The Electron and Chemical Periodicity

Thomson s discovery of the electron is one of the most celebrated events in the history of physics. What is not so well known is that Thomson had a deep interest in chemistry, which, among other things, motivated him to put forward the first explanation for the periodic table of elements in terms of electrons. Today, it is still generally believed that the electron holds the key to explaining the existence of the periodic table and the form it takes. This explanation has undergone a number of subtle changes.The extent to which the modern explanation is purely deductive or whether it is semiempirical is examined in this chapter. 

While Dimitri Mendeleev had remained strongly opposed to any attempts to reduce, or explain, the periodic table in terms of atomic structure, Julius Lothar Meyer was not so averse to the reduction of the periodic system.The latter strongly believed in the existence of primary matter and also supported William Prout s hypothesis. Lothar Meyer did not hesitate to draw curves through the numerical properties of atoms, whereas Mendeleev beheved this to be a mistake, since it conflicted with his own behef in the individuahty of the elements. 

This is how matters stood before the discovery of the electron, three years prior to the turn of the twentieth century. The atom s existence was stiU very much a matter of dispute, and its substructure had not yet been discovered. There appeared to be no way of explaining the periodic system theoretically.  

Johnston Stoney first proposed the existence and name for the electron in 1891, although he did not beheve that it existed as a firee particle. Several researchers discovered the physical electron, including Emil Wiechert in Koningsberg, who was 

In that same year, Thomson began to think specifically about how the electrons might be arranged in the atom. He concluded that the solar system-hke 

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A non exhaustive description of the history of X-ray Absorption Spectroscopy (XAS) can be found in Ref. 1. The modem EXAFS (Extended X-ray Absorption Fine Structure) technique began in the early seventies of the last century. It corresponds to the concomitance of both theoretical and experimental developments. Between 1969 and 1975, Stem, Sayers and Lytle succeeded in interpreting theoretically the X-ray Absorption Structures observed above an absorption edge [2], while during the same period, the advent of synchrotron radiation (SR) sources reduced drastically the acquisition time of a spectrum if compared to data obtained with conventional X-ray tubes. XAS provides essential information about the local atomic geometry and the electronic and chemical state of a specific atom, for almost any element of the periodic table (Z>5). This prime tool for... 

Silicon is a member of the Group IV elements in the Periodic Table. However, little of the chemistry of silicon can be inferred from carbon, one of its closest neighbors. Although silicon is the second most abundant element in Earth s crust (approximately 26%), it does not exist in nature as a free element. Silicon must be freed from its oxides through a chemical process known as carbothermic reduction. In this reaction, sihca and a carbon source (generally wood) are heated together at extremely high temperatures to yield silicon in its elemental form. The Swedish chemist Jons Jakob Berzelius (1824) was the first to isolate silicon from its natural matrix. Sificon is widely used in the electronics and chemical industries. 

The concept of chemical periodicity is central to the study of inorganic chemistry. No other generalization rivals the periodic table of the elements in its ability to systematize and rationalize known chemical facts or to predict new ones and suggest fruitful areas for further study. Chemical periodicity and the periodic table now find their natural interpretation in the detailed electronic structure of the atom indeed, they played a major role at the turn of the century in elucidating the mysterious phenomena of radioactivity and the quantum effects which led ultimately to Bohr s theory of the hydrogen atom. Because of this central position it is perhaps not surprising that innumerable articles and books have been written on the subject since the seminal papers by Mendeleev in 1869, and some 700 forms of the periodic table (classified into 146 different types or subtypes) have been proposed. A brief historical survey of these developments is summarized in the Panel opposite. 

The physical and chemical properties of the elements show regular periodic trends that can be explained using electron configurations and nuclear charges. We focus on the physical properties of the elements in this section. A preliminary discussion of the chemical properties of some of the elements appears in Section Other chemical properties are discussed after we introduce the principles of chemical bonding in Chapters 9 and 10. 

Classifying the elements by physical and chemical characteristics enabled scientists to assemble periodic tables long before their electron configurations were known. In fact, the first periodic table came before J.J. Thomson discovered the electron and long before Bohr developed electron configurations. 

