 5), and much more. However, we still have not been able to deduce why the transition metal groups and inner transition metal groups arise, why many of the transition metals have ions of different charges, how the shapes of molecules are determined, and much more. In this chapter we introduce a more detailed description of the electronic structure of the atom which begins to answer some of these more difficult questions. 

These properties of the d-shell chromophore (group) prove the necessity of the localized description of d-electrons of transition metal atom in TMCs with explicit account for effects of electron correlations in it. Incidentally, during the time of QC development (more than three quarters of century) there was a period when two directions based on two different approximate descriptions of electronic structure of molecular systems coexisted. This reproduced division of chemistry itself to organic and inorganic and took into account specificity of the molecules related to these classical fields. The organic QC was then limited by the Hiickel method, the elementary version of the HFR MO LCAO method. The description of inorganic compounds — mainly TMCs,— within the QC of that time was based on the crystal field... 

Carbon, of course, is the ideal element for simple-minded chemists. Obey a few elementary valency rules, and almost any organic structure you can doodle would exist, if you could make it. And this is perhaps the justification for my own simple-minded contribution to the story of C60. For many years I have maintained a scientific alter ego, Daedalus, whose musings used to appear in New Scientist but now appear in Nature. Daedalus launches scientific proposals which are intended to fall in that uneasy no-man s-land between the clearly feasible and the clearly fantastic. His aim is inevitably rather erratic, and many of the attempts land on one side or the other. An account of some of Daedalus s chemical proposals has appeared in Chemistry in Britain (Jones 1987). His greatest moment came late in 1966, when he proposed the hollow-shell graphite molecule (Jones 1966). 

Prompted by the structure of the periodic table of the elements, electrons were assumed to occur in concentric shells around the nucleus with a positive charge of Z units, equal to the number of extranuclear electrons. In any period of 8 elements, arranged in order of increasing Z, electrons are postulated to occupy an increasing number of sites (from 1 to 8) at the corners of a cube centred at the nucleus. Any vacancy in the shell of eight enables the relevant atom to share an electron with a neighbouring atom to form a covalent bond and to complete the octet of electrons for that shell. This view has now endured for almost hundred years and still forms the basis for teaching elementary chemistry. The simple planetary model, proposed by Bohr, allows for only one electron per orbit and has little in common with the Lewis model. 

We now turn from the use of quantum mechanics and its description of the atom to an elementary description of molecules. Although most of the discussion of bonding in this book uses the molecular orbital approach to chemical bonding, simpler methods that provide approximate pictures of the overall shapes and polarities of molecules are also very useful. This chapter provides an overview of Lewis dot structures, valence shell electron pair repulsion (VSEPR), and related topics. The molecular orbital descriptions of some of the same molecules are presented in Chapter 5 and later chapters, but the ideas of this chapter provide a starting point for that more modem treatment. General chemistry texts include discussions of most of these topics this chapter provides a review for those who have not used them recently. 

Clay-based nanocomposites generate an overall improvement in physical performances. The most widely used ones are the phyllosilicates (smectites). They have a shell-shaped crystalline stiucture with nanometric thickness. Clays are classified according to their crystalline structures and also to the quantity and position of the ions within the elementary mesh. The elementary or primitive mesh... 

The band calculation of Ce is a formidable challenge to theorists. On the one hand there is ample evidence that the f level is very close to the Fermi level so that its population can be influenced by temperature and pressure, etc. (see ch. 4 sections 2.3 and 5-and ch. 9 section 2.1), but on the other hand we can not treat the f levels as a band because the strong Coulomb correlation effect within the shell of each atom makes the f state excitations highly localized in space. For this reason it is meaningless to discuss the band structure of Ce because it implies the treatment of the elementary excitations as Bloch states. Nevertheless band calculations have been done and the results are at times rather suggestive. 

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