Wave properties, of electrons

Here, the orbital phase theory sheds new light on the regioselectivities of reactions [29]. This suggests how widely or deeply important the role of the wave property of electrons in molecules is in chemistry. 

Molecular properties and reactions are controlled by electrons in the molecules. Electrons had been thonght to be particles. Quantum mechanics showed that electrons have properties not only as particles but also as waves. A chemical theory is required to think abont the wave properties of electrons in molecules. These properties are well represented by orbitals, which contain the amplitude and phase characteristics of waves. This volume is a result of our attempt to establish a theory of chemistry in terms of orbitals — A Chemical Orbital Theory. 

Wolfgang Pauh (1900-1958), an American physicist, was awarded a Nobel Prize in 1945 for developing the exclusion principle. In essence, it states that a particular electron in an atom has only one of fom energy states and that all other electrons are excluded from this electron s energy level or orbital. In other words, no two electrons may occupy the same state of energy (or position in an orbit around the nucleus). This led to the concept that only a certain number of electrons can occupy the same shell or orbit. In addition, the wave properties of electrons are measmed in quantum amounts and are related to the physical and, thus, the chemical properties of atoms. These concepts enable scientists to precisely define important physical properties of the atoms of different elements and to more accmately place elements in the periodic table. 

Erwin Schrodinger (1887-1961) and others considered the wave properties of electrons and proposed that electrons were not orbiting around the nucleus in an atom but were in electron-cloud probability areas outside the atomic nucleus. These probability areas were designated as energy levels. 

In 1926 and 1927, Schrodinger and Heisenberg published papers on wave mechanics (descriptions of the wave properties of electrons in atoms) that used very different mathematical techniques. In spite of the different approaches, it was soon shown that their theories were equivalent. Schrodinger s differential equations are more commonly used to introduce the theory, and we will follow that practice. 

Figure 17 shows the present-day model of the atom, which takes into account both the particle and wave properties of electrons. According to this model, electrons are located in orbitals, regions around a nucleus that correspond to specific energy levels. Orbitals are regions where electrons are likely to be found. Orbitals are sometimes called electron clouds because they do not have sharp boundaries. When an orbital is drawn, it shows where electrons are most likely to be. Because electrons can be in other places, the orbital has a fuzzy boundary like a cloud. 

Clinton Joseph Davisson (1881-1958). American physicist. He and G. P. Thomson shared the Nobel Prize in Physics in 1937 for demonstrating wave properties of electrons. 

Lester Halbert Germer (1896-1972). American physicist. Discoverer (with Davisson) of the wave properties of electrons. 

In the Bohr-Sommerfeld theories of the atom, the electrons are moving in orbits which are precisely specified, and the velocities are given exactly. Those theories are therefore concerned with properties which cannot be measured precisely. This difficulty is avoided, however, if one develops theories based on the wave properties of electrons we have already seen, with reference to Figure 1.1, that such theories remove some of the arbitrariness inherent in the Bohr-Sommerfeld approach. Modern theories of atoms and molecules are, therefore, wave theories, which have led to a very considerable increase in our understanding. In the remainder of this chapter we will describe aspects of wave mechanics, or quantum mechanics, that will be of help to biologists in appreciating the nature of the molecular structures with which they are concerned. 

Rutherford and his coworkers might have discovered the wave properties of electrons. 

The wave properties of electrons are used in the operation of an electron microscope. 

George Paget Thomson (1892-1975). English physicist. Son of J. J. Thomson, he received the Nobel Prize in Physics in 1937, along with Clinton Davisson, for denaonstrating the wave properties of electrons. 

The main difference between the two models is that, while Bohr considered the electrons to be traditional particles whose motion could be described by the classical mechanics of Newton, the quantum mechanical model treats the electrons as waves. The wave properties of electrons provide a logical explanation for the existence of allowed orbits in Bohr s atomic model. 

The wave property of electrons is shown directly in electron diffraction, and the electron microscope. As described earlier the resolution of a microscope is determined by the wavelength of the analysing wave. In an electron microscope the resolution is controlled by the acceleration given to the electrons, since, from the de Broglie relationship, high velocity electrons have shorter wavelengths than low velocity electrons. The wave properties of neutrons are apparent in neutron... 

The discovery of the wave properties of electrons and other sub-atomic particles led to Heisenberg s uncertainty principle, which states that there is an inherent physical limit to the accuracy with which the position and velocity of any particle can be measured simultaneously. 

In 1926, the Austrian physicist Erwin Schrodinger used the hypothesis that electrons have a dual wave-particle nature to develop an equation that treated electrons in atoms as waves. Unlike Bohr s theory, which assumed quantization as a fact, quantization of electron energies was a natural outcome of Schrodinger s equation. Only waves of specific energies, and therefore frequencies, provided solutions to the equation. Along with the uncertainty principle, the Schrodinger wave equation laid the foundation for modem quantum theory. Quantum theory describes mathematically the wave properties of electrons and other very small particles. 

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