Modern Concept of Atomic Structure

A nuclear atom viewed in cross section. (The symbol means approximately.) 

This drawing does not show the actual scale. The nucleus is actually much smaller than the atom itself. 

A subatomic particle with no charge located in the atomic nucleus 

What if the average diameter of an atom were 1 cm How tall would you be  

The nucleus contains protons, which have a positive charge equal in magnitude to the electrons negative charge, and neutrons, which have almost the same mass as protons but no charge. The neutrons function in the nucleus is not obvious. They may help hold the protons (which repel each other) together to form the nucleus, but we will not be concerned with that here. The relative masses and charges of the electron, proton, and neutron are shown in Table 3.4. 

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The Elements Symbols for the Elements Dalton s Atomic Theory Formulas of Compounds The Structure of the Atom Introduction to the Modern Concept of Atomic Structure Isotopes... 

Structural formulas serve a key role as devices to facilitate communication of chemical information, but it is important to recognize at the outset that the relationship of a structural formula to molecular structure is a symbolic one. The current system of structural formulas arose largely as a result of chemistry done in the last half of the nineteenth century. Elemental analyses, interrelation of various compounds, and systematic investigation of the reactivity of various functional groups permitted organic chemists to deduce correctly much information about molecular structure. For many molecules, it became possible to draw conclusions as to which atoms were directly connected. Lines drawn between atoms were used to represent direct connections or bonds. These structural deductions predated modern concepts of atomic and molecular structure and of the nature of the forces that bind atoms. With the advent of quantum mechanics and new experimental techniques for accurate determination of such basic structural parameters as bond lengths and bond... 

Marie Sklodowska Curie (1867—1934) and Pierre Curie (1859—1906) are credited with discovering polonium as they sought the source of radiation in pitchblende after they removed the uranium from its ore. Their discovery in 1898 led to the modern concepts of the nucleus of the atom, its structure, and how it reacts. 

This was the state of our knowledge of the structure of the atom when Langmuir, the modern scientific conquistador, attempted to invade the tiny world of the atom. There was an unmistakable conflict between Bohr s theory of the hydrogen atom and the conception of Lewis. Chemists could see but little use in the Bohr atom. They wanted an atom which would explain chemical reactions. The first World War over, Langmuir undertook to reconcile the two theories by publishing his concentric shell theory of atomic structure. 

In the inevitably arbitrary division of any subject it is well to cboose so that it may easily be seen where each part belongs. For this reason the treatment adopted by Lothar Meyer in the later editions of his Modern Theories of Ohemistrij seemed to me appropriate for my lectures in it the whole is divided into Statics and Dynamics. Statics then deals with single substances i.e. with views on the structure of matter, the conception of atoms and molecules, and on constitution so far as the determining of molecular configuration. Dynamics is devoted to the mutual actions of several substances, i. e. to chemical change, affinity, velocity of reaction, and chemical equilibrium. 

We begin this section by considering the concept of a theoretical model chemistry and the development of a numerical spectrometer in section 1. In section 2, we briefly consider the complementary probes of matter which are used in modern science to elucidate the structure and properties of matter on an atomic scale. In section 3, we discuss how the very concept of atoms and molecules depends on the probes employed and how in modern science, computers provide one of the most powerful probes. The complementarity of different probes of matter is briefly described in section 4. The different perspectives given by complementary probes are emphasised and the greater potency acquired when probes are used in conjunction in a problem based environment is underlined. 

To systematize the structural problems of the so-called compounds of higher order he reconsidered valency. Thus, he advanced the idea of Hauptvalenz and Nebenvalenz, which could be said to contain the image of modern ionic and covalent bonding theory. Furthermore, he visualized the valence bond not as small sticks existing around atoms in a plane, as conceived by Kekul6, but as positions taken by bond arms on the surfaces of spherical atoms in three dimensions. Could this idea not be a precursor to the modern conception of bond orbitals ... 

HE STRUCTURAL THEORY of Kekulc has been the growth hormone of organic chemistry. This theory used the atom and the bond as its fundamental concepts. It assumed no knowledge of either the composition of the atom or the nature of the bond. In recent years, however, modern theodes of atoms and bonds have nearly filled these lacunae. 

The study of atomic structure in Chapter 6 provided an interesting detour into modern physics, and there s no doubt that the paradoxes of quantum mechanics are intellectually intriguing. But quite apart from pure scientific curiosity, the concepts we developed for atomic structure offer important insights into some of the most fundamental questions in chemistry. How and why do atoms combine to form molecules What do those molecules actually look like, and which factors determine their shapes In our technological society, the answers to these seemingly abstract questions can have tremendous practical and economic importance. Knowing how to control or selectively exploit the formation of chemical bonds has played a crucial role in the development of many new materials and devices. 

In the last 200 years, vast amounts of data have been accumulated to support atomic theory. When atoms were originally suggested by the early Greeks, no physical evidence existed to support their ideas. Early chemists did a variety of experiments, which culminated in Dalton s model of the atom. Because of the limitations of Dalton s model, modifications were proposed first by Thomson and then by Rutherford, which eventually led to our modern concept of the nuclear atom. These early models of the atom work reasonably well—in fact, we continue to use them to visualize a variety of chemical concepts. There remain questions that these models cannot answer, including an explanation of how atomic structure relates to the periodic table. In this chapter, we will present our modern model of the atom we will see how it varies from and improves upon the earlier atomic models. 

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. 

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