Chemistry, atomic theory

With the advent of modern atomic theory began a new major enterprise in the science of chemistry that of understanding the bulk properties of materials in terms of the properties and machinations of their component submicroscopic atoms and molecules. The enterprise moved very slowly into its current central role in the science of chemistry. Atomic theory won acceptance among chemists only slowly and, in many quarters, grudgingly. Antiatomist views were common among chemists into the late nineteenth-century, and agnostic views persisted until well into the twentieth (e.g.. Smith, 1910, pp. 224-225), despite the experimental work of Thomson, Rutherford, and other physicists on the structure of the atom. 

The dawn of the nineteenth century saw a drastic shift from the dominance of French chemistry to first English-, and, later, German-influenced chemistry. Lavoisier s dualistic views of chemical composition and his explanation of combustion and acidity were landmarks but hardly made chemistry an exact science. Chemistry remained in the nineteenth century basically qualitative in its nature. Despite the Newtonian dream of quantifying the forces of attraction between chemical substances and compiling a table of chemical affinity, no quantitative generalization emerged. It was Dalton s chemical atomic theory and the laws of chemical combination explained by it that made chemistry an exact science. 

For two thousand years atoms were considered the smallest and indivisible units of nature. At the beginning of the nineteenth century Dalton got chemistry on the path of atomic theory with his book, A New System of Chemical Philosophy, in which he argued that unbreakable atoms form compounds by linking with other atoms in simple... 

Quantum mechanics (QM) is the correct mathematical description of the behavior of electrons and thus of chemistry. In theory, QM can predict any property of an individual atom or molecule exactly. In practice, the QM equations have only been solved exactly for one electron systems. A myriad collection of methods has been developed for approximating the solution for multiple electron systems. These approximations can be very useful, but this requires an amount of sophistication on the part of the researcher to know when each approximation is valid and how accurate the results are likely to be. A significant portion of this book addresses these questions. 

The concept that all substances are composed of elements and atoms goes back at least 2000 years. Originally, only four elements were recognized air, earth, fire, and water. Each substance was thought to consist of very small particles, called atoms, that could not be subdivided any further. This early mental concept of the nature of matter was extremely prescient, considering there were no experimental results to indicate that matter should be so and none to verify that it was so. Modern atomic theory is much more rigorously based, and we even have the ability to see atoms with special tunneling microscopes. All of chemistry is based on how atoms react with each other. 

The discovery of hafnium was one of chemistry s more controversial episodes. In 1911 G. Urbain, the French chemist and authority on rare earths , claimed to have isolated the element of atomic number 72 from a sample of rare-earth residues, and named it celtium. With hindsight, and more especially with an understanding of the consequences of H. G. J. Moseley s and N. Bohr s work on atomic structure, it now seems very unlikely that element 72 could have been found in the necessary concentrations along with rare earths. But this knowledge was lacking in the early part of the century and, indeed, in 1922 Urbain and A. Dauvillier claimed to have X-ray evidence to support the discovery. However, by that time Niels Bohr had developed his atomic theory and so was confident that element 72 would be a... 

In 1808, an English scientist and schoolteacher, John Dalton, developed the atomic model of matter that underlies modem chemistry. Three of the main postulates of modem atomic theory, all of which Dalton suggested in a somewhat different form, are stated below and illustrated in Figure 2.1. 

Dalton s atomic theory explained three of the basic laws of chemistry The law of conservation of mass This states that there is no detectable change in mass in an ordinary chemical reaction. If atoms are con-... 

Let us begin by looking again at the kinds of evidence we already have for the existence of atoms—the evidence from chemistry. We shall consider, in turn, the definite composition of compounds, the simple weight relations among compounds, and the reacting volumes of gases. Each behavior provides experimental support for the atomic theory. 

This success of the atomic theory is not surprising to a historian of science. The atomic theory was first deduced from the laws of chemical composition. In the first decade of the nineteenth century, an English scientist named John Dalton wondered why chemical compounds display such simple weight relations. He proposed that perhaps each element consists of discrete particles and perhaps each compound is composed of molecules that can be formed only by a unique combination of these particles. Suddenly many facts of chemistry became understandable in terms of this proposal. The continued success of the atomic theory in correlating a multitude of new observations accounts for its survival. Today, many other types of evidence can be cited to support the atomic postulate, but the laws of chemical composition still provide the cornerstone for our belief in this theory of the structure of matter. 

The initial set of experiments and the first few textbook chapters lay down a foundation for the course. The elements of scientific activity are immediately displayed, including the role of uncertainty. The atomic theory, the nature of matter in its various phases, and the mole concept are developed. Then an extended section of the course is devoted to the extraction of important chemical principles from relevant laboratory experience. The principles considered include energy, rate and equilibrium characteristics of chemical reactions, chemical periodicity, and chemical bonding in gases, liquids, and solids. The course concludes with several chapters of descriptive chemistry in which the applicability and worth of the chemical principles developed earlier are seen again and again. 

