Atoms modern theory

The modern theory of the behavior Of matter, called quantum mechanics, was developed by several workers in the years 1925-1927. For our purposes the most important result of the quantum mechanical theory is that the motion of an electron is described by the quantum numbers and orbitals. Quantum numbers are integers that identify the stationary states of an atom the word orbital means a spatial description of the motion of an electron corresponding to a particular stationary state. 

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 modern theory of the electronic structure of the atom is based on experimental observations of the interaction of electricity with matter, studies of electron beams (cathode rays), studies of radioactivity, studies of the distribution of the energy emitted by hot solids, and studies of the wavelengths of light emitted by incandescent gases. A complete discussion of the experimental evidence for the modern theory of atomic structure is beyond the scope of this book. In this chapter only the results of the theoretical treatment will be described, These results will have to be memorized as rules of the game, but they will be used so extensively throughout the general chemistry course that the notation used will soon become familiar. 

It has also to be remembered that the band model is a theory of the bulk properties of the metal (magnetism, electrical conductivity, specific heat, etc.), whereas chemisorption and catalysis depend upon the formation of bonds between surface metal atoms and the adsorbed species. Hence, modern theories of chemisorption have tended to concentrate on the formation of bonds with localized orbitals on surface metal atoms. Recently, the directional properties of the orbitals emerging at the surface, as discussed by Dowden (102) and Bond (103) on the basis of the Good-enough model, have been used to interpret the chemisorption behavior of different crystal faces (104, 105). A more elaborate theoretical treatment of the chemisorption process by Grimley (106) envisages the formation of a surface compound with localized metal orbitals, and in this case a weak interaction is allowed with the electrons in the metal. 

This definition of a molecule soon gained popularity. Before modern theories of bonding were developed, Tyndall had clearly assimilated Cannizarro s definition of a molecule when he described the way atoms assemble, when he said, A molecule is a group of atoms drawn and held together by what chemists term affinity . 

Our modern theory of the atom describes it as an electrically neutral sphere with a tiny nucleus at the center, which holds the positively charged protons and the neutral neutrons. The negatively charged electrons move around the nucleus in complex paths, all of which comprise the electron cloud. Table 5.1 summarizes the properties of the three fundamental subatomic particles ... 

The key word in modern theory is evolution . The impressive consistency of the astro-nuclear view of the heavens has established the idea of an evolution of nuclear species which has the same significance for astrophysics as the evolution of living species for biology, it is itself preceded by an evolution of particle or corpuscular species, which would have been very short, lasting iess than i second. This process was of a quite crucial nature in determining the components available to buM up atoms, that is, those stable particles, protons and neutrons, that serve as the buMing-blocks, and the forces that bind them together. 

Modern theories of valency are electrochemical in character, arc concerned with the nature of the atom, and assume union between elements to take place by exchange or sharing of electrons, although theories of how this exchange or sharing takes place differ considerably. 

The periodic system developed from Bohr s atomic theory is of the greatest importance in chemical science because it demonstrates that the properties of the elements depend on their positions in the system. It is immediately apparent that chemical valency depends on the number of loosely-bound electrons in the atom. Thus, the alkali metals have one such electron while the divalent alkaline-earth metals have two, etc. Valency is therefore closely connected with electronic structure and provides the foundation for the modern theory of the chemical bond, the basis of which is to be found in the coupling or transfer of the valency electrons. 

Ever since the discovery by Gomberg in 1900 of the dissociation of hexaphenylethane into triphenylmethyl radicals the search for a theoretical explanation of the phenomenon has been carried on. The modern theory of the stability of the aromatic free radicals attributes it in the main to the resonance of the free valence among many atoms.28... 

G. N. Lewis (1876-1946), the renowned U.S. chemist, was the first to grasp the significance of the electron-pair in molecular structure. He laid the foundation for modern theory of structure and bonding in his treatise on Valence and the Structure of Atoms and Molecules (1923). 

A comprehensive review of non-relativistic and relativistic versions of the random phase approximation may be found in [217]. Some applications are described in [218]. A survey of various aspects of the modern theory of many-electron atoms is presented in [219]. 

Yield.—66% theoretical (10 gms.). Colourless crystals insoluble in water M.P. 61° gives a deep violet coloration with ferric chloride and a bluish-green crystalline precipitate of copper benzoyl acetone with alcoholic copper acetate. This shows the compound to be tautomeric, a little of the enol form being present at ordinary temperatures. The acidity of the hydroxyl group in the enol form is not so marked as it is in the case of the hydroxymethylene compounds nevertheless, the metallic salts of benzoyl acetone and such di-ketones are remarkably stable, and on account of their great crystallising power have been used for the determination of the valency and atomic weight of the rare elements. They are also of importance in the modern theory of co-ordination. (C., 1900,1., 588 B., 34, 2584.)... 

John Dalton did not know about subatomic particles when he developed his atomic theory. Even so, the modern atomic theory (shown on the next page) retains many of Dalton s ideas, with only a few modifications. Examine the comments to the right of each point. They explain how the modern theory differs from Dalton s. 

Then came Gilbert Newton Lewis of the University of California. Just before leaving for France in 1916 as head of the Defense Division of the Chemical Warfare Service, he published a paper which laid the basis of the modern theory of the atom In every atom, said the California professor, is an essential nucleus which remains unaltered. Around this nucleus are cubical shells containing varying numbers of electrons which occupy fixed positions. 

One of the most striking things about the chemistry of the elements is the periodic repetition of properties. There are several groups of elements that show great similarities in chemical behavior. As we saw in Chapter 2, these similarities led to the development of the periodic table of the elements. In this chapter we will see that the modern theory of atomic structure accounts for periodicity in terms of the electron arrangements in atoms. 

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. 

Bohrium (Bh) Named after Danish physicist Niels Bohr (1885-1962), who helped develop the modern theory of the atom... 

This subject is too vast to cover even superficially, but it is worthwhile to show how the modern theory of heterogeneous catalysis is related to the topics in this book. " The key reaction is between the adsorbate molecule, acting as one reactant, and the surface atoms of the catalyst, acting as the second reactant. There is a transfer of electron density between the two reactants leading to chemisorption, and bond-breaking or weakening in the adsorbate. The acid-base character of the surface is matched to that of the adsorbate. ... 

The modern theory of the atom cannot tell you exactly where the electrons in atoms are placed. However, it does define regions in space called orbitals, where there is a 95 percent probability of finding an electron. The lowest energy orbital in any atom is called the Is orbital. In this MiniLab, you will simulate the probability distribution of the Is orbital by noting the distribution of impacts or hits around a central target point. 

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