Humans began to practice chemistry--the transformation of material things--in prehistoric times, beginning with the use of fire. Primitive humans used fire to produce such chemical transformations as the burning of wood, the cooking of food, and the firing of pottery and bricks, and later to work with such ores as copper, silver, and gold. As civilization developed in China, Mesopotamia, and Egypt, artisans performed further transformations to produce a variety of dyes, drugs, glazes, glasses, perfumes, and metals. Early theoretical explanations of chemical phenomena were generally magical and mythological in character. The ancient Greeks added little to the chemical practice that they inherited from older and neighboring civilizations, but they did refine the theoretical explanations of transformations observed in the artisans' shops and in the environment. They recognized change as a universal phenomenon, to such a degree that \THeraclitus\t, in the 6th century BC, asked whether there was anything visible or invisible that did not change. After considerable discussion of this question by many Greek philosophers, \TAristotle\t, in the 4th century BC, formulated a theory that dominated scientific thinking for almost 2,000 years. He postulated the existence of a primeval matter and four qualities: heat, cold, wetness, and dryness. As these qualities were impressed on the primeval matter, four elements were produced: fire (hot and dry), air (hot and wet), earth (cold and dry), and water (cold and wet). All material things were viewed as different combinations of these four elements. Greek philosophers also introduced the theory of atomicity. \TAnaxagoras\t and \TEmpedocles\t held that all matter was composed of infinitely small seeds; \TLeucippus\t and \TDemocritus\t proposed that all matter coalesced out of indivisible atoms moving rapidly and at random in a void. \Talchemy\t, the next major phase of the history of chemistry, developed in Alexandria, Egypt, and combined aspects of Greek philosophy, Oriental artisanship, and religious mysticism. Its main objective was the transformation of base metals into gold. In the 4th and 5th centuries, the emigrant Nestorians brought the craftsmanship of Egyptian artisans to the Arabs in Anatolia. During the golden age of Arabic science (8th-11th century) the ideas of Aristotle were modified, and a number of important substances, such as sodium hydroxide and ammonium chloride, were introduced into chemical practice. This Arabic alchemy came into Western Europe between the 11th and 16th centuries through Sicily and Spain. The mystical ideas so introduced were accompanied by practical advances in chemical procedures, such as distillation, and by the discovery of new metals and compounds. The art of \Tmetallurgy\t became more sophisticated, and chemicals were introduced into medical practice by \TParacelsus\t in the 16th century. 17TH AND 18TH CENTURIES At the beginning of the 17th century chemistry became recognized as a science. The first methodical chemical textbook, Alchemia, by Andreas LIBAVIUS, was published in 1597. Alchemia was defined as the art of producing reagents and extracting pure essences from mixtures. During this century many new compounds were prepared by distilling animal and vegetable materials, and the \Tphlogiston theory\t was proposed by Georg Ernst \TStahl\t as a unified explanation of combustion and calcination (rusting): when a substance burned or a metal was converted into calx (rusted), a proposed substance known as phlogiston was lost. Easily combustible materials such as charcoal were viewed as containing large amounts of phlogiston, which could be transferred directly to calx, thereby regenerating the metal. Refined Techniques The invention of the pneumatic trough and the balance stimulated the development of chemistry in the 18th century. During the previous centuries, liquids and solids could be handled in the laboratory without difficulty, but gases produced by heating substances could be manipulated only by using animal bladders. The pneumatic trough, filled with water or mercury and containing an inverted vessel also filled with water or mercury, permitted easy collection, transfer, and study of gases. Joseph \TBlack\t discovered carbon dioxide (1756), and Karl \TScheele\t (1772) and Joseph \TPriestley\t (1774) discovered oxygen, using the pneumatic trough. The balance was used effectively by Antoine \TLavoisier\t, at the end of the 18th century, to disprove the phlogiston theory and establish the true nature of combustion, calcination, and biological respiration. Lavoisier showed that