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Branches of Chemistry
Chemistry can be divided into branches according to either the substances studied or the types of study conducted. The primary division of the first type is between inorganic chemistry and organic chemistry. Divisions of the second type are physical chemistry and analytical chemistry.
The original distinction between organic and inorganic chemistry arose as chemists gradually realized that compounds of biological origin were quite different in their general properties from those of mineral origin; organic chemistry was defined as the study of substances produced by living organisms. However, when it was discovered in the 19th cent. that organic molecules can be produced artificially in the laboratory, this definition had to be abandoned. Organic chemistry is most simply defined as the study of the compounds of carbon. Inorganic chemistry is the study of chemical elements and their compounds (with the exception of carbon compounds).
Physical chemistry is concerned with the physical properties of materials, such as their electrical and magnetic behavior and their interaction with electromagnetic fields. Subcategories within physical chemistry are thermochemistry, electrochemistry, and chemical kinetics. Thermochemistry is the investigation of the changes in energy and entropy that occur during chemical reactions and phase transformations (see states of matter). Electrochemistry concerns the effects of electricity on chemical changes and interconversions of electric and chemical energy such as that in a voltaic cell. Chemical kinetics is concerned with the details of chemical reactions and of how equilibrium is reached between the products and reactants.
Analytical chemistry is a collection of techniques that allows exact laboratory determination of the composition of a given sample of material. In qualitative analysis all the atoms and molecules present are identified, with particular attention to trace elements. In quantitative analysis the exact weight of each constituent is obtained as well. Stoichiometry is the branch of chemistry concerned with the weights of the chemicals participating in chemical reactions. See also chemical analysis.
History of Chemistry
The earliest practical knowledge of chemistry was concerned with metallurgy, pottery, and dyes; these crafts were developed with considerable skill, but with no understanding of the principles involved, as early as 3500 B.C. in Egypt and Mesopotamia. The basic ideas of element and compound were first formulated by the Greek philosophers during the period from 500 to 300 B.C. Opinion varied, but it was generally believed that four elements (fire, air, water, and earth) combined to form all things. Aristotle's definition of a simple body as “one into which other bodies can be decomposed and which itself is not capable of being divided” is close to the modern definition of element.
About the beginning of the Christian era in Alexandria, the ancient Egyptian industrial arts and Greek philosophical speculations were fused into a new science. The beginnings of chemistry, or alchemy, as it was first known, are mingled with occultism and magic. Interests of the period were the transmutation of base metals into gold, the imitation of precious gems, and the search for the elixir of life, thought to grant immortality. Muslim conquests in the 7th cent. A.D. diffused the remains of Hellenistic civilization to the Arab world. The first chemical treatises to become well known in Europe were Latin translations of Arabic works, made in Spain c.A.D. 1100; hence it is often erroneously supposed that chemistry originated among the Arabs. Alchemy developed extensively during the Middle Ages, cultivated largely by itinerant scholars who wandered over Europe looking for patrons.
Evolution of Modern Chemistry
In the hands of the “Oxford Chemists” (Robert Boyle, Robert Hooke, and John Mayow) chemistry began to emerge as distinct from the pseudoscience of alchemy. Boyle (1627–91) is often called the founder of modern chemistry (an honor sometimes also given Antoine Lavoisier, 1743–94). He performed experiments under reduced pressure, using an air pump, and discovered that volume and pressure are inversely related in gases (see gas laws). Hooke gave the first rational explanation of combustion—as combination with air—while Mayow studied animal respiration. Even as the English chemists were moving toward the correct theory of combustion, two Germans, J. J. Becher and G. E. Stahl, introduced the false phlogiston theory of combustion, which held that the substance phlogiston is contained in all combustible bodies and escapes when the bodies burn.
The discovery of various gases and the analysis of air as a mixture of gases occurred during the phlogiston period. Carbon dioxide, first described by J. B. van Helmont and rediscovered by Joseph Black in 1754, was originally called fixed air. Hydrogen, discovered by Boyle and carefully studied by Henry Cavendish, was called inflammable air and was sometimes identified with phlogiston itself. Cavendish also showed that the explosion of hydrogen and oxygen produces water. C. W. Scheele found that air is composed of two fluids, only one of which supports combustion. He was the first to obtain pure oxygen (1771–73), although he did not recognize it as an element. Joseph Priestley independently discovered oxygen by heating the red oxide of mercury with a burning glass; he was the last great defender of the phlogiston theory.
The work of Priestley, Black, and Cavendish was radically reinterpreted by Lavoisier, who did for chemistry what Newton had done for physics a century before. He made no important new discoveries of his own; rather, he was a theoretician. He recognized the true nature of combustion, introduced a new chemical nomenclature, and wrote the first modern chemistry textbook. He erroneously believed that all acids contain oxygen.
Impact of the Atomic Theory
The assumption that compounds were of definite composition was implicit in 18th-century chemistry. J. L. Proust formally stated the law of constant proportions in 1797. C. L. Berthollet opposed this law, holding that composition depended on the method of preparation. The issue was resolved in favor of Proust by John Dalton's atomic theory (1808). The atomic theory goes back to the Greeks, but it did not prove fruitful in chemistry until Dalton ascribed relative weights to the atoms of chemical elements. Electrochemical theories of chemical combinations were developed by Humphry Davy and J. J. Berzelius. Davy discovered the alkali metals by passing an electric current through their molten oxides. Michael Faraday discovered that a definite quantity of charge must flow in order to deposit a given weight of material in solution. Amedeo Avogadro introduced the hypothesis that equal volumes of gases at the same pressure and temperature contain the same number of molecules.