The physical and chemical properties of an atom are determined by the number and configuration of electrons in its electronic retinue. These are arranged in layers or shells, in a well-defined order. Some atoms have more shells than others, or indeed their shells are more complete and better organised. Chemical properties and molecule formation are determined by the outer shell. This is because only the outer electrons can mediate in chemical bonds, playing the role of a common currency. Atoms in the first column of Mendeleyev s periodic table have a single electron in their outermost shell, whilst those in the second column have two, and so on, until we reach the noble gases which have eight electrons in their outer layer (except for helium, which has two). 

From the above brief discussion it is difficult to imagine how NMR could have had the revolutionary impact on our understanding of the electronic and geometrical structures of molecules that it has had. However, in the period 1949-51, spectrometer systems (particularly magnets) were refined to the point where chemical shifts and nuclear spin-spin splittings became resolvable. It is, of course, from these two rather small effects (small energy-wise) that NMR derives its primary usefulness to chemistry. 

The physical and chemical properties of the elements are directly related to their electron configurations. For example, chemical properties such as gaining, giving and sharing of electrons are dependent on the valence electrons and nucleus structure. As a result, chemical behaviors of the elements are closely related to the nucleus structure and electron configuration of the element. Elements in the same period contain different numbers of electrons in the valence shells. 

Of course, what the students are really interested in is why thallium is poisonous. Surprisingly, thallium is toxic because it mimics potassium in the body. But why would thallium behave like potassium As we study the periodic table and chemical periodicity, there is no immediate reason to suspect that these two elements would have similar properties. A close look at the electron shell arrangement of thallium and potassium, however, reveals that both form +1 ions. Since Tl+ ions also happen to be similar in size to K+ ions, they are able to replace potassium ions in cellular processes. (Thallium poisoning is treated with a compound called Prussian blue, which binds to +1 ions and thus facilitates their removal from the body.) It is clear then that we cannot understand the toxicity of thallium without studying its atomic structure and electron distribution. But chemistry is only part of the story. The effects of thallium poisoning only make sense if the... 

The disorder of the atomic structure is the main feature which distinguishes amorphous from crystalline materials. It is of particular significance in semiconductors, because the periodicity of the atomic structure is central to the theory of crystalline semiconductors. Bloch s theorem is a direct consequence of the periodicity and describes the electrons and holes by wavefunctions which are extended in space with quantum states defined by the momentum. The theory of lattice vibrations has a similar basis in the lattice symmetry. The absence of an ordered atomic structure in amorphous semiconductors necessitates a different theoretical approach. The description of these materials is developed instead from the chemical bonding between the atom, with emphasis on the short range bonding interactions rather than the long range order. 

The precise features of real catalysts at a microscopic scale are rather unknown but in all cases the main interactions occur through a surface. Two different theoretical models are often used to describe the electronic and other microscopic features of a surface. On the one hand, there is the solid state physics approach in which a surface is considered as a slab of a given thickness, finite in the direction perpendicular to the surface and infinite in the two other dimensions with perfect two-dimensional periodical symmetry. On the other hand, one has the cluster model approach which represents the surface with a finite number of atoms and the surface-adsorbate interaction as a supermolecule this is essentially a quantum chemical approach. It is important to realize that both approaches are crude representations of physical reality because real surfaces are far from being perfect, usually... 

Fortunately the chemical formulas can be rationalized on the basis of the size of the central atoms involved it is not necessary to memorize their formulas In general, second-period atoms are hmited to a maximum total coordination number (the total coordination number counts unshared electron pairs in the p-block) of fom third and fourth-period atoms can have maximum total coordination numbers of six fifth and sixth-period atoms can exceed a total coordination number of six. These observations explain the hydrolytic inertness (and persistence in the atmosphere) of compounds such as CF4 and SFe, which contain very strongly acidic cations they also explain why the formulas of fluoro anions vary (e.g. BF4 in period 2 AlFe in period 3 WFg in period 6). Evidently, because of the influence of Jt bonding to oxygen, central atoms in 0x0 anions fail to exhibit these coordination numbers, but instead settle for lower penultimate total coordination numbers 3 in the second period, for example, COs 4 in the third and fourth periods, for example, 8104 and 0004 6 in the fifth and sixth periods. 

The f block elements Lanthanide and actinide elements. These two series often appear with a or in Group IIIB (3), but these elements do not belong to that family. (Note that the transition metals do not belong to group IIA (2), which they follow.) The most common oxidation state for the lanthanides and some of the actinides is +3, hence the popularity of the IIIB (3) position. Because of their remarkable electronic and chemical properties they should be set apart, but most periodic tables give no special numerical appellations to these elements. 

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