Atomic theory electron, discovery

This system of nomenclature has withstood the impact of later experimental discoveries and theoretical developments that have since the time of Guyton de Morveau and Lavoisier greatiy altered the character of chemical thought, eg, atomic theory (Dalton, 1802), the hydrogen theory of acids (Davy, 1809), the duahstic theory (Berzehus, 1811), polybasic acids (Liebig, 1834), Periodic Table (Mendeleev and Meyer, 1869), electrolytic dissociation theory (Arrhenius, 1887), and electronic theory and modem knowledge of molecular stmcture. 

From 50 years to 100 years after Dalton proposed his theory, various discoveries showed that the atom is not indivisible, but really is composed of parts. Natural radioactivity and the interaction of electricity with matter are two different types of evidence for this subatomic structure. The most important subatomic particles are listed in Table 3-2, along with their most important properties. The protons and neutrons occur in a very tiny nucleus (plural, nuclei). The electrons occur outside the nucleus. 

A series of episodes in the historical development of our view of chemical atoms are presented. Emphasis is placed on the key observations that drove chemists and physicists to conclude that atoms were real objects and to envision their stracture and properties. The kinetic theory of gases and measmements of gas transport yielded good estimates for atomic size. The discovery of the electrorr, proton and neutron strongly irtfluenced discttssion of the constitution of atoms. The observation of a massive, dertse nucleus by alpha particle scattering and the measrrrement of the nuclear charge resrrlted in an enduring model of the nuclear atom. The role of optical spectroscopy in the development of a theory of electronic stracture is presented. The actors in this story were often well rewarded for their efforts to see the atoms. 

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. 

The most recent advance in the theory of the helium atom was the discovery of its classically chaotic nature. In connection with modern semiclassical techniques, such as Gutzwiller s periodic orbit theory and cycle expansion techniques, it was possible to obtain substantial new insight into the structure of doubly excited states of two-electron atoms and ions. This new direction in the application of chaos in atomic physics was initiated by Ezra et al. (1991), Kim and Ezra (1991), Richter (1991), and Bliimel and Reinhardt (1992). The discussion of the manifestations of chaos in the helium atom is the focus of this chapter. 

On the basis of Dalton s atomic theory, we can define an atom as the basic unit of an element that can enter into chemical combination. Dalton imagined an atom that was both extremely small and indivisible. However, a series of investigations that began in the 1850s and extended into the twentieth century clearly demonstrated that atoms actually possess internal structure that is, they are made up of even smaller particles, which are called subatomic particles. This research led to the discovery of three such particles—electrons, protons, and neutrons. 

The influence of chemistry on physics is less direct. There have been important investigations in chemistry that led to developments in physics. The discovery of the third law of thermodynamics was a result of low temperature chemical equilibrium studies by Nernst. Chemical studies undoubtedly played a significant role in the early stages of the development of electromagnetism. In fact, it is electrochemical investigations by Faraday that led GJ. Stoney to coin the word electron and to estimate its charge (1874) before it was detected by J.J. Thomson in the gas phase (1897). To this list, we should add the modern atomic theory, which took root in chemistry before it found its way into physics. 

Thomson concluded that the mass of the charged particle was much less than that of a hydrogen atom, the lightest known atom. The conclusion was shocking because it meant there were particles smaller than the atom. In other words, Dalton had been incorrect—atoms were divisible into smaller subatomic particles. Because Daltons atomic theory had become so widely accepted and Thomsons conclusion was so revolutionary, many other scientists found it hard to accept this new discovery. But Thomson was correct. He had identified the first subatomic particle—the electron. He received a Nobel Prize in 1906 for this discovery. 

When Dalton stated his atomic theory in the early 1800s, he assumed all of the atoms of a given element were identical. This idea persisted for over a hundred years, until James Chadwick discovered that the nuclei of most atoms contain neutrons as well as protons. (This is a good example of how a theory changes as new observations are made.) After the discovery of the neutron, Dalton s statement that all atoms of a given element are identical had to be changed to "All atoms of the same element contain the same number of protons and electrons, but atoms of a given element may have different numbers of neutrons."... 

In previous chapters we have seen how Dalton s atomic theory encountered difficulties, especially during the first half of the nineteenth century. After 1860 the chemical atom became so useful, especially to organic chemists, that only a few dissenting voices were heard in this branch of the science. However, to many physicists the atomic concept seemed unnecessary, and some of the great advances in nineteenth century physics had been made without reference to atomism. Then, in a series of discoveries which were made in rapid succession around the turn of the century, the existence of atoms was established to the satisfaction of everyone. All atoms were shown to contain identical subatomic particles, which were called electrons. Furthermore, certain atoms were shown to be undergoing spontaneous and continuous transmutation into others. The atomic concept may have been vindicated, but at the cost of disproving Dalton s tenets of the indivisibility and immutability of atoms. Further advances placed the periodic table on a firm theoretical basis, and provided an explanation of the forces involved in chemical bonding. 

Accounts of some important developments in atomic theory appear on the World Wide Web for example, an interesting online history of the discovery of the electron is at www.aip.org/history/electron... 

In the late nineteenth and early twentieth centuries, major discoveries about atoms and electrons placed theories of molecular structure and bonding on a more secure, physics-based foundation. Several of these are described at the beginning of this section. 

What is scanning tunneling microscopy How does it work 2. Summarize the history of the atomic idea. How was Dalton able to convince others to accept an idea that had been controversial for 2000 years 3. State and explain the law of conservation of mass. 4. State and explain the law of definite proportions. 5. State and explain the law of multiple proportions. How is the law of multiple proportions different from the law of definite proportions 6. What are the main ideas in Dalton s atomic theory How do they help explain the laws of conservation of mass, of constant composition, and of definite proportions 7. How and by whom was the electron discovered What basic properties of the electron were reported with its discovery 8. Explain MUlikan s oil drop experiment and how it led to the measurement of the electron s charge. Why is the magnitude of the charge of the electron so important 9. Describe the plum-pudding model of the atom. 

We begin this chapter with a brief survey of early chemical discoveries, culminating in Dalton s atomic theory. This is followed by a description of the physical evidence leading to the modern picture of the nuclear atom, in which protons and neutrons are combined into a nucleus with electrons in space surrounding the nucleus. We will also introduce the periodic table as the primary means of organizing elements into groups with similar properties. Finally, we will introduce the... 

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