Atomic structure modem concept

A basic understanding of the quantum theory is essential in many areas of chemistry, especially in connection with spectroscopy and with theories of atomic and molecular structure. The present book gives an introduction to the theory, and its application to elementary atomic structure, but chemical bonding is not discussed. I have tried to put the essential ideas in their historical context, but without retaining the historical introduction which has been traditional with this topic. With the crucial and difficult concepts of wave-particle duality, it seemed to me more important to give modem illustrations to show that they have current applications in chemistry. 

The structure-properties concept has advanced so far that compounds are designed and synthesized in the laboratory with the hope that they will perform very specific functions, such as curing diseases that have been resistant to other forms of treatment. Figure 2.1 shows some of the variety of modem technology that has its roots in the understanding of the atom. 

Building on Dalton s atomic theory, the concept of chemical structure continued to be developed and refined leading, in 1874, to the concept of stereoisomerism developed by van t Hoff and Le Bel [9]. Perhaps, the most important achievement during the latter half of the nineteenth century was the development of the Periodic Table of the elements by Mendeleev [10] and Meyer [11], both of whom were students of Robert Bunsen. The form of Mendeleev s classification, which is the progenitor of today s modem Periodic Table, is based on atomic mass. However, Meyer s work, which was based on valency, also contributed to its development. 

The modem theory of valency is not simple—it is not possible to assign in an unambiguous way definite valencies to the various atoms in a molecule or crystal. It is instead necessary to dissociate the concept of valency into several new concepts—ionic valency, covalency, metallic valency, oxidation number—that are capable of more precise treatment and even these more precise concepts in general involve an approximation, the complete description of the bonds between the atoms in a molecule or crystal being given only by a detailed discussion of its electronic structure. Nevertheless, these concepts, of ionic valency, covalency, etc., have been found to be so useful as to justify our considering them as constituting the modern theory of valency. 

The ideas involved in modem structural chemistry are no more difficult and require for their understanding no more, or little more, mathematical preparation than the familiar concepts of chemistry. Some of them may seem strange at first, but with practice there can be developed an extended chemical intuition which perir-its the new concepts to be used just as confidently as the older ones of the valence bond, the tetrahedral carbon atom, etc., which form the basis of classical structural chemistry. 

Mackay had a long list covering a whole range of transitions from classical crystallographic concepts to what is termed the modem science of structure at the atomic level. This list is reproduced in Table 9-6. There is resonance of several of Mackay s ideas with other directions in modem chemistry, where the non-classical, the... 

The concept of the elements depended on two different but ultimately complementary ideas about matter. The first idea was ancient that the elements were the fundamental building blocks of nature. Whether there were 1, 2, 3, 4, or 92 elements was in a sense less important than the power of the concept to explain nature and direct research. The second idea came with the discovery of the structure of the atom and the physics that made that discovery possible that an element represented a specific combination of subatomic particles determined by physical laws. The creation of controlled nuclear fission and the invention of accelerators and cyclotrons made a kind of modem alchemy possible, allowing the creation of new elements that were not found in nature but that still met the new conditions to be considered elements. 

In the following sections we summarize how these conclusions lead to our present view of the structure of the atom and, in particular, the nature and arrangement of the electrons in the atom. We are not attempting to summarize modem physics, but only the concepts that we use in this text. You need to understand the main aspects of the nature of the chemical bond in ceramic materials what is an ionic bond, what is a covalent bond, and why do most... 

Since ancient times humans have pondered the nature of matter. Our modem ideas of the structure of matter began to take shape in the early nineteenth century with Dalton s atomic theory. We now know that all ordinary matter is made up of atoms, molecules, and ions. All of chemistry is concerned with the nature of these species and their transformations. Over the past two centuries, chemists have developed a lexicon of terms and concepts that allows them to accurately and efficiently discuss chemical ideas among themselves and communicate these ideas to others. This language of chemistry provides us a way to visualize and quantify chemical transformations at the molecular level while simultaneously understanding the consequences of these transformations in the maaoscopic world in which we live. 

Concepts and Methods in Modem Theoretical Chemistry Electronic Structure and Reactivity, the first book in a two-volume set, focuses on the structure and reactivity of systems and phenomena. A new addition to the series Atoms, Molecules, and Clusters, this book offers chapters written by experts in their fields. It enables readers to learn how concepts from ab initio quantum chemistry and density functional theory (DFT) can be used to describe, understand, and predict electronic structure and chemical reactivity. 

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