# chemistry

American Heritage Dictionary:

## chem·is·try

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(kĕm'ĭ-strē)
n., pl., -tries.
1. The science of the composition, structure, properties, and reactions of matter, especially of atomic and molecular systems.
2. The composition, structure, properties, and reactions of a substance.
3. The elements of a complex entity and their dynamic interrelation: "Now that they had a leader, a restless chemistry possessed the group" (John Updike).
4. Mutual attraction or sympathy; rapport: The chemistry was good between the partners.

Gale Encyclopedia of US History:

## Chemistry

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Chemistry is the study of the chemical and physical change of matter.

Early U.S. Chemistry and Chemical Societies

The beginning of chemistry in the United States came in the form of manufacturing goods such as glass, ink, and gunpowder. In the mid-1700s, some academic instruction in chemistry started in Philadelphia. The earliest known academic institution to formally teach chemistry was the medical school of the College of Philadelphia, where Benjamin Rush was appointed the chair of chemistry in 1769. Not only was Rush the first American chemistry teacher, he may have been the first to publish a chemistry textbook written in the United States. In 1813, the Chemical Society of Philadelphia published the first American chemical journal, Transactions. Although other chemical societies existed at that time, the Philadelphia Chemical Society was the first society to publish its own journal. Unfortunately, the journal and chemical society lasted only one year.

Sixty years later, in 1874, at Joseph Priestley's home in Northumberland, Pennsylvania, a number of renowned scientists gathered to celebrate Priestley's 1774 discovery of oxygen. It was at this gathering that Charles F. Chandler proposed the concept of an American chemical society. The proposal was turned down, in part because the American Association for the Advancement of Science (AAAS) had a chemical section that provided an adequate forum for assembly and debate. Two years later, a national society, based in New York and called the American Chemical Society, was formed with John W. Draper as its first president. Since New York chemists dominated most of the meetings and council representative positions, the Washington Chemical Society was founded in 1884 by two chemists based in Washington, D.C., Frank W. Clarke and Harvey W. Wiley. In 1890, the American Chemical Society constitution was changed to encourage the formation of local sections, such as New York, Washington, and other chemical societies in the United States, thereby leading to a national organization. By 1908, the society had approximately 3,400 members, outnumbering the German Chemical Society, which at that time was the center of world chemistry. Today, the American Chemical Society has some 163,503 members, and the United States is considered the center of world chemistry. In addition to its premier journal, the Journal of the American Chemical Society, the society also publishes several other journals that are divisional in nature, including the Journal of Organic Chemistry, Analytical Chemistry, Journal of Physical Chemistry, Inorganic Chemistry, and Biochemistry. The society also produces a publication called Chemical Abstracts, which catalogs abstracts from thousands of papers printed in chemical journals around the world.

European Influences

Although the various disciplines of chemistry—organic, inorganic, analytical, biochemistry, and physical chemistry—have a rich American history, they have also been influenced by European, especially German, chemists. The influence of physical chemistry on the development of chemistry in the United States began with American students who studied under a number of German chemists, most notably the 1909 Nobel Prize winner, Wilhelm Ostwald. In a 1946 survey by Stephen S. Visher, three of Ostwald's students were recognized as influential chemistry teachers—Gilbert N. Lewis, Arthur A. Noyes, and Theodore W. Richards. Of these three, Richards would be awarded the 1914 Nobel Prize for his contributions in accurately determining the atomic weight of a large number of chemical elements. Lewis and Noyes would go on to play a major role in the development of academic programs at institutions such as the University of California at Berkeley, the Massachusetts Institute of Technology, and Caltech. While at these institutions Lewis and Noyes attracted and trained numerous individuals, including William C. Bray, Richard C. Tolman, Joel H. Hildebrand, Merle Randall, Glenn T. Seaborg, and Linus Pauling. These individuals placed physical chemistry at the center of their academic programs and curricula. Students from these institutions, as well as other universities across America, took the knowledge they gained in physical chemistry to places like the Geophysical Laboratories, General Electric, Pittsburgh Plate Glass Company, and Bausch and Lomb.

Influence could also flow from America to Europe, as it did with one of the earliest great American chemists, J. Willard Gibbs (1839–1903). Gibbs, educated at Yale University, was the first doctor of engineering in the United States. His contribution to chemistry was in the field of thermodynamics—the study of heat and its transformations. Using thermodynamic principles, he deduced the Gibbs phase rule, which relates the number of components and phases of mixtures to the degrees of freedom in a closed system. Gibbs's work did not receive much attention in the United States due to its publication in a minor journal, but in Europe his ideas were well received by the physics community, including Wilhelm Ostwald, who translated Gibbs's work into German.

A second influence from Europe came around the 1920s, when a very bright student from Caltech named Linus Pauling went overseas as a postdoctoral fellow for eighteen months. In Europe, Pauling spent time working with Niels Bohr, Erwin Schrödinger, Arnold Sommerfeld, Walter Heitler, and Fritz London. During this time, Pauling trained himself in the new area of quantum mechanics and its application to chemical bonding. A significant part of our knowledge about the chemical bond and its properties is due to Pauling. Upon his return from Europe, Pauling went to back to Caltech and in 1950 published a paper explaining the nature of helical structures in proteins.

At the University of California at Berkeley, Gilbert N. Lewis directed a brilliant scientist, Glenn T. Seaborg. Seaborg worked as Lewis's assistant on acid-base chemistry during the day and at night he explored the mysteries of the atom. Seaborg is known for leading the first group to discover plutonium. This discovery would lead him to head a section on the top secret Manhattan Project, which created the first atomic bomb. Seaborg's second biggest achievement was his proposal to modify the periodic table to include the actinide series. This concept predicted that the fourteen actinides, including the first eleven transuranium elements, would form a transition series analogous to the rare-earth series of lanthanide elements and therefore show how the transuranium elements fit into the periodic table. Seaborg's work on the transuranium elements led to his sharing the 1951 Nobel Prize in chemistry with Edwin McMillan. In 1961, Seaborg became the chairman of the Atomic Energy Commission, where he remained for ten years.

Perhaps Seaborg's greatest contribution to chemistry in the United States was his advocacy of science and mathematics education. The cornerstone of his legacy on education is the Lawrence Hall of Science on the Berkeley campus, a public science center and institution for curriculum development and research in science and mathematics education. Seaborg also served as principal investigator of the well-known Great Explorations in Math and Science (GEMS) program, which publishes the many classes, workshops, teacher's guides, and handbooks from the Lawrence Hall of Science. To honor a brilliant career by such an outstanding individual, element 106 was named Seaborgium.

