The discovery of electrons

At the time of its discovery, the electron was considered a particle and this is still a popular opinion. The reason was that the electron has a well-defined charge and mass as other particles. For most practical purposes, the electron behaves as a particle, thus it is easy to assume that it is a particle. The same applies to the photon. 

As we will see next, electrons are quantum mechanical wave packets. Several wavelengths are present in the wave packet. The wave packet can be proven to have velocity and acceleration consistent with electronic mass and charge, measured under circumstances where the particle characteristics of the electron are unquestioned (such as the Millikan oil drop experiment). 

From the time of Dalton, atoms were indivisible. The discovery of the electron by J. J. Thomson in 1897 was the first hint of the existence of particles smaller than atoms. Thomson s discovery allowed speculation about the interior structure of the atom and extended the hope that such speculation could be verified experimentally. 

In the highly evacuated tube, cathode rays are emitted from the cathode C. Two slotted metal plates A and A serve as anodes. Passage through the two slots collimates the beam, which then moves in a straight line to hit the spot P at which the fluorescence appears. An 

The electrical field E is applied, which pulls the beam downward and deflects the spot to P the magnetic field, with a flux density B is applied and adjusted so that the spot returns to the original position P. If the beam consists of particles of charge e and mass m, then the force on the beam due to the electrical field is eE, and that due to the magnetic field is Bev, where v is the horizontal component of velocity of the particle. Since these forces are in balance, eE = Bev, and we obtain the horizontal velocity component in terms of E and B  

In the second experiment, the magnetic field is turned off, and the deflection PP under the electrical field only is measured. Since the force is eE, the vertical acceleration is eE/m. The time to pass through the field is t = L/v. After this time, the vertical component of velocity w = (eE/m)t in this same time the vertical displacement is s = j(eE/m)t. The value of 5 can be calculated from the displacement PP and the length L. Using the value for t, we have e/m = 2sv /EL, and using the value for v from Eq. (19.1) 

The experiment yields the value of e/m for the particles. The present value of this ratio is 

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Cathodoluminescence (CL), i.e., the emission of light as the result of electron-beam bombardment, was first reported in the middle of the nineteenth century in experiments in evacuated glass tubes. The tubes were found to emit light when an electron beam (cathode ray) struck the glass, and subsequendy this phenomenon led to the discovery of the electron. Currendy, cathodoluminescence is widely used in cathode-ray tube-based (CRT) instruments (e.g., oscilloscopes, television and computer terminals) and in electron microscope fluorescent screens. With the developments of electron microscopy techniques (see the articles on SEM, STEM and TEM) in the last several decades, CL microscopy and spectroscopy have emerged as powerfirl tools for the microcharacterization of the electronic propenies of luminescent materials, attaining spatial resolutions on the order of 1 pm and less. Major applications of CL analysis techniques include ... 

Thomson s momentous discovery of the electron 100 years ago this year is a story familiar to anyone who has enrolled in an undergraduate chemistry course. His experiments with cathode-ray tubes allowed him to determine the charge-to-mass ratio of the electron—with a mass some 1,000 times less than the smallest particle previously found—and to establish that it was a component of all matter. Thus Thomson earned a place in the annals of physics—and the honor of a centenary. We might also, however, take note of another contribution Thomson made, one that is not so widely known. 

Mendeleev s reluctance toward reduction was not widely shared. One of the codiscoverers of the periodic system, the German Lothar Meyer, accepted the possibility of primary matter and supported Prouf s hypothesis. He was also happy to draw curves through numerical data, including his famous plot of atomic volumes that showed such remarkable periodicity that it helped in the acceptance of the periodic system. Nonetheless, prior to Thomson s discovery of the electron, no accepted model of atomic substructure existed to explain the periodic system, and the matter was still very much in dispute. 

Electrons. If the discovery of isotopes threatened ro undermine the periodic system, the discovery of the electron explained many of the periodic properties on which the table was based. J. J. Thomson attempted to explain the periodic system by postulating rings of electrons embedded in the positive charge that made up his phim pudding model of the atom. Thomson s model was quickly superseded by more sophisticated and elaborate mod-... 

After the discovery of the electron many efforts were made to develop an electronic theory of the chemical bond. A great contribution was made in 1916 by Gilbert Newton Lewis, who proposed that the chemical bond, such as... 

The discovery of the electron prompted a series of more sophisticated experiments. J. J. Thomson experimented with a device called a cathode-ray tube, illustrated in Figure 2-13. A cathode ray is a beam of electrons. Because an electron beam is a collection of moving electrical charges, electrical and magnetic forces affect the beam. Application of either type of force at right angles to the direction of electron motion causes the beam to bend. The... 

I.W. Griffiths, J.J. Thomson—the centenary of his discovery of the electron and of his invention of mass spectrometry, Rapid Commn. Mass Spectrom., 11 (1997) 2-16. 

Griffiths, I.W. J. J. Thomson - the Centenary of His Discovery of the Electron and of His Invention of Mass Spectrometry. Rapid Commun. Mass Spectrom. 1997,11, 1-16. 

