The Atomic and Molecular Structure of Matter

The properties of any kind of matter are most easily and clearly learned and understood when they are correlated with its structure, in terms of the molecules, atoms, and still smaller particles that compose it. This subject, the atomic structure of matter, will be taken up in this chapter. 

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The basic law of viscosity was formulated before an understanding or acceptance of the atomic and molecular structure of matter although just like Hooke s law for the elastic properties of solids the basic equation can be derived from a simple model, where a flnid is assumed to consist of hypothetical spherical molecules. Also like Hooke s law, this theory predicts linear behavior at low rates of strain and deviations at high strain rates. But we digress. The concept of viscosity was first introduced by Newton, who considered what we now call laminar flow and the frictional forces exerted between layers within a fluid. If we have a fluid placed between a stationary wall and a moving wall and we assume there is no slip at the walls (believe it or not, a very good assumption), then the velocity profile illustrated in Figure... 

Our knowledge of the structure of matter and of its electric, magnetic, and optical properties is based on the theory of the electron, " quantum theories, quantum and dassical electrodynanucs, statistical mechanics, " and the theory of molecular interactions. " The fundamentals of electron theory were first stated in the classical work of Lorentz and then developed in a modem approach by Rosenfeld. The quantum-mechanical theory of the electromagnetic properties of matter is presented semi-dassically in the work of Born and Jordan, and to Heitler is due the complete quantum theory of interaction between matter and the electromagnetic field. The above-named methods have permitted the determination of the atomic and molecular structure of matter, in... 

Strange as it may seem, the key to progress towards a reliable atomic conception of matter was a very humble instrument, the balance. It was in fact through weight laws - Boyle and Lavoisier - that modern science arrived at a proper understanding of the atomic and molecular structure of matter. The discovery of the electron and quantum mechanics completed the job at the beginning of the last century. 

Pure thermodynamics is developed, without special reference to the atomic or molecular structure of matter, on the basis of bulk quantities like internal energy, heat, and different types of work, temperature, and entropy. The understanding of the latter two is directly rooted in the laws of thermodynamics— in particular the second law. They relate the above quantities and others derived from them. New quantities are defined in terms of differential relations describing material properties like heat capacity, thermal expansion, compressibility, or different types of conductance. The final result is a consistent set of equations and inequalities. Progress beyond this point requires additional information. This information usually consists in empirical findings like the ideal gas law or its improvements, most notably the van der Waals theory, the laws of Henry, Raoult, and others. Its ultimate power, power in the sense that it explains macroscopic phenomena through microscopic theory, thermodynamics attains as part of Statistical Mechanics or more generally Many-body Theory. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

An energy content can be ascribed to any thermodynamic system of matter -the internal energy U of the system which is boimd up with phenomena in the electronic, atomic and molecular structure of the stem. For systems of matter at room temperature, changes of the internal energy AU will mainly be caused by activation of the following energy forms in the system ... 

Chemistry is the science of the combination of atoms, and physics is the science of the forces between atoms. Simply stated, chemistry deals with matter and its transformations, and physics deals witli energy and its transformations. These transformations may be temporaiy, such as a change in phase, or seemingly penmnent, such as a change in the form of matter resulting from a chemical reaction. The study of atomic and molecular structure deals witli tliese transformations, and can be used to make a preliminary identification of a healtli liazard. 

Much of chemistry occurs at small scales. As Dalton and other scientists realized, atoms are the basic units of matter and combine chemically to form molecules. Although chemists usually work with substantially larger amounts of matter, researchers in the field of nanotechnology are beginning to develop the techniques to manipulate matter on the atomic and molecular scale. By finding the right conditions in which atoms and molecules will assemble into functional structures, or by constructing tiny machines that oscillate, researchers have entered the domain of the atom. 

PHYSICAL CHEMISTRY. Application of the concepts and laws of physics to chemical phenomena in order to describe in quantitative (mathematical) terms a vast amount of empirical (observational) information. A selection of only the most important concepts of physical chemistiy would include the electron wave equation and the quantum mechanical interpretation of atomic and molecular structure, the study of the subatomic fundamental particles of matter. Application of thermodynamics to heats of formation of compounds and the heats of chemical reaction, the theory of rate processes and chemical equilibria, orbital theory and chemical bonding. surface chemistry (including catalysis and finely divided particles) die principles of electrochemistry and ionization. Although physical chemistry is closely related to both inorganic and organic chemistry, it is considered a separate discipline. See also Inorganic Chemistry and Organic Chemistry. 

What fundamental property of atoms is responsible for the periodic variations we observe in atomic radii and in so many other characteristics of the elements This question occupied the thoughts of chemists for more than 50 years after Mendeleev, and it was not until well into the 1920s that the answer was established. To understand how the answer slowly emerged, it s necessary to look first at the nature of visible light and other forms of radiant energy. Historically, studies of the interaction of radiant energy with matter have provided immense insight into atomic and molecular structure. 

Thermodynamics and statistical mechanics, as well as the structure of matter, can hardly be understood without a study of atomic and molecular structure, and of the quantum theory which underlies them. Suggested texts in this general field are... 

The highly interesting and now classical monograph of Born contains the theory of the optical properties of matter covering atomic and molecular structural considerations. The fundamentals of molecular optics, presented by Bom, have been developed by Volkenshteyn in tensorial form. Various monographs have been written on the laws of classical optics and the fundamentals of physical optics, including natural optical activity," optical rotatory dispersion and circular dichroism," " i.r. spectroscopy, and other related topics. 

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