William Prout suggested that as all elements seemed to have atomic weights that were multiples of the atomic weight of hydrogen, they could all be in some way different combinations of hydrogen atoms. This contributed to the concept of the periodic table of the elements, the culmination of a long effort to find regular, systematic properties among the elements. Periodic laws were put forward almost simultaneously and independently by J. L. Meyer in Germany and D. I. Mendeleev in Russia (1869). An early triumph of the new theory was the discovery of new elements that fit the empty spaces in the table. William Ramsay's discovery, in collaboration with Lord Rayleigh, of argon and other inert gases in the atmosphere extended the periodic table
Organic Chemistry and the Modern Era
Organic chemistry developed extensively in the 19th cent., prompted in part by Friedrich Wohler's synthesis of urea (1828), which disproved the belief that only living organisms could produce organic molecules. Other important organic chemists include Justus von Liebig, C. A. Wurtz, and J. B. Dumas. In 1852 Edward Frankland introduced the idea of valency (see valence), and in 1858 F. A. Kekule showed that carbon atoms are tetravalent and are linked together in chains. Kekule's ring structure for benzene opened the way to modern theories of organic chemistry. Henri Louis Le Châtelier, J. H. van't Hoff, and Wilhelm Ostwald pioneered the application of thermodynamics to chemistry. Further contributions were the phase rule of J. W. Gibbs, the ionization equilibrium theory of S. A. Arrhenius, and the heat theorem of Walther Nernst. Ernst Fischer's work on the amino acids marks the beginning of molecular biology.
At the end of the 19th cent., the discovery of the electron by J. J. Thomson and of radioactivity by A. E. Becquerel revealed the close connection between chemistry and physics. The work of Ernest Rutherford, H. G. J. Moseley, and Niels Bohr on atomic structure (see atom) was applied to molecular structures. G. N. Lewis, Irving Langmuir, and Linus Pauling developed the electronic theory of chemical bonds, directed valency, and molecular orbitals (see molecular orbital theory). Transmutation of the elements, first achieved by Rutherford, has led to the creation of elements not found in nature; in work pioneered by Glenn Seaborg elements heavier than uranium have been produced. With the rapid development of polymer chemistry after World War II a host of new synthetic fibers and materials have been added to the market. A fuller understanding of the relation between the structure of molecules and their properties has allowed chemists to tailor predictively new materials to meet specific needs.
See I. Asimov, A Short History of Chemistry (1965); D. A. McQuarrie and P. A. Rock, General Chemistry (1984); L. Pauling, General Chemistry (3d ed. 1991); R. C. Weast, ed., CRC Handbook of Chemistry and Physics (published annually).
Chemistry is the branch of natural science that studies the chemical elements (atoms), the simple and complex compounds formed by atoms (molecules), and the transformations of these substances and the laws governing these transformations (seeATOM and MOLECULE). According to D. I. Mendeleev’s definition (1871), “chemistry in its current state may be … called the study of the elements.” [The origin of the word “chemistry” remains unresolved. Many conjecture that it originated from the ancient Greek name for Egypt, Chemia, found in Plutarch, which is derived from Chem or Chame, “black,” and means “the science of the black land” (Egypt), that is, “Egyptian science.”]
Modern chemistry is closely related to the other sciences, as well as to all branches of the national economy.
The qualitative nature of the chemical form of the motion of matter and its transformation into other forms of motion results in the diversity of the science of chemistry and its relationship to disciplines studying both lower and higher forms of motion. Knowledge of the chemical form of the motion of matter enriches man’s general understanding of the development of nature and the evolution of matter in the universe and facilitates the formation of an integral materialist view of the world. The close relationship between chemistry and other sciences has generated specific areas of mutual interest. Thus, the transitional areas between chemistry and physics are physical chemistry and chemical physics (seePHYSICS; PHYSICAL CHEMISTRY; and ). Special areas have arisen on the boundary between chemistry and biology and chemistry and geology, namely, geochemistry, biochemistry, biogeochemistry, and molecular biology (seeGEOCHEMISTRY; BIOCHEMISTRY; BIOGEOCHEMISTRY; and MOLECULAR BIOLOGY). The most important laws of chemistry are formulated in mathematical language, and consequently theoretical chemistry cannot develop without mathematics. Chemistry has influenced and continues to influence the development of philosophy and chemistry itself, in turn, has been influenced by philosophy.
Historically, two major divisions of chemistry have emerged: inorganic chemistry, which primarily studies the chemical elements and the simple and complex compounds, with the exception of carbon compounds, formed by these elements, and organic chemistry, which studies organic compounds, that is, compounds of carbon and other elements (see INORGANIC CHEMISTRY and ORGANIC CHEMISTRY). Until the end of the 18th century, the terms “inorganic chemistry” and “organic chemistry” indicated only from which kingdom of nature (mineral, plant, or animal) the compounds were obtained. In the 19th century, these terms came to indicate the presence or absence of carbon in a given compound, and subsequently they acquired new and broader meaning. Inorganic chemistry is allied with geochemistry, mineralogy, and geology, that is, with sciences concerned with inorganic nature. Organic chemistry studies the various compounds of carbon, including the most complex biopolymer compounds; through organic and bio-organic chemistry (seeBIO-ORGANIC CHEMISTRY), the science of chemistry borders on biochemistry and biology, that is, on all the life sciences. The study of hetero-organic compounds is on the boundary between organic and inorganic chemistry.
As chemistry developed, concepts of the structural levels of the organization of matter were gradually formulated. Matter exists at various levels of increasing complexity, beginning from the lowest, atomic, level and proceeding to the level of molecular and macromolecular (or high-molecular-weight) compounds (polymers), then to the level of intermolecular structures (complexes, clathrates, and catenanes), and finally to the level of various macrostructures (crystals and micelles), including indefinite nonstoichiometric formations. The following disciplines gradually emerged and became defined: the chemistry of complexes, polymer chemistry, crystal chemistry, the study of disperse systems and surface phenomena, and the study of alloys (seeCOMPLEXES; POLYMER; CRYSTAL CHEMISTRY; DISPERSE SYSTEMS; and SURFACE PHENOMENA).
Physical chemistry is concerned with the study of chemical objects and effects by the methods of physics and with the determination of the nature of chemical transformations predicated on the basis of the general principles of physics. A group of rather independent disciplines is related to physical chemistry, including chemical thermodynamics, chemical kinetics, electrochemistry, colloid chemistry, quantum chemistry, the study of the structure and properties of molecules, ions, and radicals, radiation chemistry, photochemistry, and the study of catalysis, chemical equilibria, and solutions (seeTHERMODYNAMICS, CHEMICAL; KINETICS, CHEMICAL; COLLOID CHEMISTRY; QUANTUM CHEMISTRY; RADIATION CHEMISTRY; and CATALYSIS).