The first of these chapters, by Carmen Giunta, concentrates on the evidence that atoms are composite—not the ultimate particles of matter. Evidence for the divisibility and impermanence of atoms was collected even while some chemists and physicists continued to doubt their very existence. The chapter focuses on discoveries of the electron, the nucleus, and the heavy particles of the nucleus. Girmta is Professor of Chemistry at Le Moyne College in Syracuse, New York, and he maintains the Classic Chemistry website. 

Arabatzis, T. Rethinking the discovery of the electron. Studies in the History and Philosophy of Modem Physics 1996, 27, 405 35. 

In the nineteenth century the valence bond was represented by a line drawn between the symbols of two chemical elements, which expressed in a concise way many chemical facts, but which had only qualitative significance with regard to molecular structure. The nature of the bond was completely unknown. After the discovery of the electron numerous attempts were made to develop an electronic theory of the chemical bond. These culminated in the work of Lewis, who in... 

Evidence for the existence of neutrons did not come until many years after the discoveries of the electron and the proton. Give a possible explanation. 

Models for the electronic structure of polynuclear systems were also developed. Except for metals, where a free electron model of the valence electrons was used, all methods were based on a description of the electronic structure in terms of atomic orbitals. Direct numerical solutions of the Hartree-Fock equations were not feasible and the Thomas-Fermi density model gave ridiculous results. Instead, two different models were introduced. The valence bond formulation (5) followed closely the concepts of chemical bonds between atoms which predated quantum theory (and even the discovery of the electron). In this formulation certain reasonable "configurations" were constructed by drawing bonds between unpaired electrons on different atoms. A mathematical function formed from a sum of products of atomic orbitals was used to represent each configuration. The energy and electronic structure was then... 

Atomic spectra, which historically contributed extensively to the development of the theory of the structure of the atom and led 10 the discovery of the electron and nuclear spin, provide a method of measuring ionization potentials, a method for rapid and sensitive qualitative and quantitative analysis, and data for the determination of the dissociation energy of a diatomic molecule. Information about the type of coupling of electron spin and orbital momenta in the atom can be obtained with an applied magnetic field. Atomic spectra may be used to obtain information about certain regions of interstellar space from the microwave frequency emission by hydrogen and to examine discharges in thermonuclear reactions. 

The idea of electronegativity was bom as soon as chemists suspected that the formation of chemical compounds involved electrical forces (before the discovery of the electron) metals and nonmetals were seen to possess opposite appetites for the electrical fluid(s) of eighteenth century physics. This electrochemical dualism is most strongly associated with Berzelius [136], and is clearly related to our qualitative notion of electronegativity as the tendency of a species to attract electrons. Parr and Yang have given a sketch of attempts to quantify the idea [137]. Electronegativity is a central notion in chemistry. 

These laws (determined by Michael Faraday over a half century before the discovery of the electron) can now be shown to be simple consequences of the electrical nature of matter. In any electrolysis, an oxidation must occur at the anode to supply the electrons that leave this electrode. Also, a reduction must occur at the cathode removing electrons coming into the system from an outside source (battery or other DC source). By the principle of continuity of current, electrons must be discharged at the cathode at exactly the same rate at which they are supplied to the anode. By definition of the equivalent mass for oxidation-reduction reactions, the number of equivalents of electrode reaction must be proportional to the amount of charge transported into or out of the electrolytic cell. Further, the number of equivalents is equal to the number of moles of electrons transported in the circuit. The Faraday constant (F) is equal to the charge of one mole of electrons, as shown in this equation ... 

The discovery of the electron first showed that the physics and chemistry of the nineteenth century were inadequate. The electron is a particle nearly two thousand times lighter than a hydrogen atom and electrons are contained in every kind of matter. At oms contain electrons and so are not merely indi-visible hard parthJeg The existence of electrons was established about 1897 by J. J. Thomson in the Cavendish Laboratory at Cambridge, England. Wiechert and Kaufmann in Germany at the same time also carried out experiments which indicated the existence of electrons. The theory of electrons was rapidly developed by J. J. Thomson, H. A. Lorentz, and many others. 

Here was the counterpart of the negative electron. This positive charge of electricity was like the electron it could be deflected by powerful magnets, it obeyed the same laws, The great difference between them lay in their different masses— the positive particles in the nucleus were almost two thousand times as heavy as the electron. A few months later, at the Cardiff meeting of the British Association in Wales, Rutherford christened the new arrival proton, just as twenty-two years before Thomson had announced the discovery of the electron. 

Even before the discovery of the electron, Hendrik A. Lorentz of Amsterdam had come to the conclusion that these... 

Of the 20th century s development of structural chemistry, we mention the discovery of the electron-pair covalent bond by Lewis [22] which remains a fundamental tenet. It is remembered in every line we have drawn to represent a linkage and is present in most models of molecular structure, such as, for example, the valence shell electron pair repulsion (VSEPR) model [23]. 

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