With remarkable accuracy, Democritus in the fifth century B.C. set the stage for modem chemistry. His atomic theory of matter, which he formulated without experimental verification, still stands, more or less intact, and encapsulates the profound truth that nature s stunning wealth boils down to atoms and molecules. As science uncovers the mysteries of the world around us, we stand ever more in awe of nature s ingenious molecular designs and biological systems nucleic acids, saccharides, proteins, and secondary metabolites are four classes of wondrous molecules that nature synthesizes with remarkable ease, and uses with admirable precision in the assembly and function of living systems. 

In chemistry, perhaps because of the significance in visualizing molecular strac-ture, there has been a focus on how students perceive three-dimensional objects from a two-dimensional representation and how students mentally manipulate rotated, reflected and inverted objects (Stieff, 2007 Tuckey Selvaratnam, 1993). Although these visualization skills are very important in chemistry, it is evident that they are not the only ones needed in school chemistry (Mathewson, 1999). For example, conceptual understanding of nature of different types of chemical bonding, atomic theory in terms of the Democritus particle model and the Bohr model, and... 

The sub-micro level is real, but is not visible and so it can be difficult to comprehend. As Kozma and Russell (1997) point out, understanding chemistry relies on making sense of the invisible and the untouchable (p. 949). Explaining chemical reactions demands that a mental picture is developed to represent the sub-micro particles in the substances being observed. Chemical diagrams are one form of representation that contributes to a mental model. It is not yet possible to see how the atoms interact, thus the chemist relies on the atomic theory of matter on which the sub-micro level is based. This is presented diagrammatically in Fig. 8.2. The links from the sub-micro level to the theory and representational level is shown with the dotted line. 

Chemistry is a quantitative science, and chemists frequentiy measure amounts of matter. As the atomic theory states, matter consists of atoms, so measuring amounts means measuring numbers of atoms. Counting atoms is difficuit, but we can easiiy measure the mass of a sampie of matter. To convert a mass measurement into a statement about the number of atoms in a sampie, we must know the mass of an individuai atom. 

Many decades ago, with the advent of convincing atomic theory, it was thought that a universal model for hardness could be found. This is not the case given the present state of solid-state physics. Much of physics, and therefore chemistry, is based on interactions between pairs of particles. This is adequate for understanding changes of sizes of objects, but hardness involves changes of shape, and this requires more complex interactions. 

Loring published chemistry books—Studies in Valency (1913), Atomic Theory (1921), Definition of the Principle of Equivalence (1922), and The Chemical Elements (1923). During the brief existence of the Alchemical Society, he published twenty articles (eight of them lead articles) in Chemical News on such subjects as atomic weight, the radio-atoms, the evolution of chemical elements, and a five-part Introduction to the Theory of Relativity. He also published seven correspondences in the journal, and Chemical News reviewed his Studies in Valency positively. 

But in spite of Theosophical vitalism (to which I shall return below) and the Theosophical attack on mechanistic science and philosophy, the atomic theory Besant and Leadbeater developed in Occult Chemistry and elsewhere was in many ways a mechanical theory involving interactions of ever-more-rarified particles and their vibrations. It adapted many assumptions of Victorian ether mechanics. 

To acquaint the reader with the potential uses of the theory in medicinal chemistry, a theory couched in the language and concepts of chemistry a theory of atoms with characteristic properties, of the bonds that link them, and of structure and its stability. 

Chemical analysis is an indispensable servant of modern technology whilst it partly depends on that modem technology for its operation. The two have in fact developed hand in hand. From the earliest days of quantitative chemistry in the latter part of the eighteenth century, chemical analysis has provided an important basis for chemical development. For example, the combustion studies of La Voisier and the atomic theory proposed by Dalton had their bases in quantitative analytical evidence. The transistor... 

Volume 3 explains the systems of molecular and atomic weight, valences, the atomic theory, the system of classification of the elements, and the laws of chemical equilibrium. Here we find Lespieau s view that the goal of chemistry is the formule developee, not the formule brute, and that the atomic hypothesis gives us a striking interpretation and creates a language that is now adopted by all chemists, even those who reject the hypothesis of an indivisible primordial particle.30... 

William Thomson [Lord Kelvin], "On Vortex Atoms," 1617 and Ludwig Boltzmann, "On the Necessity of Atomic Theories in Physics," 7374. There seemed to be less resentment among chemists of J. J. Thomson s application of the ether vortex atom to chemistry in his 1882 book. 

Toward the close of the nineteenth century, chemists had two invaluable conceptual tools to aid them in their understanding of matter. The first was John Dalton s atomic theory, which you have studied intensively in previous chemistry courses. Dalton s atomic theory, first published in 1809, provided chemists with a framework for describing and explaining the behaviour of matter during chemical reactions. As you can see in Figure 3.1, the model of the atom that resulted from this theory was very simple. 

As you know, Dalton s atomic theory no longer applies in its original form, and Mendeleev s periodic table has undergone many changes. For example, scientists later discovered that atoms are not the most basic unit of matter because they are divisible. As well, the modern periodic table lists the elements in order of their atomic number, not their atomic mass. Of course, it also includes elements that had not been discovered in Mendeleev s time. Even so, in modified form, both of these inventions are still studied and used today in every chemistry course around the world. 

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