Analytical chemistry, whose methods are used in all areas of chemistry and in the chemical industry, has acquired an independent character (seeANALYTICAL CHEMISTRY). Interest in the practical applications of chemistry has resulted in the development of various new scientific disciplines, such as chemical engineering with its many branches, metallurgy, agricultural chemistry, medical chemistry, and forensic chemistry (see; METALLURGY; and AGRICULTURAL CHEMISTRY).
As an area of practical activity, chemistry traces its origins to ancient Egypt, India, China, and other countries. Long before the Common Era, transformations of various substances were recognized and their uses for various purposes learned. One of the most ancient branches of chemistry is metallurgy. The smelting of copper from ores dates to the fourth or third millennium B.C., followed later by the production of bronze, an alloy of copper and tin. In the second millennium B.C., man began producing iron from ores using the bloomery process. Circa 1600 B.C., indigo came to be used as a natural fabric dye, followed later by Tyrian purple and alizarin. Also obtained during this period were vinegar, drugs from plants, and other substances whose production is in some way related to a chemical process. The two alternative theories of ancient natural philosophy—the atomistic and elemental theories—sprang from the same roots as chemistry.
Alchemy arose in the third or fourth century A.D. in Alexandria (seeALCHEMY). Alchemists believed that the conversion of nonprecious metals into precious metals, namely gold and silver, was possible by using the philosopher’s stone. The chemistry of this period was primarily concerned with the observation of the individual properties of substances and with the explanation of these properties on the basis of the principles (constituents) of the substances.
During the Renaissance, the industrial and, in general, the practical aspects of alchemy steadily acquired greater importance as manufacturing developed. The practical aspects included metallurgy, glassmaking, and the production of ceramics and dyes (the work of V. Biringuccio, G. Agricola, and B. Palissy). Iatrochemistry emerged as a special medical discipline (P. A. Paracelsus, J. B. van Helmont). These two trends were characteristic of chemistry of the 16th and first half of the 17th centuries and were instrumental in the emergence of chemistry as a science. This period was marked by the development of new experimental methods in chemistry and the accumulation of new observation results. In particular, new furnace designs were developed and improved, and new laboratory instruments and methods for the purification of compounds (crystallization and distillation) were worked out. New chemical preparations were also obtained.
At the beginning of the second half of the 17th century, R. Boyle proved the invalidity of alchemical concepts and gave the first scientific definition of a chemical element, thereby for the first time raising chemistry to the level of a science. The development of chemistry into a science took more than 100 years, a process that ended with the discoveries of A. L. Lavoisier. The first theory in chemistry was the phlogiston theory, based on a special hypothetical principle of combustibility (phlogiston). Although erroneous, the phlogiston theory nevertheless encompassed a broad range of facts relating to combustion and to the roasting of metals. The second half of the 17th century was marked by the rapid development of chemical analysis, in particular of qualitative analysis, beginning with Boyle’s work; quantitative analysis began developing intensively in the mid-18th century with the work of M. V. Lomonosov and J. Black.
In 1748, Lomonosov and, later, Lavoisier discovered the law of the conservation of mass in chemical reactions. Lavoisier’s formulation of the oxygen theory, according to F. Engels, “for the first time set on its feet all of chemistry, which in its phlogiston form had stood on its head” (K. Marx and F. Engels, Soch., 2nd ed., vol. 24, p. 20). At the end of the 18th century, chemistry finally acquired the character of a true science.
Chemistry of the 19th century was characterized by the development of atomism (seeATOMISM). In the 17th and 18th centuries, the study of the atom proceeded from a highly abstract, mechanistic viewpoint. Lomonosov was the first to perceive the possibility of applying the atomic hypothesis to problems in chemistry. Atomistic chemistry arose from the coalescence of natural philosophy’s idea about atoms and experimental analytical data on the quantitative chemical composition of compounds. During the first two thirds of the 19th century, the fundamental chemical concepts of atomic weight and valence, or “atomicity,” were formulated (seeATOMIC MASS and VALENCE), and in 1869, Mendeleev discovered the relationship between the two. In 1803, J. Dalton derived the law of multiple proportions on the basis of the atomic theory and in 1804 confirmed it experimentally. He then developed the concept of atomic weights and compiled the first table of atomic weights, taking the combining weight of hydrogen as unity. However, the values of the atomic weights of the elements were still quite inaccurate, partly because of the primitiveness of the measuring instruments and partly because the correct relationship between the concepts of atoms and molecules had not yet been established.
The concept of molecules and their difference from atoms was advanced by A. Avogadro (1811) and A. Ampère (1814), but it was not accepted by other chemists at the time. J. J. Berzelius, making use of a vast amount of experimental data, substantiated Dalton’s law of multiple proportions and extended it to organic compounds. He published (1814) a table of the atomic weights of 46 elements that was more accurate than Dalton’s table and introduced new chemical symbols (seeCHEMICAL SYMBOLS). On the basis of the first data indicating a relationship between electrical and chemical processes, Berzelius proposed the “dualistic” theory (1812–19), according to which chemical interactions result from the action of electrical forces, since it was assumed that there were two electrical poles in each atom and in any atomic grouping. Attempts were made to apply this theory to organic chemistry, more specifically, to the radical theory, according to which organic compounds also consist of two parts, one of which is a radical, that is, a group of atoms that behaves like a single atom and is transferred without change from one chemical compound to another. The unitary (molecular) theory of C. F. Gerhardt replaced the dualistic theory, and the theory of types, correspondingly, replaced the radical theory (seeRADICAL THEORY and TYPES, THEORY OF). The theory of types did not reflect the structure of a compound but rather its chemical transformations, indicating the reactivity of molecular functional groups. Thus, several different “type” formulas could exist for one compound. Organic compounds themselves were considered as the products of the substitution of hydrogen atoms by other atoms or groups (residues) in the hydrogen, water, hydrogen chloride, and ammonia molecules. The theory of types, particularly the concept of homologous series introduced by Gerhardt, played an important role in the classification of organic compounds and their transformations (seeHOMOLOGOUS SERIES).