Twentieth-Century Research and Discoveries

Research in the American chemical industry started in the early twentieth century with the establishment of research laboratories such as General Electric, Eastman Kodak, AT&T, and DuPont. The research was necessary in order to replace badly needed products and chemicals that were normally obtained from Germany. Industry attracted re-search chemists from their academic labs and teaching assignments to head small, dynamic research groups. In 1909, Irving Langmuir was persuaded to leave his position as a chemistry teacher at Stevens Institute of Technology to do research at General Electric. It was not until World War I that industrial chemical research took off. Langmuir was awarded a Nobel Prize for his industrial work. In the early 1900s, chemists were working on polymer projects and free radical reactions in order to synthesize artificial rubber. DuPont hired Wallace H. Carothers, who worked on synthesizing polymers. A product of Carothers's efforts was the synthesis of nylon, which would become DuPont's greatest moneymaker. In 1951, modern organometallic chemistry began at Duquesne University in Pittsburgh with the publication of an article in the journal Nature on the synthesis of an organo-iron compound called dicyclopentadienyliron, better known as ferrocene. Professor Peter Pauson and Thomas J. Kealy, a student, were the first to publish its synthesis, and two papers would be published in 1952 with the correct predicted structure. One paper was by Robert Burns Woodward, Geoffrey Wilkinson, Myron Rosenblum, and Mark Whiting; the second was by Ernst Otto Fischer and Wolfgang Pfab. Finally, a complete crystal structure of ferrocene was published in separate papers by Phillip F. Eiland and Ray Pepinsky and by Jack D. Dunitz and Leslie E. Orgel. The X-ray crystallographic structures would confirm the earlier predicted structures. Ferrocene is a "sandwich" compound in which an iron ion is sandwiched between two cyclopentadienyl rings. The discovery of ferrocene was important in many aspects of chemistry, such as revisions in bonding concepts, synthesis of similar compounds, and uses of these compounds as new materials. Most importantly, the discovery of ferrocene has merged two distinct fields of chemistry, organic and inorganic, and led to important advances in the fields of homogeneous catalysis and polymerization.

Significant American achievements in chemistry were recognized by the Nobel Prize committee in the last part of the twentieth century and the first years of the twenty-first century. Some examples include: the 1993 award to Kary B. Mullis for his work on the polymerase chain reaction (PCR); the 1996 award to Robert F. Curl Jr. and Richard E. Smalley for their part in the discovery of C60, a form of molecular carbon; the 1998 award to John A. Pople and Walter Kohn for the development of computational methods in quantum chemistry; the 1999 award to Ahmed H. Zewail for his work on reactions using femtosecond (10-14 seconds) chemistry; the 2000 award to Alan G. MacDiarmid and Alan J. Heeger for the discovery and development of conductive polymers; and the 2001 award to William S. Knowles and K. Barry Sharpless for their work on asymmetric synthesis. The outcomes of these discoveries are leading science in the twenty-first century. The use of PCR analysis has contributed to the development of forensic science. The discovery C60 and related carbon compounds, known as nanotubes, is leading to ideas in drug delivery methods and the storage of hydrogen and carbon dioxide. The computational tools developed by Pople and Kohn are being used to assist scientists in analyzing and designing experiments. Femtosecond chemistry is providing insight into how bonds are made and broken as a chemical reaction proceeds. Heeger and MacDiarmid's work has led to what is now known as plastic electronics—devices made of conducting polymers, ranging from light-emitting diodes to flat panel displays. The work by Knowles and Sharpless has provided organic chemists with the tools to synthesize compounds that contain chirality or handedness. This has had a tremendous impact on the synthesis of drugs, agrochemicals, and petrochemicals.

Bibliography

Brock, William H. The Norton History of Chemistry. New York: Norton, 1993.

———. The Chemical Tree: A History of Chemistry. New York: Norton, 2000.

Greenberg, Arthur. A Chemical History Tour: Picturing Chemistry from Alchemy to Modern Molecular Science. New York: Wiley, 2000.

Servos, John W. Physical Chemistry from Ostwald to Pauling: The Making of Science in America. Princeton, N.J.: Princeton University Press, 1990.

chemistry, branch of science concerned with the properties, composition, and structure of substances and the changes they undergo when they combine or react under specified conditions.

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.

Bibliography

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).

Gale Encyclopedia of the Early Modern World:

## Chemistry

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The history of early modern chemistry, understood as a body of ideas and practices related to compounding and decomposing material substances, takes us to alchemy and apothecary laboratories, artisans' workshops, metallurgists and manufacturers, scientific societies, arsenals, royal courts, and public squares. It should not be understood in terms of the victory of scientific theory over arcane beliefs, but of the changing employment of its various technologies and the contexts in and by which they were legitimized.

Material and Bodily Technology

Chemistry's material technology—that is, its instruments and laboratory equipment—remained stable throughout most of the period, but was augmented by precision-oriented apparatus in the second half of the eighteenth century as the study of heat and gases, along with early industrial innovations, redirected chemical investigations. Increasingly accurate measuring devices helped bring about standardization in manufacturing ventures (e.g. Josiah Wedgwood's pottery works) while feeding debates over how to organize chemistry as an investigative enterprise. Should the heterogeneous chemical world be disciplined by analyzing qualitative or quantitative data?

The way chemical operators and investigators used their own bodies was part of this historical development and debate. As long as chemical determination rested on examining colors, smells, tastes and textures, the human senses served as crucial chemical instruments. As experimental claims increasingly relied on precise measurements by the late eighteenth century (a hallmark of the chemical revolution), sense evidence became "subjective" and, hence, a questionable foundation for proof. Chemists continued to rely on their senses, but proof became increasingly a matter of quantitative determination.