heating the calx of mercury produced oxygen, with a loss of weight that was regained when oxygen and mercury were heated at a lower temperature. Refined Theory Through these experiments Lavoisier demonstrated that the process of combustion was the reaction of oxygen with carbonaceous material to form carbon dioxide and water, and that respiration was biological combustion. More generally, Lavoisier defined a chemical \Telement\t as a substance that could not be decomposed into simpler substances by heat or chemical reaction. A compound was defined as a combination of two or more elements in a definite proportion by weight. This innovative concept placed chemistry on a quantitative basis. Each element could be assigned a number, or combining weight, such that this number or any integral multiple of it represented the weight in which the element combined. Lavoisier can be considered the father of modern chemistry. The atomic theory of John \TDalton\t, at the beginning of the 19th century, further extended Lavoisier's theories. Dalton assumed that each element was composed of very small particles, called \Latom\ls, which have a characteristic weight, and that chemical reactions resulted from the combination or reshuffling of atoms. For almost 50 years, however, there was no clear way of distinguishing combining weights, which were variable, from atomic weights. Furthermore, there was confusion concerning \Tmolecular weight\t, the weight of a standard number of molecules of any given compound. 19TH AND EARLY 20TH CENTURY Discovery of New Elements and Their Systematization During the first half of the 19th century new elements were discovered at an increasing rate. \Telectrolysis\t--the decomposition of compounds by an electric current--broke up compounds that hitherto could not be decomposed by heat or chemical reactions. The alkali metals, alkaline earth metals, silicon, and the halogens were isolated and studied. Michael \TFaraday\t showed that the amount of current necessary to liberate an \Tequivalent weight\t (combining weight) of an element was the same for all elements. Electrolysis deeply influenced the thinking of chemists of the time, including Jons Jakob \TBerzelius\t, who formulated the dualistic, or electrochemical, theory of atomic combination. According to this theory, all atoms are either positively or negatively charged, and molecules are formed by the electrostatic attraction of oppositely charged atoms. A clearer insight into the distinction between combining weights and atomic weights was offered by Pierre \TDulong\t and A. T. Petit (1791-1820), who stated (1819) that the product of the \Tatomic weight\t and the specific heat was a constant. Thus, once the specific heat of an element was determined, an approximate atomic weight could be obtained. The exact atomic weight could then be obtained by multiplying the combining weight by an integer. This weight, referred to oxygen, was precisely determined. The integer used in this calculation came to be known as the \Tvalence\t of the element. Using this method, Berzelius was able to draw up a table of atomic weights. The distinction between combining weights and atomic weights was further clarified at the First International Chemical Congress (1860) at Karlsruhe, Germany. Stanislao \TCannizzaro\t showed how the hypothesis of Amadeo \TAvogadro\t--that equal volumes of gases at the same temperature and pressure contain the same number of molecules--could be used to determine molecular and atomic weights. The culmination of the atomic weight program was the formulation in 1869 by Dmitry \TMendeleyev\t and in 1871 by Lothar \TMeyer\t of the \Tperiodic table\t, which systematized the large number of elements according to their atomic weights and correlated their physical and chemical properties. Medeleyev carried this systemization still further and used his table to produce the properties of three elements not known at that time. These elements (gallium, scandium, and germanium) were discovered in 1875, 1879, and 1886, respectively. Their properties were found to be strikingly similar to those predicted by Mendeleyev. The periodic table was subsequently extended to accommodate the inert gases, such as helium, argon, and neon, discovered by Lord \TRayleigh\t and Sir William \TRamsay\t. Theories of Chemical Bonding During the first half of the 19th century, several carbon compounds were isolated, purified, and characterized. The dualistic electrochemical theory of Berzelius was inadequate to explain the chemical bonding found in most of these compounds. This was strikingly shown in the ability of chlorine, an