In 1852, E. Frankland, in the study of metallo-organic compounds, laid the basis for the concept of valence. He demonstrated that atoms have a definite combining capacity, satisfied by the same number of additive atoms. The valence of hydrogen was taken as the unit of valence. Subsequently, F. A. Kekulé introduced the concept of methane-type compounds, which implied that the carbon atom was tetravalent. He also proposed (1858) that carbon atoms may combine in the form of a chain. In 1858, A. Couper, for the first time, drew graphic formulas of organic compounds on the basis of the tetravalency of carbon. However, Kekulé was unable to completely overcome the limitation of “type” concepts. This was done by A. M. Butlerov, who proposed (1861) the theory of chemical structure, according to which the chemical properties of a compound are determined by its molecular composition and structure, while a compound’s reactivity depends on the specific sequence by which the atoms are bonded in a given molecule, as well as on the interaction of these atoms (see).
The first international chemistry congress, held in Karlsruhe in 1860, clearly defined the concepts of atom, molecule, and equivalent, which facilitated the further development of chemistry. In the period 1859–61, chemistry was enriched by spectral analysis (seeSPECTRAL ANALYSIS), a highly precise method that made possible the discovery of several chemical elements in the composition of astronomical bodies; consequently, a relationship between physics (optics), astronomy, and chemistry was established.
With the discovery of an ever-growing number of new chemical elements, the need for their systematization became increasingly evident. In 1869, D. I. Mendeleev discovered the interrelationship of the elements: he developed the periodic system of the elements and discovered the underlying law (seePERIODIC SYSTEM OF ELEMENTS and PERIODIC LAW). Mendeleev’s work proved to be the theoretical synthesis of all the preceding developments in chemistry. Mendeleev correlated the physical and chemical properties of all the 63 chemical elements then known with their atomic weights and discovered the relationship between two very important quantitatively measured atomic properties, namely, atomic weight and valence (“compound forms”), which are the foundation on which chemistry is constructed.
On the basis of the periodic system, Mendeleev corrected the existing values of the atomic weights of many elements and predicted a series of elements not yet discovered, also providing a detailed description of the properties of three of the undiscovered elements. These predictions and the periodic law itself were soon brilliantly confirmed by experiments. Subsequently, the periodic law served as the basis for the development of chemistry and the entire study of matter.
With the development of physics and chemistry, basic concepts and laws were established, which, on the one hand, placed both these sciences on a higher level and, on the other, served as the groundwork for the formulation of physical chemistry, individual branches of which had already emerged at the end of the 18th and first half of the 19th centuries. The chemical industry, which by the 1880’s had achieved considerable progress, expressed a keen interest in the study of general chemical relationships.
The study of the thermal effects of chemical processes became firmly grounded after the discovery of a basic law of chemical processes by G. I. Gess (also G. H. Hess; seeHESS’S LAW). In the second half of the 19th century, extensive work on the determination of the heats of chemical reactions was carried out by P. E. M. Berthelot, H. P. Thomsen, and N. N. Beketov. This work culminated at the end of the 19th century in the founding of thermochemistry, a branch of physical chemistry (seeTHERMOCHEMISTRY). The appearance of thermodynamics (seeTHERMODYNAMICS) and its close development with thermochemistry led to the development in the second half of the 19th century of chemical thermodynamics, which is concerned with the study of the energy effects that accompany chemical processes and the feasibility, direction, and limits of such processes, as well as other thermodynamic effects in physicochemical systems (the work of J. W. Gibbs, J. van’t Hoff, H. Le Châtelier).
Electrochemical studies, successfully initiated by H. Davy, obtained quantitative precision in the work of M. Faraday, who discovered (1833–34) the laws of electrolysis. In the second half of the 19th century, the study of the mechanism of the passage of an electrical current through electrolytic solutions was begun by R. Clausius, J. W. Hittorf, and F. Kohlrausch, which led to S. Arrhenius’ derivation (1883–87) of the theory of electrolytic dissociation, according to which electrolytes in solution dissociate into ions. The application of the laws of thermodynamics to electrochemistry also made it possible to establish the cause of the generation of an electromotive force in galvanic circuits.
The study of solutions developed concurrently. Advances were made by van’t Hoff in his work on dilute solutions carried out in the period 1885–89. In these studies the properties of solutes were compared with the properties of gases, while the solvent was considered an indifferent medium. According to Mendeleev’s theory of aqueous solutions, developed between 1865 and 1887, the solute and solvent interact in a solution. Mendeleev did not draw a sharp distinction between chemical reactions and the interactions of solute and solvent in a solution. This view was further developed by N. S. Kurnakov and his school of physicochemical analysis (seePHYSICOCHEMICAL ANALYSIS).
Observations of liquid systems showed that in addition to true solutions, in which the solute is present as separate molecules and ions, there exist systems in which the “dissolved” substance is present in the form of aggregates consisting of an enormous number of molecules. In 1861, T. Graham introduced the term “colloid” for such systems. Subsequent studies of disperse systems led to the development of colloid chemistry.
As early as the beginning of the 19th century, the first evidences were obtained of the acceleration of chemical reactions by the addition of small amounts of certain substances. Such processes were called contact processes (1833) by E. Mitscherlich and catalytic processes (1835) by Berzelius. Interest in such processes increased greatly in the 1860’s after the catalytic production of sulfuric acid was developed. At the end of the 19th century, the study of catalysis and the practical use of catalysts occupied an important place in chemistry (seeCATALYSTS). Adsorption, which was discovered by T. E. Lovits (J. T. Lowitz) in 1785, is closely related to catalysis. In 1878, Gibbs established the fundamental laws of surface phenomena, adsorption, and the formation of new phases.
The study of the rates of chemical reactions and chemical equilibria emerged in the second half of the 19th century. The significance of the active mass (concentration) of reactants had been noted as early as 1801–03 by C. Berthollet. The subsequent study of the equilibrium of chemical reactions led C. Guldberg and P. Waage to the discovery (1864–67) of the law of mass action which formed the basis of the study of reaction rates. The systematic studies of N. A. Menshutkin, begun in 1877, made significant contributions to the establishment of the kinetic regularities of chemical reactions. In 1884, van’t Hoff summarized the data accumulated in this field in the form of kinetic equations.
The end of the 19th century was marked by three outstanding discoveries in physics—X-rays, the phenomenon of radioactivity, and the electron—which eventually led to proof of the complex structure of the atom, previously considered to be indivisible. These discoveries heralded a new era in the development of chemistry. E. Rutherford’s establishment of the existence of atomic nuclei and derivation of the planetary model of the atom (1911) led to the start of the successful development of the theory of atomic structure and the appearance of new concepts of the electrical nature of chemical forces (seeATOMIC PHYSICS).