Theory and Practice

The question of what constituted a primary chemical element was not a part of practical chemists' daily routine. Neither, prior to the late eighteenth century, was there a direct correlation between one's theoretical views and how one actually carried out chemical procedures, which can be seen by examining the impact of the mechanical philosophy on chemistry. Textbook writers such as Nicolas Lémery (1645–1715) attributed a substance's qualities to the shape of particles that composed it. But authors left such explanations behind when dealing with actual chemical operations. Robert Boyle (1627–1691), often labeled a mechanical philosopher, made a bigger impact on chemistry through his interests in practical knowledge and alchemy. Even Isaac Newton's (1642–1727) mechanism, which married particles to short-range forces, hardly touched chemical practice—although theorists such as Georges-Louis Leclerc de Buffon (1707–1788) hoped chemical attraction (affinities) could be explained mathematically with Newtonian forces. Working chemists continued to learn their trade through apprenticeship and to be guided by practical recipes. Acquiring tacit knowledge and practical skills, then, were certainly as important for the historical development of chemistry as theoretical knowledge.

It is, however, historically important that matter theory became linked to chemical research in an increasingly instrumental way by the eighteenth century. Paracelsus (1493–1541), who argued for the chemical foundation of medicine (iatrochemistry), claimed that Aristotle's four elements appeared in bodies as mercury, sulfur, and salt. Mercury was the principle of volatility and fusibility, sulfur of inflammability, and salt of incombustibility. Therefore, chemists might recognize a compound not only as heavy or wet, but also as liable to specific chemical processes.

Johann Joachim Becher (1635–1682) substituted three categories of earth for Paracelsus's principles and explained material change largely in terms of their combination with and release from compounds through processes such as combustion. His student George Ernst Stahl (1660–1734) further codified Becher's work, giving the name "phlogiston" (from the Greek verb "to inflame") to Becher's terra pinguis (the sulfur of inflammability) and teaching that phlogiston's presence was responsible for characteristics including metallicity, color, and inflammability. In France, the influential chemistry lecturer Guillaume François Rouelle (1703–1770) popularized the idea of phlogiston, associating it with fire. Others such as Joseph Priestley (1733–1804) identified it variously with electricity and hydrogen. Phlogiston was used to explain phenomena including combustion, calcination, and the quality of air, thereby organizing a number of research activities under a set of interconnecting theories and emphasizing the potential reversibility of chemical processes.

Others began considering the Aristotelian elements as material instruments. Stephen Hales (1677–1761) focused on the expansion of air and the way in which it could become "fixed" in bodies. Herman Boerhaave (1668–1738) went further, organizing his chemistry lectures largely around the investigative consequences of considering earth, water, air, and fire as instruments that afforded specific chemical processes. A Newtonian by public pronouncement, Boerhaave actually did much more to stimulate chemical research by focusing on the reactive effects of these elemental instruments. He related fire (the substance of heat) to the primary processes of expansion and repulsion. He presented air and water as providing containers in which other particles were suspended. It wasn't long before these "instruments" themselves were subjected to chemical analysis, as investigators sought to understand whether their "instrumental" presence was chemically passive or active. Research in the second half of the eighteenth century was marked by investigations of newly discovered gases (qualitatively distinct "airs"), the role of heat, and, in the 1780s, the composition of water.

Literary Technology

Chemical theory and instrumental research practices were also linked in the way chemical knowledge came to be organized nomenclaturally and in analytical tables (chemistry's literary technology). Related to the heritage of alchemy and the various contexts in which chemical substances were discovered and used, chemical nomenclature was traditionally a colorfully unsystematic affair. Growing interest in chemical research in the second half of the eighteenth century, especially the investigation of a number of new "airs," led chemists to consider nomenclatural reform. Standard conventions for naming new substances would allow researchers from various communities to communicate. In 1787 Antoine Laurent Lavoisier (1743–1794), Louis Bernard Guyton de Morveau (1737–1816), Antoine François Fourcroy (1755–1809), and Claude Simon Berthollet (1748–1822) revamped chemistry's nomenclature totally, enunciating in their Méthode de nomenclature chimique a revolutionary way to structure chemistry's investigative knowledge and practices.

Oxygen's discovery and naming provides a good example. Recognized in the 1770s as a distinct "air" responsible for combustion, supporting respiration, and the process of calcination, it was variously named the "purest part of air," "fire air," "eminently respirable air," and "dephlogisticated air." Lavoisier focused on what he considered its most far-reaching characteristic and argued that it should be called "oxygen," the "generator" of acids. Not only did he use oxygen's causal properties to argue against the existence of phlogiston, he named the substance in a way that simultaneously reflected how the relation between these properties ought to be understood and how chemists ought to pursue future research.

Traditionally, the secretive nature of many alchemical and artisanal practices had combined with chemistry's lack of institutional and disciplinary unity to work against the development of a public, systematic means of recording compositional data. This began changing when Étienne François Geoffroy (1672–1731) presented his "Table of the different relationships observed between different substances" to the French Academy of Sciences in 1718. Recording and publishing these relationships, often called affinities, provided a handy way for chemists to share and expand empirical knowledge without having to agree on their theoretical explanation. As the century progressed, affinity and solvent tables became more sophisticated (recording, for example, how relations were observed), leading chemists to hope that their field might thereby gain the certainty of a scientific discipline. As was true with nomenclatural reform, this was largely achieved by Lavoisier and his colleagues, with revolutionary results. Lavoisier's 1789 textbook Traitéélémentaire de chimie included tables whose structures redirected research along the same lines as chemistry's new nomenclature.

Lavoisier began his textbook by arguing that humans live in a Condillacian world; chemists should therefore build their discipline on a foundation of sensible facts. Chemistry's nomenclature should express only what chemists actively observed; its basic elements should be defined by laboratory procedures. In fact, Lavoisier began his "table of simple substances" with five elements that could never be isolated, but which he made responsible for fundamental chemical processes. Oxygen "generates" acidity, hydrogen "generates" water. Caloric, the substance of heat, interacts with chemical affinities to regulate composition and decomposition. In place of affinity and solvent tables, Lavoisier filled his textbook with tables that simultaneously recorded and predicted the combinatorial powers of elements such as oxygen. Together they formed an integrated research program intended to discipline chemistry.

Chemistry's Instrumentalization

Lavoisier's laboratory practices reflected what appeared on the pages of his book, the last third of which treated laboratory instruments. If primary elements couldn't be isolated, Lavoisier argued that their active presence could be quantitatively traced. Unmeasurable phlogiston was out, precision balances were in, as seen in his proof that water is compounded of hydrogen and oxygen. Affinities could not yet be quantified, but the effect of caloric on composition and decomposition could be quantitatively inferred by the melting of ice in an ice calorimeter—an instrument designed by Lavoisier. In general, nomenclature, instrumental theory, and measurement provided a research program for future chemists, in terms of both questions and methods for resolving them.