electronegative element, to replace hydrogen, an electropositive element, in methane. A theory of radicals and types was formulated to explain this situation. It was postulated that certain groups of atoms (radicals) could act as a unit in chemical reactions, replacing hydrogen, for example, in HCl. The HCl represented what was called a type. Other types were water, ammonia, hydrogen, and methane. Thus a radical could replace one hydrogen in water to form alcohol and two to form an ether. Although the radical and type theories systematized many organic compounds, they could not systematize all the compounds without the invention of more and more types. The confusion was compounded by the failure at this time to distinguish between atomic, equivalent, and molecular weights. The theory of types and the study of metal organic compounds, however, suggested that each atom could bind only a fixed number of atoms or radicals: hydrogen one, oxygen two, and nitrogen three. Friedrich \TKekule von Stradonitz\t and A. S. Couper (1831-92) proposed that carbon not only could bind four atoms but also could bond with other carbon atoms to form chains and rings. The number of such bonds was called the valence of the element. Simplistically it could be visualized as the number of "hooks" that the element possessed. Valence theory led to the structural theory of organic \Tchemistry\t expounded by Aleksandr \TButlerov\t. This theory explained the differences between two compounds, such as dimethyl ether and ethyl alcohol, having the same composition, molecular weight, and molecular formula but markedly different chemical properties, by assigning them different structural formulas. In 1874, Jacobus van't HOFF (1852-1911) and Joseph LE BEL extended these formulas into three dimensions, opening up the new field of \Tstereochemistry\t. Stereochemistry not only explained the puzzling difference between compounds that had the same structure and different properties (stereoisomers), but also explained the optical activity of certain compounds important in life processes. The ideas of stereochemistry were soon applied to complex compounds of the transition metals by Alfred \TWerner\t. By the end of the 19th century, organic chemistry had not only acquired a comprehensive theory of structure but had also developed new methods for the synthesis of dyes, perfumes, explosives, and medicines. The starting materials for these syntheses were obtained from the coal-tar industry. Contributions of Physics During the 19th century, chemistry developed for the most part independently of physics, where progress was being made in mechanics, electricity, magnetism, thermodynamics, and optics. Nevertheless, there were interactions between the two fields. Electrolysis and chemical batteries involved electricity. Some new elements were detected by \Tspectroscopy\t, notably by Robert \TBunsen\t. Electric conductivity of aqueous salt solutions showed that on dissolution salts break up into charged particles. In 1884 Svante \TArrhenius\t formulated the theory of electrolytic dissociation. Thermodynamics was applied to chemistry during the middle of the century in the measurement of heats of reaction by Germain \THess\t, Marcelin \TBerthelot\t, and Julius Thomson (1826-1909). J. Willard \TGibbs\t, Jr., developed the thermodynamics of heterogeneous equilibria and formulated (1876) the phase rule. He also formulated the discipline of statistical mechanics. Chemical Analysis through Spectroscopy The ability to analyze gases spectroscopically by passing electricity through them proved to be a major new analytical tool. The use of chemical electrical batteries to pass electricity through molten solids and liquids had led to the isolation of new elements and the formulation of the dualistic theory of molecular structure. Electrolysis of gases, however, was not possible until the invention of an efficient vacuum pump to provide low pressure gases in a glass tube. When an electric potential is applied to such a gas it becomes conductive and produces visible radiation that can be analyzed with a spectroscope. The development of this discharge tube was essential to the development of modern physics and chemistry. The electric charge carriers inside the discharge tube were found to include both positive (canal) rays and negative (cathode) rays. The positive rays were composed of different ionized atoms when different gases were in the discharge tube, but the \Lcathode ray\ls were always the same no matter what residual gas was in the tube. The cathode rays were identified as electrons by Sir Joseph John \TThomson\t