The law discovered by H. Moseley in 1913 established a relationship between the position of an element in the periodic system and its characteristic X-ray emission. This led to the conclusion that the atomic number of a chemical element was numerically equal to the charge of the atomic nucleus of the element and thus equal to the total number of electrons in the shell of the neutral atom. An even deeper understanding of the periodic law was achieved through the research of N. Bohr and others, which showed that in proceeding from elements with lower to higher atomic numbers, the filling of shells (levels and sublevéis) occurred at an ever-greater distance from the nucleus. Furthermore, similar structures of the outer electron configurations were periodically repeated, which, in general, determined the periodicity of the chemical properties and most of the physical properties of the elements and their compounds. The Pauli exclusion principle was instrumental in leading to an understanding of the regularities in the filling of the electron shells and in providing an explanation of atomic and molecular spectra. The various discoveries permitted the solution of many problems related to the further development and theoretical grounding of Mendeleev’s periodic system. The discovery of isotopes showed that nuclear charge rather than atomic mass determines an element’s position in the periodic system.
The discovery of neutrons by J. Chadwick (1932) and of artificial radioactivity by I. Joliot-Curie and F. Joliot-Curie (1934) served as the basis for obtaining new radioactive isotopes and new elements not present in nature and the subsequent synthesis of the transuranium elements.
The problem of elucidating the nature of the chemical bond was related to the solution of the problem of atomic structure. W. Kossel and G. Lewis both proposed (1916) the first electron theories of valence and the chemical bond. Kossel’s theory concerned the formation of the ionic bond, while Lewis’ theory concerned the formation of the covalent bond. The descriptive aspect of both theories, which correlated well with Bohr’s model of the atom and explained some characteristics of the chemical bond, has retained its importance to this day. Later concepts, in particular the resonance theory of L. Pauling, made it possible to obtain qualitative and semiquantitative data on molecular symmetry, on the equivalence of specific bonds and structural elements in molecules, and on the stability and reactivity of molecules (seeRESONANCE THEORY).
However, it was not until the advent of quantum mechanics (seeQUANTUM MECHANICS) that it became possible to explain the nature of the chemical bond and to calculate accurately the bond energy for the simplest molecule—the hydrogen molecule (the German scientists W. Heitler and F. London in 1927)—and many physical parameters of other diatomic molecules and some polyatomic molecules (such as H2O, HF, LiH, and NH3), including interatomic distances, the formation energy from atoms, spectral vibration frequencies, electrical and magnetic properties, and the saturation and direction of bonds.
The modern stage of the development of chemistry is characterized by a rapid development of the spatial conceptions of the structure of matter and stereochemical conceptions. As early as 1874–75, J. A. Le Bel and van’t Hoff had proposed that the four atoms or radicals bonded to a carbon atom do not lie in one plane but rather are located in space at the corners of a tetrahedron, at the center of which is the carbon atom. This conception expanded the concept of isomerism and made it possible to establish several types of isomerism; it also served as the basis for stereochemistry (seeISOMERISM and STEREOCHEMISTRY). The stable spatial configurations of many molecules were determined and labile molecular configurations were later discovered; the latter arise as a result of some hindrance to free rotation of the atomic groups about single bonds (seeCONFORMATIONAL ANALYSIS).
The contemporary theory of chemistry is based on the general physical theory of the structure of matter and on advances in quantum theory, thermodynamics, and statistical physics. The application of quantum-mechanical methods to chemical problems led to the development of quantum chemistry with the goal of solving the Schrödinger wave equation for the many-electron systems of molecules. One of the first results was the theory of valence bonds, which still retained the traditional concept of an electron pair as the carrier of the chemical bond (Heitler, London, J. Slater, Pauling). Subsequently, the molecular orbital (MO) method was developed, which considers the whole electronic structure of the molecule; each molecular orbital (wave function) takes into account the contribution to it of all the atomic electron orbitals (seeMOLECULAR ORBITAL METHOD). The most common variant of the MO method is based on an approximate description of molecular orbitals by a linear combination of atomic orbitals (LCAO MO). In a number of cases, highly complex molecular calculations may be performed using modern computer technology without any prior simplifications for simple molecules. On the basis of the given method, the molecular energy is calculated, as are the electron parameters (distribution of electron density, bond energy, bond length, bond order, and some physical properties of compounds). The MO method has now been extended to organic chemistry. In inorganic chemistry, the ligand field theory was developed by H. Bethe, which combines the MO method and the theory of the crystal field.
The quantum-chemical consideration of the kinetic equations formulated by Arrhenius and van’t Hoff led to the theory of the absolute rates of chemical reactions, which underlies chemical kinetics. This made it possible to focus on a very important problem of contemporary chemistry, namely, the nature of the transition state and the intermediate activated complex (seeACTIVATED COMPLEX), within which occur molecular rearrangement processes, which still remain largely unclear.
The detailed study of the kinetics and mechanisms of reactions and the investigation of the simple steps of chemical reactions are important problems of chemical physics. Studies of chain reactions, the theory of which is based on the work of N. N. Semenov and C. Hinshelwood, have acquired great importance. Kinetic studies have played an important role in the development of the technology of petroleum refining and fuel combustion and the synthesis of high-molecular-weight compounds. The feasibility of the chemical fixation of nitrogen at ordinary temperature and pressure has been demonstrated, a discovery that may significantly alter future technology.
Nuclear transformations and the accompanying physicochemical effects, nuclear reaction products, and radioactive isotopes, elements, and substances are studied in nuclear chemistry and radiochemistry (seeRADIOCHEMISTRY). Research in these areas is of great importance for the production and extraction of radioactive raw materials, the separation of isotopes, and the use of fissionable materials.
The interaction between matter and radiation and matter and various types of high-energy particles leading to chemical transformations is studied in radiation chemistry. Radiation initiates many processes, including the synthesis of high-molecular-weight compounds from monomers. In particular, photochemical reactions occur under the action of light. Photochemistry studies both the capture of the energy of electromagnetic radiation (for example, in photosynthesis performed by green plants) and the numerous reactions of synthesis, decomposition, isomerization, and rearrangement that arise in the course of a given interaction (seePHOTOCHEMISTRY). The industrial applications of high-energy lasers are being developed.
