CHEMISTRY

the science that deals with the properties, composition, and structure of substances (defined as elements and compounds), the transformations they undergo, and the energy that is released or absorbed during these processes. Every substance, whether naturally occurring or artificially produced, consists of one or more of the hundred-odd species of atoms that have been identified as elements. Although these atoms, in turn, are composed of more elementary particles, they are the basic building blocks of chemical substances; there is no quantity of oxygen, mercury, or gold, for example, smaller than an atom of that substance. Chemistry, therefore, is concerned not with the subatomic domain but with the properties of atoms and the laws governing their combinations and how the knowledge of these properties can be used to achieve specific purposes. The great challenge in chemistry is the development of a coherent explanation of the complex behaviour of materials, why they appear as they do, what gives them their enduring properties, and how interactions among different substances can bring about the formation of new substances and the destruction of old ones. From the earliest attempts to understand the material world in rational terms, chemists have struggled to develop theories of matter that satisfactorily explain both permanence and change. The ordered assembly of indestructible atoms into small and large molecules, or extended networks of intermingled atoms, is generally accepted as the basis of permanence, while the reorganization of atoms or molecules into different arrangements lies behind theories of change. Thus chemistry involves the study of the atomic composition and structural architecture of substances, as well as the varied interactions among substances that can lead to sudden, often violent reactions. Chemistry also is concerned with the utilization of natural substances and the creation of artificial ones. Cooking, fermentation, glass making, and metallurgy are all chemical processes that date from the beginnings of civilization. Today, vinyl, Teflon, liquid crystals, semiconductors, and superconductors represent the fruits of chemical technology. The 20th century has seen dramatic advances in the comprehension of the marvelous and complex chemistry of living organisms, and a molecular interpretation of health and disease holds great promise. Modern chemistry, aided by increasingly sophisticated instruments, studies materials as small as single atoms and as large and complex as DNA (deoxyribonucleic acid), which contains millions of atoms. New substances can even be designed to bear desired characteristics and then synthesized. The rate at which chemical knowledge continues to accumulate is remarkable. Over time more than 8,000,000 different chemical substances, both natural and artificial, have been characterized and produced. The number was less than 500,000 as recently as 1965. Intimately interconnected with the intellectual challenges of chemistry are those associated with industry. In the mid-19th century the German chemist Justus von Liebig commented that the wealth of a nation could be gauged by the amount of sulfuric acid it produced. This acid, essential to many manufacturing processes, remains today the leading chemical product of industrialized countries. As Liebig recognized, a country that produces large amounts of sulfuric acid is one with a strong chemical industry and a strong economy as a whole. The production, distribution, and utilization of a wide range of chemical products is common to all highly developed nations. In fact, one can say that the "iron age" of civilization is being replaced by a "polymer age," for in some countries the total volume of polymers now produced exceeds that of iron. the science that deals with the properties, composition, and structure of substances (defined as elements and compounds), the transformations that they undergo, and the energy that is released or absorbed during these processes. Every substance, whether naturally occurring or artificially produced, consists of one or more of the hundred-odd species of atoms that have been identified as elements. Although these atoms, in turn, are composed of more elementary particles, they are the basic building blocks of chemical substances; there is no quantity of oxygen, mercury, or gold, for example, smaller than an atom of that substance. Chemistry, therefore, is concerned not with the subatomic domain but with the properties of atoms and the laws governing their combinations and with how the knowledge of these properties can be used to achieve specific purposes. Chemistry is treated in a number of articles. For the history and principal treatment of the discipline, see chemistry. For the primary subjects of study, see atom; chemical bonding; chemical compound; chemical element; chemical reaction; phase. For the major classes of substances that occur in life forms, see protein; carbohydrate; lipid; nucleic acid; vitamin; and hormone. For methodology and instrumentation, see analysis. For the role of chemistry in the study of plant and animal life, see biology. For its applications in industry, see chemical industry. Additional reading Historical developments in chemistry through the 17th century are explored in Robert P. Multhauf, The Origins of Chemistry (1966). Cecil J. Schneer, Mind and Matter: Man's Changing Concepts of the Material World (1970, reprinted 1988), gives an interesting account of the history of chemistry in relation to the structure of matter; Aaron J. Ihde, The Development of Modern Chemistry (1964, reprinted 1984), is a comprehensive history covering the 18th to the middle of the 20th century; and Frederic Lawrence Holmes, Lavoisier and the Chemistry of Life: An Exploration of Scientific Creativity (1985), is a study of chemical experimentation.Concise explanations of chemical terms can be found in Douglas M. Considine and Glenn D. Considine (eds.), Van Nostrand Reinhold Encyclopedia of Chemistry, 4th ed. (1984), a specialized reference work. Comprehensive treatment of chemical theories and reactivity is presented in Donald A. McQuarrie and Peter A. Rock, General Chemistry, 2nd ed. (1987); and in John C. Kotz and Keith F. Purcell, Chemistry & Chemical Reactivity (1987). Studies of common applications of chemistry, intended for the general reader, include William R. Stine et al., Applied Chemistry, 2nd ed. (1981); and John W. Hill, Chemistry for Changing Times, 5th ed. (1988). An overview of modern chemistry and discussion of its prospects is found in George C. Pimentel and Janice A. Coonrod, Opportunities in Chemistry: Today and Tomorrow (1987). P.W. Atkins, Molecules (1987), is a pictorial examination of chemical structure. Lionel Salem, Marvels of the Molecule, trans. from French (1987), presents the molecular orbital theory of chemical bonding in simple terms. The extent to which molecular structure has become central to biochemistry is demonstrated in the articles in The Molecules of Life: Readings from Scientific American (1985). The fundamental principles governing chemical change and the laws of thermodynamics are presented, with a minimum of mathematics, in P.W. Atkins, The Second Law (1984); and John B. Fenn, Engines, Energy, and Entropy: A Thermodynamics Primer (1982). Social aspects of developments in chemistry, especially the environmental costs of use of chemical products, are studied in Luciano Caglioti, The Two Faces of Chemistry, trans. from Italian (1983).