in 1897. Using an appropriate arrangement of magnetic and electric fields, Francis \TAston\t constructed a mass spectroscope, which he used to separate ions of the positive rays according to their atomic weight. In this way, not only were atoms of different species separated from each other, but also some elements were found to consist of atoms with differing weights (\Lisotope\ls). Radiation emitted from the discharge tube consisted of a visible glow that, when analyzed by a spectrograph, showed discrete spectral lines characteristic of traces of gas remaining in the evacuated discharge tube. The surprising result was that even the spectrum of hydrogen, the simplest atom, appeared complex, consisting of a series of discrete lines whose wavelengths could be determined with a high degree of precision. Almost four decades elapsed before this complexity was explained. In 1885, Johann J. \TBalmer\t showed that there was a simple mathematical relationship between the lines of hydrogen in the visible spectrum. A similar relationship was found in the extreme ultraviolet and in the infrared. Thus each spectral line at wavelength lambda of the hydrogen atom could be represented by one formula. A similar formula was discovered for spectral lines of alkali metals and alkaline earth elements. Structure of the Atom In 1913, Niels \TBohr\t proposed his atomic theory for the hydrogen atom. This theory postulated that the hydrogen atom consisted of a positive massive nucleus and an electron traveling in definite discrete orbits around it. These orbits are characterized by integers called quantum numbers, represented by the letter n. Bohr also related the energy of the atom to the orbit of the electron, determining it to be equal to r/nn, where r is the radius of the electron orbit; he further maintained that emission and absorption of light were characterized by a "quantum jump" between two orbits in the atom. Bohr was able to derive the Rydberg constant, R, in terms of known physical constants and to calculate its value to within several percentage points. The simple Bohr theory was extended to other atoms and made more sophisticated for hydrogen by the introduction of a set of quantum numbers. Despite its many virtues, the Bohr theory had several shortcomings; particularly a lack of self-consistency. In 1925, Louis \Tde Broglie\t proposed that electrons have wave properties; Clinton \TDavisson\t and Lester Germer (1896-1972) in the United States and Sir George \TThomson\t in England confirmed this by showing diffraction of electrons by crystals. Erwin \TSchrodinger\t developed this wave concept into \Twave mechanics\t, which was given a self-consistent formulation by Paul \TDirac\t, Werner \THeisenberg\t, and John von \TNeumann\t. The discovery of the electron spin by George Uhlenbeck (1900-88) and Samuel Goudsmit (1902-78) in 1925 represented a major advance in the understanding of atomic and molecular structure, and had important repercussions in the theory of magnetism and chemical bonding. Another important development was the formulation of the \Texclusion principle\t (1925) by Wolfgang \TPauli\t and the formulation of the \Tuncertainty principle\t (1927) by Heisenberg. These developments constituted the new quantum theory. The gas discharge tube also led to new knowledge about the structure of matter. In 1895, Wilhelm \TRoentgen\t discovered a penetrating invisible radiation (X RAY) that was emitted from the discharge tube. The characteristics of this radiation were determined by the electrode in the discharge tube. In 1913, Henry \TMoseley\t used X-ray spectroscopy to show that each element could be assigned a characteristic integer, the atomic number, which was equal to the positive charge on the nucleus and also corresponded to the element's position in the periodic table. Thus the simple Bohr theory was extended to complex atoms. In the period from 1916 to 1920, these ideas of atomic structure were used by Gilbert \TLewis\t, Walter Kossel (1882-1956), Irving \TLangmuir\t, and Nevil \TSidgwick\t to formulate a quantum theory of valence. A distinction was made between an ionic, a covalent, and a coordinate bond. In 1927, Walter Heitler (1904-81) and Fritz London (1907-76) formulated a quantum mechanical theory of bonding. Their ideas were further developed by Linus \TPauling\t, Erich Huckel (1896-1980), and Henry Eyring (1901-81). The first half of the 20th century was marked by far-reaching discoveries concerning the nucleus. In experiments indirectly associated with the penetrating radiation from the discharge tube, Antoine \TBecquerel\t discovered that all uranium