Atomic theory historical development

Alan J. Rocke, Atoms and Equivalents The Early Development of the Chemical Atomic Theory, Historical Studies in the Physical Sciences 9, 1978, 225-263 idem. Chemical Atomism in the Nineteenth Century (Ohio State University Press, 1984). 

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. 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

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. 

There are several reasons for starting this account with a discussion of electromagnetic radiation. Historically, it was in this area that the quantum theory first developed. It is easier here to understand the evidence for the theory, and to appreciate some of its paradoxical consequences, than it is in the quantum theory of matter. The applications of the light-quantum hypothesis, as it was first called, also provide key pieces of evidence for the quantization of energy in atoms and molecules. Studies of the absorption and emission of radiation—the field of spectroscopy—and of the effect of light on chemical reactions—photochemistry—are very important areas of modem chemistry, in which the quantum nature of radiation is crucial. 

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. 

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... 

He was one of the earliest supporters of the doctrine of bodies combining in definite proportions, or of what is commonly called the atomic theory. Previous to introducing a paper of his in which this doctrine is happily illustrate by new and curious facts, I shall give a short historical sketch equally candid and perspicuous of the origin of this doctrine, and of its progress, vdiich forms a part of the Discourse he delivered as President of the Royal Society on the awardment of the first Royal medal to Mr. Dalton "for the developement of Ae chemical theory of definite proportions." -He proceeds, - "What Mr Daltons merits are, I shall briefly endeavor to state to you, though it is impossible to do justice to them in the time necessarily allotted to this address.""... 

If, contrary to the order of historical development, we have discussed the quantum theory of the atom before quantum statistics, we have our reasons. In the first place, the failure of- the classical theory displays itself in atomic mechanics—for instance, in the explanation of line spectra or the diffraction of electrons—even more immediately than in the attempts to fit the law of radiation into the frame of classical physics. In the second place, it is an advantage to understand the mechanism of the individual particles and the elementary processes before proceeding to set up a system of statistics based upon the quantum idea. 

Historically important in the development of modern atomic theory was the recognition that although polyatomic molecules show more or less broad bands of absorption and emission in the visible and ultraviolet regions of the spectrum, the characteristic light absorption or emission by individual atoms occurs at fairly narrow lines of the spectrum, which correspond to sharply defined wavelengths. The line spectrum of each element is so uniquely characteristic of that element that atomic spectroscopy can be used for precise elementary analysis of many types of chemically complex materials. 

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. 

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