This culmination of chemistry's instrumentalization was, arguably, the essence of the chemical revolution. Whether others adopted Lavoisier's theories or followed the specifics of his research proposals, the modern discipline of chemistry was permanently marked by the instrumental bounds he prescibed.

Bibliography

Bensaude-Vincent, Bernadette. Lavoisier. Memoires d'une révolution. Paris, 1993.

Golinski, Jan. Science as Public Culture: Chemistry and Enlightenment in Britain 1760–1820. Cambridge, U.K., 1992.

Hannaway, Owen. The Chemist and the Word: The Didactic Origins of Chemistry. Baltimore, 1985.

Holmes, Frederick Lawrence. Eighteenth-Century Chemistry as an Investigative Enterprise. Berkeley, 1989.

Roberts, Lissa. "The Death of the Sensuous Chemist: the 'New' Chemistry and the Transformation of Sensuous Technology." Studies in History and Philosophy of Science 26 (1995): 503–529.

——. "Setting the Table: The Disciplinary Development of Eighteenth-Century Chemistry as Read through the Changing Structure of its Tables." In The Literary Structure of Scientific Argument, edited by Peter Dear, pp. 99–132. Philadelphia, 1991.

—LISSA ROBERTS

Word Tutor:

## chemistry

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IN BRIEF: The science in which substances are examined to find out what they are made of and how they act under different conditions.

To think is to practice brain chemistry. — Deepak Chopra.

LearnThatWord.com is a free vocabulary and spelling program where you only pay for results!

Wiley Dictionary of Flavors:

## Chemistry

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The study of matter, its reactions, composition, and properties.

## categories related to 'chemistry'

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For a list of words related to chemistry, see:
• Branches and Disciplines - chemistry: science dealing with the composition, structure, properties, and reactions of substances
• Love and Romance - chemistry: animal attraction; sympathetic understanding; rapport

Rhymes:

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## Chemistry

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Chemicals in flasks (including ammonium hydroxide and nitric acid) lit in different colors

Chemistry, a branch of physical science, is the study of the composition, properties and behavior of matter.[1][2] Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds. Chemistry is also concerned with the interactions between atoms (or groups of atoms) and various forms of energy (e.g. photochemical reactions, changes in phases of matter, separation of mixtures, properties of polymers, etc.).

Chemistry is sometimes called "the central science" because it bridges other natural sciences like physics, geology and biology with each other.[3][4] Chemistry is a branch of physical science but distinct from physics.[5]

The etymology of the word chemistry has been much disputed.[6] The genesis of chemistry can be traced to certain practices, known as alchemy, which had been practiced for several millennia in various parts of the world, particularly the Middle East.[7]

## Etymology

The word chemistry comes from the word alchemy, an earlier set of practices that encompassed elements of chemistry, metallurgy, philosophy, astrology, astronomy, mysticism and medicine; it is commonly thought of as the quest to turn lead or another common starting material into gold.[8] Alchemy, which was practiced around 330, is the study of the composition of waters, movement, growth, embodying, disembodying, drawing the spirits from bodies and bonding the spirits within bodies (Zosimos).[9] An alchemist was called a 'chemist' in popular speech, and later the suffix "-ry" was added to this to describe the art of the chemist as "chemistry".

The word alchemy in turn is derived from the Arabic word al-kīmīā (الكيمياء). The Arabic term is borrowed from the Greek χημία or χημεία.[10][11] This may have Egyptian origins. Many believe that al-kīmīā is derived from χημία, which is in turn derived from the word Chemi or Kimi, which is the ancient name of Egypt in Egyptian.[10] Alternately, al-kīmīā may be derived from χημεία, meaning "cast together".[12]

### Definition

In retrospect, the definition of chemistry has changed over time, as new discoveries and theories add to the functionality of the science. The term "chymistry", in the view of noted scientist Robert Boyle in 1661, meant the subject of the material principles of mixed bodies.[13] In 1663, "chymistry" meant a scientific art, by which one learns to dissolve bodies, and draw from them the different substances on their composition, and how to unite them again, and exalt them to a higher perfection - this definition was used by chemist Christopher Glaser.[14]

The 1730 definition of the word "chemistry", as used by Georg Ernst Stahl, meant the art of resolving mixed, compound, or aggregate bodies into their principles; and of composing such bodies from those principles.[15] In 1837, Jean-Baptiste Dumas considered the word "chemistry" to refer to the science concerned with the laws and effects of molecular forces.[16] This definition further evolved until, in 1947, it came to mean the science of substances: their structure, their properties, and the reactions that change them into other substances - a characterization accepted by Linus Pauling.[17] More recently, in 1998, the definition of "chemistry" was broadened to mean the study of matter and the changes it undergoes, as phrased by Professor Raymond Chang.[18]

## History

Ancient Egyptians pioneered the art of synthetic "wet" chemistry up to 4,000 years ago.[19] By 1000 BC ancient civilizations were using technologies that formed the basis of the various branches of chemistry such as; extracting metal from their ores, making pottery and glazes, fermenting beer and wine, making pigments for cosmetics and painting, extracting chemicals from plants for medicine and perfume, making cheese, dying cloth, tanning leather, rendering fat into soap, making glass, and making alloys like bronze.

Democritus' atomist philosophy was later adopted by Epicurus (341–270 BCE).

The genesis of chemistry can be traced to the widely observed phenomenon of burning that led to metallurgy—the art and science of processing ores to get metals (e.g. metallurgy in ancient India). The greed for gold led to the discovery of the process for its purification, even though the underlying principles were not well understood—it was thought to be a transformation rather than purification. Many scholars in those days thought it reasonable to believe that there exist means for transforming cheaper (base) metals into gold. This gave way to alchemy and the search for the Philosopher's Stone which was believed to bring about such a transformation by mere touch.[20]

Greek atomism dates back to 440 BC, arising in works by philosophers such as Democritus and Epicurus. In 50 BC, the Roman philosopher Lucretius expanded upon the theory in his book De Rerum Natura (On The Nature of Things).[21][22] Unlike modern concepts of science, Greek atomism was purely philosophical in nature, with little concern for empirical observations and no concern for chemical experiments.[23]

Much of the early development of purification methods is described by Pliny the Elder in his Naturalis Historia.[citation needed]

A tentative outline is as follows:

1. Alchemy in Greco-Roman Egypt [ – 642 CE], the earliest Western alchemists such as Mary the Jewess, Cleopatra the Alchemist, and Zosimos of Panopolis described early laboratory equipment. They are estimated to have lived between the first and third centuries.
2. Islamic alchemy [642 CE – 1200], the Muslim conquest of Egypt; development of alchemy by Jābir ibn Hayyān, al-Razi and others; Jābir modifies Aristotle's theories; advances in processes and apparatus.[24]
3. European alchemy [1300 – present], Pseudo-Geber builds on Arabic chemistry.[citation needed] From the 12th century, major advances in the chemical arts shifted from Arab lands to western Europe.[24]
4. Chemistry [1661], Boyle writes his classic chemistry text The Sceptical Chymist.
5. Chemistry [1787], Lavoisier writes his classic Elements of Chemistry.
6. Chemistry [1803], Dalton publishes his Atomic Theory.
7. Chemistry [1869], Dmitri Mendeleev presented his Periodic table being the framework of the modern chemistry

The earliest pioneers of chemistry and the scientific method,[25] were medieval Arab and Persian scholars. They introduced precise observation and controlled experimentation into the field and discovered numerous chemical substances.[26][verification needed]

"Chemistry as a science was almost created by the Muslims; for in this field, where the Greeks (so far as we know) were confined to industrial experience and vague hypothesis, the Saracens introduced precise observation, controlled experiment, and careful records. They invented and named the alembic (al-anbiq), chemically analyzed innumerable substances, composed lapidaries, distinguished alkalis and acids, investigated their affinities, studied and manufactured hundreds of drugs. Alchemy, which the Muslims inherited from Egypt, contributed to chemistry by a thousand incidental discoveries, and by its method, which was the most scientific of all medieval operations."[26]

The most influential Muslim chemists were Jābir ibn Hayyān (Geber, d. 815), al-Kindi (d. 873), al-Razi (d. 925), al-Biruni (d. 1048) and Alhazen (d. 1039).[27] Their works became more widely known in Europe in the twelfth and thirteenth centuries, beginning with the Latin translation of Jābir’s Kitab al-Kimya in 1144. The contribution of Indian alchemists and metallurgists in the development of chemistry was also quite significant.[28]

Antoine-Laurent de Lavoisier is considered the "Father of Modern Chemistry".[29]

For some practitioners alchemy was an intellectual pursuit, and over time they got better at it. Paracelsus (1493–1541), for example, rejected the 4-elemental theory, and with only a vague understanding of his chemicals and medicines formed a hybrid of alchemy and science in what was to be called iatrochemistry. Similarly, the influences of philosophers such as Sir Francis Bacon (1561–1626) and René Descartes (1596–1650), who demanded more rigor in mathematics and in removing bias from scientific observations, led to a scientific revolution. In chemistry this began with Robert Boyle (1627–1691) who came up with an equation known as Boyle's Law about the characteristics of gaseous state.[30]

Chemistry came of age when Antoine Lavoisier (1743–1794) developed the theory of Conservation of mass in 1783; and the development of the Atomic Theory by John Dalton around 1800. The Law of Conservation of Mass resulted in the reformulation of chemistry based on this law[citation needed] and the oxygen theory of combustion, which was largely based on the work of Lavoisier. Lavoisier's fundamental contributions to chemistry were a result of a conscious effort[citation needed] to fit all experiments into the framework of a single theory.

Lavoisier established the consistent use of the chemical balance, used oxygen to overthrow the phlogiston theory, and developed a new system of chemical nomenclature and made contribution to the modern metric system. Lavoisier also worked to translate the archaic and technical language of chemistry into something that could be easily understood by the largely uneducated masses, leading to an increased public interest in chemistry. All these advances in chemistry led to what is usually called the chemical revolution. The contributions of Lavoisier led to what is now called modern chemistry—the chemistry that is studied in educational institutions all over the world. It is because of these and other contributions that Antoine Lavoisier is often celebrated as the "Father of Modern Chemistry".[31] The later discovery of Friedrich Wöhler that many natural substances, organic compounds, can indeed be synthesized in a chemistry laboratory also helped the modern chemistry to mature from its infancy.[32]

The discovery of the chemical elements has a long history from the days of alchemy and culminating in the creation of the periodic table of the chemical elements by Dmitri Mendeleev (1834–1907)[33] and later discoveries of some synthetic elements.

Jöns Jacob Berzelius, Joseph Priestley, Humphry Davy, Linus Pauling, Gilbert N. Lewis, Josiah Willard Gibbs, Robert Burns Woodward, and Fritz Haber also made notable contributions.

The year 2011 was declared by the United Nations as the International Year of Chemistry.[34] It was an initiative of the International Union of Pure and Applied Chemistry, and of the United Nations Educational, Scientific, and Cultural Organization and involves chemical societies, academics, and institutions worldwide and relied on individual initiatives to organize local and regional activities.

## Principles of modern chemistry

Laboratory, Institute of Biochemistry, University of Cologne.

The current model of atomic structure is the quantum mechanical model.[35] Traditional chemistry starts with the study of elementary particles, atoms, molecules,[36] substances, metals, crystals and other aggregates of matter. This matter can be studied in solid, liquid, or gas states, in isolation or in combination. The interactions, reactions and transformations that are studied in chemistry are usually the result of interactions between atoms, leading to rearrangements of the chemical bonds which hold atoms together. Such behaviors are studied in a chemistry laboratory.

The chemistry laboratory stereotypically uses various forms of laboratory glassware, but glassware is not central to chemistry, and a great deal of experimental (as well as applied/industrial chemistry) is done without it.

A chemical reaction is a transformation of some substances into one or more different substances.[37] The basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through a chemical equation, which usually involves atoms as subjects. The number of atoms on the left and the right in the equation for a chemical transformation is equal (when unequal, the transformation by definition is not chemical, but rather a nuclear reaction or radioactive decay). The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.

Energy and entropy considerations are invariably important in almost all chemical studies. Chemical substances are classified in terms of their structure, phase, as well as their chemical compositions. They can be analyzed using the tools of chemical analysis, e.g. spectroscopy and chromatography. Scientists engaged in chemical research are known as chemists.[38] Most chemists specialize in one or more sub-disciplines. Several concepts are essential for the study of chemistry; some of them are:[39]

### Matter

In chemistry, matter is defined as anything that has rest mass and volume (it takes up space), as well as being made up of particles. The particles that make up matter have rest mass as well - not all particles have rest mass, such as the photon.