Extensive experimental electrochemical data have been accumulated on electrolytes, electrolytic conductance, and electrochemical processes. Electrochemical kinetics has emerged, and nonequilibrium electrode potentials and metal corrosion processes are being studied. New chemical sources of current are being developed. Advances in theoretical electrochemistry have provided a firmer scientific basis for many industrial electrochemical processes.
The effect of magnetic fields on the chemical behavior of molecules is examined in magnetochemistry (seeMAGNETOCHEMISTRY). Thermochemical studies have expanded as a result of the investigation of the interaction of matter and plasma, in particular, for the purpose of applications in plasma chemistry technology. Plasma chemistry arose in the 1960’s, and fundamental work in this field has been carried out in the USSR, USA, and Federal Republic of Germany (seePLASMA and PLASMA CHEMISTRY).
Chemical transformations occur in all states of aggregation of matter, namely, in the liquid, gaseous, and solid states. The study of the chemical reactions of solids (seeTOPOCHEMICAL REACTION) is acquiring greater significance.
Data are accumulating in contemporary chemistry on the evolution of matter in the universe, which has contributed to a general understanding of the evolution of nature. Contemporary nuclear physics and astrophysics have provided theories of the origin of the chemical elements. A picture of the chemical differentiation of matter at the planetary stage of development, in particular, geochemical evolution, has gradually emerged as a result of the study of meteorites, volcanic rocks, and lunar soil (seeGEOCHEMISTRY and COSMOCHEMISTRY).
A theory of the stages of the chemical evolution of matter preceding the origin of life has been developed as a result of the discovery of complex organic molecules in interstellar space, in meteorites, and in the most ancient terrestrial rocks, as well as modeling experiments of the synthesis of complex organic compounds from very simple compounds, such as CH4, CO2, NH3, and H2O, under spark discharge conditions and radioactive and ultraviolet irradiation.
The geochemistry of volcanogenic and sedimentary rocks, hydrochemistry (seeHYDROCHEMISTRY), the chemistry of the atmosphere, and biogeochemistry are gradually developing concepts of the planetary migrations of chemical elements, while biochemistry is contributing to the understanding of life cycles. Concrete results are steadily being obtained supporting V. I. Vernadskii’s theory of the importance of the vital processes of living organisms for understanding the fate of the chemical elements on our planet.
Organic chemistry has made great advances. Automatic methods for the synthesis of many proteins have been developed, and the structures of some important natural compounds, such as tetrodotoxin, hemoglobin, and aspartate-aminotransferase, which contains 412 amino-acid residues, have been established. Highly complex natural compounds, such as quinine, vitamin B12, and even chlorophyll, have been synthesized. Organic chemistry has had an enormous influence on the development of molecular biology and has served as the basis for the creation of a vast heavy organic synthesis industry.
The chemistry of polymers, which emerged as an independent discipline only in the 1930’s, studies all the concepts of the pathways of the synthesis of high-molecular-weight compounds and the properties and transformations of these compounds, as well as the properties of substances formed from macromolecules. Contemporary polymer chemistry is characterized by a detailed study of the mechanisms of catalytic polymerization produced by metallo-organic compounds, in particular, the synthesis of stereoregular polymers and the study of the microstructure of high-molecular-weight compounds. It has been established that the properties of polymers depend not only on the chemical composition, structure, and dimensions of the macromolecules but also, to an equal extent, on their mutual arrangement and packing (supermolecular structure). An important advance has been the creation of heat-resistant polymers, such as organosilicon and polyimide polymers. Advances in polymer chemistry have been instrumental in the creation of such important branches of the chemical industry as the production of plastics, synthetic rubber, chemical fibers, paints, varnishes, ion-exchange resins, and adhesives.
Certain chemical processes are very important at all the structural levels of biology. The continuous metabolism in an organism is a highly complex system of coordinated chemical reactions that are carried out with the participation of specific protein catalysts called enzymes.
Studied within the framework of chemical ecology are the action of chemical processes occurring in the environment, the community of organisms (biocenosis), the chemical migration of elements within ecosystems, and the chemical stimulation or repression of symbiotic or competing species. The behavior of organisms within communities depends to a considerable extent on the chemical agents for information transfer, such as pheromones, which are used by animals to attract or frighten other animals and which help regulate activities within beehives and ant colonies.
Neurochemical studies, which have a long tradition in biochemistry, have grown into a new discipline that investigates the effects of chemical compounds on mental processes; molecular psychobiology, which relates molecular biology to the behavioral sciences, is developing (see alsoPSYCHOPHARMACOLOGY).
In the mid-20th century, radical changes took place in chemical research methods, which now include a broad range of physical and mathematical methods. The classical problem of chemistry—the determination of the composition and structure of compounds—is being solved with increasingly greater ease using modern physical methods. The use of high-speed computers has become an integral feature of theoretical and experimental chemistry for quantum-chemical calculations, the determination of kinetic regularities, the processing of spectroscopic data, and the determination of the structure and properties of complex molecules.
Of all the purely chemical methods developed in the 20th century, microchemical analysis should be singled out (seeMICROCHEMICAL ANALYSIS). This method permits the determination of amounts hundreds of times smaller than the amounts determined through ordinary chemical analysis. Chromatography, which is used not only in chemical analysis but also in the separation of compounds with very similar chemical properties, both on a laboratory scale and an industrial scale, has aquired great importance (see). Physicochemical analysis plays an important role in the determination of the chemical composition and nature of the interaction of components in solutions, melts, and other systems. Graphic methods, such as phase diagrams and composition-property diagrams, are commonly used in physico-chemical analysis. The classification of composition-property diagrams permits the clarification of chemical species, the compositions of which may be constant or variable. The class of nonstoichiometric compounds, predicted by Kurnakov, has acquired great importance in materials science and in the new field of solid-state chemistry.
Luminescence analysis, the method of tagged atoms, X-ray diffraction analysis, electron diffraction, polarography, and other physicochemical analytical methods are widely used in analytical chemistry (seeLUMINESCENCE ANALYSIS; X-RAY DIFFRACTION ANALYSIS; and POLAROGRAPHY). The use of radiochemical techniques has permitted the discovery of the presence of only a few atoms of a radioactive isotope (for example, in the synthesis of transuranium elements).