salts emitted penetrating radiation. This discovery ushered in the age of radioactivity. Marie and Pierre \TCurie\t discovered the radioactive elements polonium and radium. Sir Ernest \TRutherford\t unraveled the complex nature of radioactive radiation and formulated the spontaneous nuclear disintegration theory of natural \Tradioactivity\t. In 1919, Rutherford produced the first artificial transmutation of elements, realizing the dream of alchemists. By bombarding nitrogen with alpha particles, he transformed the nitrogen into oxygen and the alpha particles into protons. This opened up the new field of nuclear chemistry. In 1932, Sir James \TChadwick\t discovered the neutron, which in turn led to the discovery of artificial radioactivity by Irene and Frederic \TJoliot-Curie\t, to the synthesis of transuranium elements, and to the realization of nuclear fission. The periodic table was extended to atomic number 103 by Glenn \TSeaborg\t and to negative atomic numbers by the discovery of antimatter. All gaps in the periodic table were filled by an increasing number of isotopes of varying nuclear properties. RECENT ADVANCES Inorganic Chemistry World War II research spurred important advances in \Tinorganic chemistry\t. The atomic weapon and nuclear power projects intensified studies of uranium and the transuranium elements, the chemistry of fluorine compounds, and the metallurgy of fuel element components such as zirconium. Rare earths, produced by nuclear fission, were separated in pure state by \Tchromatography\t and were made available for chemical study. Neil Bartlett (1932- ) prepared compounds of inert gases. Modern electronics has become highly dependent on inorganic chemistry. Vacuum tubes have been replaced by solid-state devices, with the ultrapure solid matrix replacing the vacuum. Growth of single crystals of germanium, silicon, and other semiconductors has become an industry. Compound semiconductors are used in color television screens, solar batteries, lasers, photoconductors, in photocopying processes, and thermoelectric devices. Modern computers and audio and video recorders use magnetic materials for information storage. The field of microelectronics depends on inorganic chemical techniques for producing high-purity films on single crystal chips. A group of compounds known as \Tcoordination compounds\t, derived from simpler inorganic substances, were found to contain atoms or groups of atoms united by bonds supplementary to classical valence bonds. In 1891, Alfred Werner (1866-1919) had classified such amines, hydrates, double cyanides, and double salts, and had postulated the existence of secondary bonds uniting these groupings of atoms. In the 1920s, Nevil Sidgwick reformulated the Werner theory in terms of a central acceptor atom (usually a transition element such as copper) and a definite number of donor molecules such as ammonia and the chloride ion. The secondary valence bond was formulated in terms of a coordinate bond involving pairs of electrons. Stereochemistry, isomerism, and the optical properties of coordinate compounds were widely studied. Organometallic compounds in which the donor was an organic radical were extensively investigated during the middle of the 20th century. A new chemistry linking both inorganic and organic compounds was developed. The organometallic compounds have found extensive use as polymers, plastics (silicones), antioxidants, insecticides, and herbicides. They have also been used as catalysts for hydrogenation and polymerization. During the 20th century several techniques were developed for structure determination. The most important of these is X-RAY \Tdiffraction\t. Max von \TLaue\t obtained (1912), the first diffraction pattern of a single crystal of zinc sulfide. Von Laue, William H. Bragg, and his son, William L. Bragg, showed how these diffraction patterns could be used to determine the arrangement of atoms in crystals, a technique now widely employed. The structure of gaseous molecules was determined by electron diffraction and by infrared and microwave spectroscopy. Electric charge distribution within molecules was deduced from the dielectric properties of materials. Important structural information was obtained from magnetic measurements. Electron paramagnetic resonance, discovered by E. K. Zavoiski in 1945, has proved to be an important research tool in the inorganic chemistry of transition metals and in studying radicals. Organic Chemistry Organic chemistry has grown rapidly during the 20th century, as evidenced by the increase in the number of known organic compounds from approximately 12,000 