#### Atom

A diagram of an atom based on the Rutherford model

The atom is the basic unit of chemistry. It consists of a dense core called the atomic nucleus surrounded by a space called the electron cloud. The nucleus is made up of positively charged protons and neutrons that have no charge, while the electron cloud consists of negatively-charged electrons which orbit the nucleus. In a neutral atom, the negatively-charged electrons balance out the positive charge of the protons.

The atom is also the smallest entity that can be envisaged to retain the chemical properties of the element, such as electronegativity, ionization potential, preferred oxidation state(s), coordination number, and preferred types of bonds to form (e.g., metallic, ionic, covalent).

#### Element

Standard form of the periodic table of chemical elements. The colors represent different categories of elements

The concept of chemical element is related to that of chemical substance. A chemical element is specifically a pure substance which is composed of a single type of atom. A chemical element is characterized by a particular number of protons in the nuclei of its atoms. This number is known as the atomic number of the element. For example, all atoms with 6 protons in their nuclei are atoms of the chemical element carbon, and all atoms with 92 protons in their nuclei are atoms of the element uranium.

Although all the nuclei of all atoms belonging to one element will have the same number of protons, they may not necessarily have the same number of neutrons; such atoms are termed isotopes. In fact several isotopes of an element may exist. Ninety–four different chemical elements or types of atoms based on the number of protons are observed on earth naturally, having at least one isotope that is stable or has a very long half-life. A further 18 elements have been recognised by IUPAC after they have been made in the laboratory.

The standard presentation of the chemical elements is in the periodic table, which orders elements by atomic number and groups them by electron configuration. Due to its arrangement, groups, or columns, and periods, or rows, of elements in the table either share several chemical properties, or follow a certain trend in characteristics such as atomic radius, electronegativity, etc. Lists of the elements by name, by symbol, and by atomic number are also available.

#### Compound

Carbon dioxide (CO2), an example of a chemical compound

A compound is a pure chemical substance that consists of two or more elements combined together. In a compound, there is a particular ratio of atoms of particular chemical elements which determines its composition, and a particular organization which determines its chemical properties. For example, water is a compound containing hydrogen and oxygen in the ratio of two to one, with the oxygen atom between the two hydrogen atoms, and an angle of 104.5° between them. Compounds are formed and interconverted by chemical reactions.

#### Substance

 Examples of pure chemical substances. From left to right: the elements tin (Sn) and sulfur (S), diamond (an allotrope of carbon), sucrose (pure sugar), and sodium chloride (salt) and sodium bicarbonate (baking soda), which are both ionic compounds.

A chemical substance is a kind of matter with a definite composition and set of properties.[40] Strictly speaking, a mixture of compounds, elements or compounds and elements is not a chemical substance, but it may be called a chemical. Most of the substances we encounter in our daily life are some kind of mixture; for example: air, alloys, biomass, etc.

Nomenclature of substances is a critical part of the language of chemistry. Generally it refers to a system for naming chemical compounds. Earlier in the history of chemistry substances were given name by their discoverer, which often led to some confusion and difficulty. However, today the IUPAC system of chemical nomenclature allows chemists to specify by name specific compounds amongst the vast variety of possible chemicals.

The standard nomenclature of chemical substances is set by the International Union of Pure and Applied Chemistry (IUPAC). There are well-defined systems in place for naming chemical species. Organic compounds are named according to the organic nomenclature system.[41] Inorganic compounds are named according to the inorganic nomenclature system.[42] In addition the Chemical Abstracts Service has devised a method to index chemical substances. In this scheme each chemical substance is identifiable by a number known as its CAS registry number.

#### Molecule

A ball-and-stick representation of the caffeine molecule (C8H10N4O2).

A molecule is the smallest indivisible portion of a pure chemical substance that has its unique set of chemical properties, that is, its potential to undergo a certain set of chemical reactions with other substances. However, this definition only works well for substances that are composed of molecules, which is not true of many substances (see below). Molecules are typically a set of atoms bound together by covalent bonds, such that the structure is electrically neutral and all valence electrons are paired with other electrons either in bonds or in lone pairs.

Thus, molecules exist as electrically neutral units, unlike ions. When this rule is broken, giving the "molecule" a charge, the result is sometimes named a molecular ion or a polyatomic ion. However, the discrete and separate nature of the molecular concept usually requires that molecular ions be present only in well-separated form, such as a directed beam in a vacuum in a mass spectrograph. Charged polyatomic collections residing in solids (for example, common sulfate or nitrate ions) are generally not considered "molecules" in chemistry.

A 2-D skeletal model of a benzene molecule (C6H6)

The "inert" or noble gas elements (helium, neon, argon, krypton, xenon and radon) are composed of lone atoms as their smallest discrete unit, but the other isolated chemical elements consist of either molecules or networks of atoms bonded to each other in some way. Identifiable molecules compose familiar substances such as water, air, and many organic compounds like alcohol, sugar, gasoline, and the various pharmaceuticals.

However, not all substances or chemical compounds consist of discrete molecules, and indeed most of the solid substances that makes up the solid crust, mantle, and core of the Earth are chemical compounds without molecules. These other types of substances, such as ionic compounds and network solids, are organized in such a way as to lack the existence of identifiable molecules per se. Instead, these substances are discussed in terms of formula units or unit cells as the smallest repeating structure within the substance. Examples of such substances are mineral salts (such as table salt), solids like carbon and diamond, metals, and familiar silica and silicate minerals such as quartz and granite.

One of the main characteristics of a molecule is its geometry often called its structure. While the structure of diatomic, triatomic or tetra atomic molecules may be trivial, (linear, angular pyramidal etc.) the structure of polyatomic molecules, that are constituted of more than six atoms (of several elements) can be crucial for its chemical nature.

#### Mole and amount of substance

The mole is a unit of measurement that denotes an amount of substance (also called chemical amount). Specifically it is the amount of a substance that contains as many elementary entities (atoms, molecules or ions) as there are atoms in 0.012 kilogram (or 12 grams) of carbon-12, where the carbon-12 atoms are unbound, at rest and in their ground state.[43] The number of entities per mole is known as the Avogadro constant, and is determined empirically. The currently accepted value is 6.02214179(30)×1023 mol−1 (2007 CODATA). One way to understand the meaning of the term "mole" is to compare and contrast it to terms such as dozen. Just as one dozen eggs contains 12 individual eggs, one mole contains 6.02214179(30)×1023 atoms, molecules or other particles. The term is used because it is much easier to say, for example, 1 mole of carbon, than it is to say 6.02214179(30)×1023 carbon atoms, and because moles of chemicals represent a scale that is easy to experience.