Of great importance in establishing the structures of chemical compounds has been molecular spectroscopy, which is used for determining distances between atoms, symmetry, the presence of functional groups, and other characteristics of molecules (seeMOLECULAR SPECTRA). It is also used in the study of the mechanisms of chemical reactions. The electron energy structure of atoms and molecules and the value of effective charges are ascertained using emission and absorption X-ray spectroscopy (seeX-RAY SPECTROSCOPY). Molecular geometry is studied by X-ray diffraction analysis.
The discovery of the interaction between electrons and atomic nuclei (resulting in the hyperfine structure of atomic spectra) and the interaction between outer and inner electrons was instrumental in the development of various methods for establishing molecular structure, such as nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR), nuclear quadrupole resonance (NQR), and gamma-resonance spectroscopy. NMR spectroscopy has acquired a special role because of its broad range of applications. Optical methods, such as spectral polarimetry, circular dichroism, and optical rotatory dispersion, are acquiring increasing importance for elucidating the spatial structural characteristics of molecules. The decomposition of molecules in a vacuum as a result of electron collision and the subsequent identification of the fragments are useful in establishing molecular structure by mass spectrometry (seeMASS SPECTROMETRY). The arsenal of kinetic methods has been augmented by methods related to the use of EPR and NMR spectroscopy (chemically induced nuclear polarization), flash photolysis, and pulse radiolysis, all of which permit the study of very fast processes, occurring in 10–9 sec or less.
Spectral analysis in various regions of the electromagnetic spectrum has been used successfully in the study of objects in space. In particular, radio-astronomical methods have detected clouds of chemical compounds in interstellar space, which include such relatively complex compounds as formaldehyde, thiourea, methylamine, and cyanoacetylene. With the development of space flight, experimental chemical methods have been used for such nonterrestrial objects as the moon, Venus, and Mars.
The requirements of society have engendered chemical technology. According to Berthelot, chemistry generates its own object of study, creating hundreds of thousands of compounds that are unknown in nature. In the development of chemical technology, historically the first methods involved the separation and decomposition of available natural forms, such as the production of metals from ores, the separation of salts from complex systems, and the distillation of wood. The basis of chemical technology was the production of starting materials for many more complex technologies: sulfuric acid, hydrochloric acid, nitric acid, ammonia, alkali bases, soda, and several other products, all of which make up the basic chemical industry. The second major stage in the history of chemical technology was the transition to the methods of synthesis and the production of increasingly more complex systems, which is based not only on empirical data but also on a theoretical understanding of the nature, structure, and properties of chemical compounds and the regularities of their formation (seeSYNTHESIS, CHEMICAL).
The technology of synthesis in chemistry has evolved from the use of available natural compounds, to the more complex modifications of these compounds, and finally to the production of new chemical products, unknown in nature. Thus, the technology of fiber production began with the processing of natural cellulose, then proceeded to the production of chemically modified forms of cellulose, such as viscose and acetate silk, and finally made the leap to the manufacture of synthetic materials, such as polyesters, polyamides, and polyacrylonitrile. The technology of synthetic fibers has developed more rapidly than the technology of man-made fibers from natural polymers.
An important trend in the development of chemical technology is the advance beyond the physicochemical conditions that exist on our planet to a broader use of extreme conditions and unusual factors, such as high temperatures, superhigh pressures, and the action of plasma, the electrical and magnetic fields, and radiation. Chemical technology has set as one of its goals the production of compounds with unusual and very valuable properties, such as ultrapure, superhard, heat-resistant, and thermally stable materials, semiconductors, phosphors, photochromes, thermochromic substances, catalysts, inhibitors, biological stimulants, and drugs.
The rapid expansion of the sources of chemical raw materials is envisioned. An increasing number of chemical elements (up to the transuranium elements) are being used in industry, and a more completely integrated processing of natural products is being achieved. Plans are being developed for the use of such sources of raw materials as the oceans.
The increase in artificial chemical effects on natural processes often leads to disruptions and changes in natural chemical cycles. This complicates the ecological problem of maintaining and scientifically regulating the environment. Important for the solution of this problem is the creation of chemical production processes that generate no wastes and the development of regulated chemical cycles in the nature-society system as an important part of measures for the protection of the environment (seeCONSERVATION).
The increasing role of chemistry as a science has been accompanied by the rapid development of basic, applied, and combined research and the creation of new materials to meet required specifications and new engineering processes. A major feature of the modern development of productive capacity is the chemicalization of the national economy. (See also; ; SCIENTIFIC AND TECHNOLOGICAL PROGRESS; SCIENTIFIC AND TECHNOLOGICAL REVOLUTION; and MATERIAL AND TECHNICAL BASIS FOR COMMUNISM.)
Scientific work in chemistry is carried out at institutes and laboratories of the academies of sciences, at industrial institutes, and at the laboratories of universities, technical higher educational institutions, and industrial conglomerates and firms.
The international organization that maintains ties between the chemical research centers of various countries is the International Union of Pure and Applied Chemistry, of which the National Committee of Soviet Chemists of the Academy of Sciences of the USSR is a member. (See alsoCHEMICAL SCIENTIFIC SOCIETIES AND UNIONS and .)
In the USSR, the leading chemical organization is the D. I. Mendeleev All-Union Chemical Society, which regularly holds meetings on theoretical and applied chemistry and publishes its own journals.
The major periodical publications in chemistry are discussed in CHEMICAL JOURNALS.
REFERENCESBibliography, history, and methodology of chemistry
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Khimiia v izdaniiakh Akademii nauk SSSR, vol. 1, fases. 1–2. Moscow-Leningrad, 1947–51.
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Hjelt, E. Istoriia organicheskoi khimii s drevneishikh vremen do nastoiashchego vremeni. Kharkov-Kiev, 1937. (Translated from German.)
Schorlemmer, C. Vozniknovenie i razvitie organicheskoi khimii. Moscow, 1937. (Translated from English.)
Glavy iz istorii organicheskoi khimii. [Collection of articles.] Moscow, 1975.
Bykov, G. V. Istoriia organicheskoi khimii. Moscow, 1976.
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Menshutkin, N. A. Ocherk razvitiia khimicheskikh vozzrenii. St. Petersburg, 1888.
Meyer, E. Istoriia khimii ot drevneishikh vremen do nastoiashchikh dnei. Foreword by D. I. Mendeleev. St. Petersburg, 1899. (Translated from German.)