in 1880 to 150,000 in 1910; 500,000 in 1940; and 1,000,000 in 1960. Before World War II organic chemistry was based on the coal-tar industry, but after the war petroleum became the major source of organic compounds. Petrochemicals were produced in thousand-ton lots for the plastics, fiber, and solvent industries. Valuable new pharmaceuticals were synthesized: Salvarsan by Paul \TEhrlich\t in 1909; tropinone (1917) and anthocyanines (1931) by Sir Robert \TRobinson\t in 1917; vitamin C (1933) by Sir Walter \THaworth\t, E. L. Hirst and Thadeus Reichstein; complex alkaloids by R. B. Woodward; and nucleotides and coenzymes by Lord Alexander \TTodd\t. The first sulfa drug, prontosil, was synthesized in 1932 by Gerhard \TDomagk\t; sulfanilamide, synthesized by J. Trefoel (1897- ), followed in 1936. The \Tantibiotic\t penicillin was made in the laboratory in 1957. Structure determination and analysis in organic chemistry has been facilitated by modern techniques, including ultraviolet, visible, and infrared spectroscopy; mass spectrometry; and magnetic resonance spectrometry. Separation of complex mixtures has been simplified by a variety of chromatographic methods. Since World War II, studies of the structure and mechanism of reactions have occupied the attention of a large group of scientists identified as physico-organic chemists. Physical Chemistry \Tphysical chemistry\t, a discipline on the border between physics and chemistry, is concerned with the macroproperties of chemical substances and the changes they undergo when subjected to pressure, temperature, light, and electric and magnetic forces. It also investigates changes produced by dissolution in a solvent or chemical reactivity. Wilhelm \TOstwald\t was instrumental in identifying this area of knowledge as a distinct science. Subjects of particular study in the 20th century have included adsorption of gases, by Irving Langmuir; catalysis, by Giulio \TNatta\t and Karl \TZiegler\t; and the kinetics of chemical reactions, by Sir Cyril \THinshelwood\t and Nikolai \TSemenov\t. The kinetic theory of gases, and the liquid state, solution theory, electrochemistry, and thermodynamics were studied by Lars \TOnsager\t. Ilya \TPrigogine\t researched the areas of phase equilibrium, photochemistry, and the electric and magnetic properties of substances. The low-temperature phenomenon called \Tsuperconductivity\t was first observed in 1911; in the late 1980s, scientists discovered materials that became superconductive at relatively high temperatures. Chemical Physics During the second third of the 20th century, chemical physics was identified as a new discipline. Chemical physics differs from physical chemistry in that it deals with the microscopic properties of different chemical substances (spectra, X-ray structures, microwave spectroscopy, and the study of magnetic resonance), and interprets the results in terms of atomic and molecular theories. Such interpretations are based on quantum mechanics, quantum chemistry, and statistical mechanics. Analytical Chemistry During the last half of the 20th century, instrumental methods have replaced the standard gravimetric and volumetric procedures in analysis. Instruction in these classical analytical techniques occupies a very small part of today's chemical curriculum. Microtechniques, introduced in 1917 by Fritz \TPregl\t to enable analysis of several milligrams of a sample, was the first stage in the development of microchemistry, which can now analyze as few as a hundred atoms using neutron activation, mass spectrography, or fluorescence spectrometry. X-ray absorption spectrometry enables researchers to elucidate the atomic and molecular structure of chemical substances. Scientists use the scanning tunneling microscope, a type of \Telectron microscope\t, to manipulate matter at the atomic level. To observe the details of rapid chemical reactions, researchers use lasers that emit short pulses of light. Thus chemistry has become a highly sophisticated branch of science, involving complex apparatuses for experimental work, a highly refined theoretical approach in the interpretation of results, and an impact on every segment of the world economy. John Turkevich Bibliography: Asimov, Isaac, A Short History of Chemistry (1965; repr. 1975); Brown, Theodore L., and Lemay, H. E., Chemistry: the Central Science (1988); Leicester, Henry M., Historical Background of Chemistry (1971); Multhauf, R. P., The Origins of Chemistry (1967); Neubauer, Alfred, Chemistry Today: The Portrait of a Science (1983); Von Meyer, Ernst, A History of Chemistry from Earliest Times to the Present Day (1975).