The amount of substance of a solute per volume of solution is known as amount of substance concentration, or molarity for short. Molarity is the quantity most commonly used to express the concentration of a solution in the chemical laboratory. The most commonly used units for molarity are mol/L (the official SI units are mol/m3).

### Properties

#### Ions and salts

The crystal lattice structure of potassium chloride (KCl), a salt which is formed due to the attraction of K+ cations and Cl- anions. Note how the overall charge of the ionic compound is zero.

An ion is a charged species, an atom or a molecule, that has lost or gained one or more electrons. When an atom loses an electron and thus has more protons than electrons, the atom is a positively-charged ion or cation. When an atom gains an electron and thus has more electrons than protons, the atom is a negatively-charged ion or anion. Cations and anions can form a crystalline lattice of neutral salts, such as the Na+ and Cl- ions forming sodium chloride, or NaCl. Examples of polyatomic ions that do not split up during acid-base reactions are hydroxide (OH) and phosphate (PO43−).

Ions in the gaseous phase are often known as plasma.

#### Acidity and basicity

A substance can often be classified as an acid or a base. There are several different theories which explain acid-base behavior. The simplest is Arrhenius theory, which states than an acid is a substance that produces hydronium ions when it is dissolved in water, and a base is one that produces hydroxide ions when dissolved in water. According to Brønsted–Lowry acid-base theory, acids are substances that donate a positive hydrogen ion to another substance in a chemical reaction; by extension, a base is the substance which receives that hydrogen ion.

A third common theory is Lewis acid-base theory, which is based on the formation of new chemical bonds. Lewis theory explains that an acid is a substance which is capable of accepting a pair of electrons from another substance during the process of bond formation, while a base is a substance which can provide a pair of electrons to form a new bond. According to this theory, the crucial things being exchanged are charges.[44][unreliable source?] There are several other ways in which a substance may be classified as an acid or a base, as is evident in the history of this concept [45]

Acid strength is commonly measured by two methods. One measurement, based on the Arrhenius definition of acidity, is pH, which is a measurement of the hydronium ion concentration in a solution, as expressed on a negative logarithmic scale. Thus, solutions that have a low pH have a high hydronium ion concentration, and can be said to be more acidic. The other measurement, based on the Brønsted–Lowry definition, is the acid dissociation constant (Ka), which measure the relative ability of a substance to act as an acid under the Brønsted–Lowry definition of an acid. That is, substances with a higher Ka are more likely to donate hydrogen ions in chemical reactions than those with lower Ka values.

#### Phase

Example of phase changes

In addition to the specific chemical properties that distinguish different chemical classifications chemicals can exist in several phases. For the most part, the chemical classifications are independent of these bulk phase classifications; however, some more exotic phases are incompatible with certain chemical properties. A phase is a set of states of a chemical system that have similar bulk structural properties, over a range of conditions, such as pressure or temperature.

Physical properties, such as density and refractive index tend to fall within values characteristic of the phase. The phase of matter is defined by the phase transition, which is when energy put into or taken out of the system goes into rearranging the structure of the system, instead of changing the bulk conditions.

Sometimes the distinction between phases can be continuous instead of having a discrete boundary, in this case the matter is considered to be in a supercritical state. When three states meet based on the conditions, it is known as a triple point and since this is invariant, it is a convenient way to define a set of conditions.

The most familiar examples of phases are solids, liquids, and gases. Many substances exhibit multiple solid phases. For example, there are three phases of solid iron (alpha, gamma, and delta) that vary based on temperature and pressure. A principal difference between solid phases is the crystal structure, or arrangement, of the atoms. Another phase commonly encountered in the study of chemistry is the aqueous phase, which is the state of substances dissolved in aqueous solution (that is, in water).

Less familiar phases include plasmas, Bose-Einstein condensates and fermionic condensates and the paramagnetic and ferromagnetic phases of magnetic materials. While most familiar phases deal with three-dimensional systems, it is also possible to define analogs in two-dimensional systems, which has received attention for its relevance to systems in biology.

#### Bonding

An animation of the process of ionic bonding between sodium (Na) and chlorine (Cl) to form sodium chloride, or common table salt. Ionic bonding involves one atom taking valence electrons from another (as opposed to sharing, which occurs in covalent bonding

Atoms sticking together in molecules or crystals are said to be bonded with one another. A chemical bond may be visualized as the multipole balance between the positive charges in the nuclei and the negative charges oscillating about them.[46] More than simple attraction and repulsion, the energies and distributions characterize the availability of an electron to bond to another atom.

A chemical bond can be a covalent bond, an ionic bond, a hydrogen bond or just because of Van der Waals force. Each of these kinds of bonds is ascribed to some potential. These potentials create the interactions which hold atoms together in molecules or crystals. In many simple compounds, Valence Bond Theory, the Valence Shell Electron Pair Repulsion model (VSEPR), and the concept of oxidation number can be used to explain molecular structure and composition.

An ionic bond is formed when a metal loses one or more of its electrons, becoming a positively charged cation, and the electrons are then gained by the non-metal atom, becoming a negatively charged anion. The two oppositely charged ions attract one another, and the ionic bond is the electrostatic force of attraction between them. For example, sodium (Na), a metal, loses one electron to become an Na+ cation while chlorine (Cl), a non-metal, gains this electron to become Cl-. The ions are held together due to electrostatic attraction, and that compound sodium chloride (NaCl), or common table salt, is formed.

In the methane molecule (CH4), the carbon atom shares a pair of valence electrons with each of the four hydrogen atoms. Thus, the octet rule is satisfied for C-atom (it has eight electrons in its valence shell) and the duet rule is satisfied for the H-atoms (they have two electrons in their valence shells.

In a covalent bond, one or more pairs of valence electrons are shared by two atoms: the resulting electrically neutral group of bonded atoms is termed a molecule. Atoms will share valence electrons in such a way as to create a noble gas electron configuration (eight electrons in their outermost shell) for each atom. Atoms that tend to combine in such a way that they each have eight electrons in their valence shell are said to follow the octet rule. However, some elements like hydrogen and lithium need only two electron in their outermost shell to attain this stable configuration; these atoms are said to follow the duet rule, and in this way they are reaching the electron configuration of the noble gas helium, which has two electrons in its outer shell.