Ladenburg, A. Lektsii po istorii razvitiia khimii ot Lavuaz’e do nashego vremeni. (Translated from German.) With addendum “Ocherk istorii khimii v Rossii” by P. I. Val’den. Odessa, 1917.
Menshutkin, N. A. Khimiia i puti ee razvitiia. Moscow-Leningrad, 1937.
Figurovskii, N. A. Ocherk obshchei istorii khimii. Ot drevneishikh vremen do nachala XIX v. Moscow, 1969.
Giua, M. Istoriia khimii. Moscow, 1975. (Translated from Italian. Contains references.)
Ocherki po istorii khimii. [A collection.] Moscow, 1963.
Trudy Instituta istorii estestvoznaniia i tekhniki AN SSSR, vols. 2, 6, 12 18 30, 35, 39. Moscow, 1954–62.
Kuznetsov, V. I. Evoliutsiia predstavlenii ob osnovnykh zakonakh khimii. Moscow, 1967.
Manolov, K. Velikie khimiki, vols. 1–2. [Moscow] 1976. (Translated from Bulgarian.)
Zhurnal Vsesoiuznogo khimicheskogo obshchestva im. D. I. Mendeleeva, 1975, vol. 20, no. 6. (Issue devoted to Nobel Prize winners in chemistry.)
Kuznetsov, V. I. Dialektika razvitiia khimii. Moscow, 1973.
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Kedrov, B. M. Engel’s o khimii. Moscow, 1971.
Filosofskie problemy sovremennoi khimii: Sb. perevodov. Moscow, 1971.
Gnoseologicheskie i sotsial’nye problemy razvitiia khimii. Kiev, 1974.
Zhdanov, Iu. A. Ocherki metodologii organicheskoi khimii. Moscow, 1960.
Metodologicheskie problemy sovremennoi khimii: Sb. perevodov. Moscow, 1967.
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Hoefer, Fr. Histoire de la Chimie, 2nd ed., vols. 1–2. Paris, 1867–69.
Partington, J. R. A History of Chemistry, vols. 1–4. London-New York, 1961–70.
Graebe, C. Geschichte der Organischen Chemie, vol. 1. Berlin, 1920.
Walden, P. Geschichte der Organischen Chemie seit 1880. Berlin, 1941.
Szabadváry, F. Geschichte der Analytischen Chemie. Budapest, 1966.
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Great Chemists. Edited by E. Farber. New York-London, 1961.
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Lomonosov, M. V. Izbr. trudy po khimii i fizike. Moscow, 1961.
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Butlerov, A. M. “Vvedenie k polnomy izucheniiu organicheskoi khimii.” Soch., vol. 2. Moscow, 1953.
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Stoletie teorii khimicheskogo stroeniia: Sb. statei A. M. Butlerova, A. Kekule, A. S. Kupera, V. V. Markovnikova. Moscow, 1961.
Markovnikov, V. V. Izbr. trudy. Moscow, 1955.
Mendeleev, D. I. “Osnovy khimii.” Soch., vols. 13–14. Leningrad-Moscow, 1949.
Mendeleev, D. I. Periodicheskii zakon. [Principal articles.] Moscow, 1958.
Mendeleev, D. I. Periodicheskii zakon: Dopolnitel’nye materialy. Moscow, 1960.
Gibbs, J. W. Termodinamicheskie raboty. Moscow-Leningrad, 1950. (Translated from English.)
Van’t Hoff, J. H. Ocherki po khimicheskoi dinamike. Leningrad, 1936. (Translated from French.)
Tsvet, M. S. Khromatograficheskii adsorbtsionnyi analiz. Moscow-Leningrad, 1946. (In the series Klassiki nauki.)
Brave, O. Izbr. nauchnye trudy. Leningrad, 1974. (In the series Klassiki nauki.)
Gedroits, K. K. Izbr. nauchnye trudy. Moscow, 1975. (In the series Klassiki nauki.)
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Kratkaia khimicheskaia entsiklopediia, vols. 1–5. Editor in chief, I. L. Knuniants. Moscow, 1961–67.
Spravochnik khimika, 2nd ed., vols. 1–6. Edited by B. P. Nikol’skii. Moscow-Leningrad, 1965–68.
Lur’e, Iu. Iu. Spravochnik po analiticheskoi khimii. Moscow, 1962.
Kratkii spravochnik po khimii, 4th ed. Edited by O. D. Kurilenko. Kiev, 1974.
Khimiia: Spravochnoe rukovodstvo. Leningrad, 1975. (Translated from German.)
Neorganicheskaia khimiia: Entsiklopediia shkol’nika. Editor in chief, I. P. Alimarin. Moscow, 1975.
Gordon, A. J., and R. Ford. Sputnik khimika: Fiziko-khimicheskie svoistva. Moscow, 1976. (Translated from English.)
Landolt, H., and R. Börnstein. Zahlenwerte und Funktionen aus Physik, Chemie, Astronomie, Geophysik und Technik, 6th ed., vols. 1–4. Berlin, 1966–75—.
Gmelins Handbuch der anorganischen Chemie, 8th ed., Sys.-Num. 1–73—. Berlin-London, 1926–74—.
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Traite de chimie organique, vols. 1–23. Edited by V. Grignard. Paris, 1935–54.
Contemporary university textbooks
Nekrasov, B. V. Osnovy obshchei khimii, 3rd ed. [vols.] 1–2. Moscow, 1973.
Pauling, L. Obshchaia khimiia. Moscow, 1974. (Translated from English.)
Campbell, J. Sovremennaia obshchaia khimiia [vols.] 1–3. Moscow, 1975. (Translated from English.)
Glinka, N. L. Obshchaia khimiia, 18th ed. Leningrad, 1976.
Kurs fizicheskoi khimii, 2nd ed., vols. 1–2. Edited by Ia. I. Gerasimov. Moscow, 1969–73.
Kireev, V. A. Kurs fizicheskoi khimii, 3rd ed. Moscow, 1975.
Nesmeianov, A. N., and N. A. Nesmeianov. Nachala organicheskoi khimii, books 1–2. Moscow, 1969–70.
Kreshkov, A. P. Osnovy analiticheskoi khimii, 4th ed. [books] 1–2. Moscow, 1976.
IU. A. ZHDANOV and B. M. KEDROV