Historical Development of Atomic Theory

Although the Greek philosophers Democritus (460-370 bce) and Epicurus (341-270 bce) presented views of nature that included atoms, many centuries passed before experimental studies could establish the quantitative relationships needed for a coherent atomic theory. In 1808, John Dalton published A New System of Chemical Philosophyf in which he proposed that 

When two measures of hydrogen and one of oxygen gas are mixed, and fired by the electric spark, the whole is converted into steam, and if the pressure be great, this steam becomes water. It is most probable then that there is the same number of particles in two measures of hydrogen as in one of oxygen.  

Because Dalton was not aware of the diatomic nature of the molecules H2 and O2, which he assumed to be monatomic H and 0, he did not find the correct formula of water, and therefore his surmise about the relative numbers of particles in measures of the gases is inconsistent with the modem concept of the mole and the chemical equation 2H2 + 02 2H2O. 

In fact, he then changed his mind about the number of molecules in equal volumes of different gases  

At the time I formed the theory of mixed gases, I had a confused idea, as many have, I suppose, at this time, that the particles of elastic fluids are all of the same size that a given volume of oxygenous gas contains just as many particles as the same volume of hydrogenous  

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Systematic presentation of the historic development of atomic theory helps students understand chemical thought and appreciate central ideas of the science. 

The first two chapters serve as an introduction to quantum theory. It is assumed that the student has already been exposed to elementary quantum mechanics and to the historical events that led to its development in an undergraduate physical chemistry course or in a course on atomic physics. Accordingly, the historical development of quantum theory is not covered. To serve as a rationale for the postulates of quantum theory, Chapter 1 discusses wave motion and wave packets and then relates particle motion to wave motion. In Chapter 2 the time-dependent and time-independent Schrodinger equations are introduced along with a discussion of wave functions for particles in a potential field. Some instructors may wish to omit the first or both of these chapters or to present abbreviated versions. 

The book is designed to introduce fundamental knowledge in three areas the history of the atom, the periodic table and radioactivity. We will study the historical development of atomic structure theories, the tendencies of elements in periods and groups, and the types of emissions and uses of radioactivity. 

Acmally, other experimental evidence, such as splitting of atomic spectral lines due to applied magnetic fields, was also available. Furthermore, experience with the quantum theory of orbital angular momentum played a role in the treatment of electron spin. The reader should not think that the historical development of quantum theory of spin was as naive or simple as we make it appear here. 

For this reason, the book has two introductions a historical and philosophical introduction for those interested in the philosophical aspects of the development of atomic theories and a brief technical introduction for those who are not. 

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. 

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. 

While the Epicurean model appeals to us as being closer to the modern view of atoms and is sometimes pointed to as the origin of atomic theory, in historical terms, it had little effect on the development of natural philosophy. The reason for this was twofold. First, the Epicureans were known less for their physical theories than for their ideas about social and personal behavior, favoring the pursuit of pleasure. Our modern use of the term epicure for someone who enjoys good food and drink reflects this. The second reason that... 

In the 1960s and 1970s, Kuhn and others showed philosophers of science that it was futile to insist on a normative view of scientific theories which did not bear much relationship to the historical development of real science. Similarly, the case of atomic orbitals, which I continue to concentrate upon, shows us that it is somewhat unhelpful to insist only on the normative view from quantum mechanics. One needs to also consider what is actually done in chemistry and the fact that chemists get by very well by thinking of orbitals are real objects. In fact we need both views, the normative and the descriptive. Without the normative recommendation orbitals are used a little too naively as in the case of many chemical educators who do so without the slightest idea that orbitals are strictly no more than mathematical fictions. Hopefully my previous work was not in vain if I have managed to convince some people in chemical education to be a little more careful about how far an explanation based on orbitals can be taken. 

Skill 20.2 Demonstrate knowledge of the historical progression in the development of the theory of the atom, including the contributions of Dalton, Thomson, Rutherford, and Bohr. 

The separation of molecular conformations using differences in their ion mobility is generally perceived to require substantial differences in their size and/or shape and thus their cross section for interaction with the buffer gas. Perhaps not as obviously, appreciable changes in cross section can also be induced by variations in the electronic state of an atomic ion, which otherwise would seem to have very similar sizes and shapes. Indeed the development of ion mobility mass spectrometry as we know it today has its roots in this particular application. Exciting prospects for studying electronic state-specific chemistry, especially that of heavier metal elements, is made possible by this interesting observation. In this chapter, the theory behind such separations and the historical development of this area along with recent advances are reviewed. Table 2.1 lists the mobilities of the various atomic cations discussed in the remainder of this chapter. 

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