Similarly, theories from classical physics can be used to predict many ionic structures. With more complicated compounds, such as metal complexes, valence bond theory is less applicable and alternative approaches, such as the molecular orbital theory, are generally used. See diagram on electronic orbitals.

### Reaction

During chemical reactions, bonds between atoms break and form, resulting in different substances with different properties. In a blast furnace, iron oxide, a compound, reacts with carbon monoxide to form iron, one of the chemical elements, and carbon dioxide.

When a chemical substance is transformed as a result of its interaction with another substance or with energy, a chemical reaction is said to have occurred. A chemical reaction is therefore a concept related to the 'reaction' of a substance when it comes in close contact with another, whether as a mixture or a solution; exposure to some form of energy, or both. It results in some energy exchange between the constituents of the reaction as well with the system environment which may be designed vessels which are often laboratory glassware.

Chemical reactions can result in the formation or dissociation of molecules, that is, molecules breaking apart to form two or more smaller molecules, or rearrangement of atoms within or across molecules. Chemical reactions usually involve the making or breaking of chemical bonds. Oxidation, reduction, dissociation, acid-base neutralization and molecular rearrangement are some of the commonly used kinds of chemical reactions.

A chemical reaction can be symbolically depicted through a chemical equation. While in a non-nuclear chemical reaction the number and kind of atoms on both sides of the equation are equal, for a nuclear reaction this holds true only for the nuclear particles viz. protons and neutrons.[47]

The sequence of steps in which the reorganization of chemical bonds may be taking place in the course of a chemical reaction is called its mechanism. A chemical reaction can be envisioned to take place in a number of steps, each of which may have a different speed. Many reaction intermediates with variable stability can thus be envisaged during the course of a reaction. Reaction mechanisms are proposed to explain the kinetics and the relative product mix of a reaction. Many physical chemists specialize in exploring and proposing the mechanisms of various chemical reactions. Several empirical rules, like the Woodward-Hoffmann rules often come handy while proposing a mechanism for a chemical reaction.

According to the IUPAC gold book a chemical reaction is a process that results in the interconversion of chemical species".[48] Accordingly, a chemical reaction may be an elementary reaction or a stepwise reaction. An additional caveat is made, in that this definition includes cases where the interconversion of conformers is experimentally observable. Such detectable chemical reactions normally involve sets of molecular entities as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e. 'microscopic chemical events').

#### Redox

Redox (reduction-oxidation) reactions include all chemical reactions in which atoms have their oxidation state changed by either gaining electrons (reduction) or losing electrons (oxidation). Substances that have the ability to oxidize other substances are said to be oxidative and are known as oxidizing agents, oxidants or oxidizers. An oxidant removes electrons from another substance. Similarly, substances that have the ability to reduce other substances are said to be reductive and are known as reducing agents, reductants, or reducers.

A reductant transfers electrons to another substance, and is thus oxidized itself. And because it "donates" electrons it is also called an electron donor. Oxidation and reduction properly refer to a change in oxidation number—the actual transfer of electrons may never occur. Thus, oxidation is better defined as an increase in oxidation number, and reduction as a decrease in oxidation number.

#### Equilibrium

Although the concept of equilibrium is widely used across sciences, in the context of chemistry, it arises whenever a number of different states of the chemical composition are possible. For example, in a mixture of several chemical compounds that can react with one another, or when a substance can be present in more than one kind of phase.

A system of chemical substances at equilibrium, even though having an unchanging composition, is most often not static; molecules of the substances continue to react with one another thus giving rise to a dynamic equilibrium. Thus the concept describes the state in which the parameters such as chemical composition remain unchanged over time.

#### Energy

In the context of chemistry, energy is an attribute of a substance as a consequence of its atomic, molecular or aggregate structure. Since a chemical transformation is accompanied by a change in one or more of these kinds of structures, it is invariably accompanied by an increase or decrease of energy of the substances involved. Some energy is transferred between the surroundings and the reactants of the reaction in the form of heat or light; thus the products of a reaction may have more or less energy than the reactants.

A reaction is said to be exergonic if the final state is lower on the energy scale than the initial state; in the case of endergonic reactions the situation is the reverse. A reaction is said to be exothermic if the reaction releases heat to the surroundings; in the case of endothermic reactions, the reaction absorbs heat from the surroundings.

Chemical reactions are invariably not possible unless the reactants surmount an energy barrier known as the activation energy. The speed of a chemical reaction (at given temperature T) is related to the activation energy E, by the Boltzmann's population factor $e^{-E/kT}$ - that is the probability of a molecule to have energy greater than or equal to E at the given temperature T. This exponential dependence of a reaction rate on temperature is known as the Arrhenius equation. The activation energy necessary for a chemical reaction to occur can be in the form of heat, light, electricity or mechanical force in the form of ultrasound.[49]

A related concept free energy, which also incorporates entropy considerations, is a very useful means for predicting the feasibility of a reaction and determining the state of equilibrium of a chemical reaction, in chemical thermodynamics. A reaction is feasible only if the total change in the Gibbs free energy is negative, $\Delta G \le 0 \,$; if it is equal to zero the chemical reaction is said to be at equilibrium.

There exist only limited possible states of energy for electrons, atoms and molecules. These are determined by the rules of quantum mechanics, which require quantization of energy of a bound system. The atoms/molecules in a higher energy state are said to be excited. The molecules/atoms of substance in an excited energy state are often much more reactive; that is, more amenable to chemical reactions.

The phase of a substance is invariably determined by its energy and the energy of its surroundings. When the intermolecular forces of a substance are such that the energy of the surroundings is not sufficient to overcome them, it occurs in a more ordered phase like liquid or solid as is the case with water (H2O); a liquid at room temperature because its molecules are bound by hydrogen bonds.[50] Whereas hydrogen sulfide (H2S) is a gas at room temperature and standard pressure, as its molecules are bound by weaker dipole-dipole interactions.

The transfer of energy from one chemical substance to another depends on the size of energy quanta emitted from one substance. However, heat energy is often transferred more easily from almost any substance to another because the phonons responsible for vibrational and rotational energy levels in a substance have much less energy than photons invoked for the electronic energy transfer. Thus, because vibrational and rotational energy levels are more closely spaced than electronic energy levels, heat is more easily transferred between substances relative to light or other forms of electronic energy. For example, ultraviolet electromagnetic radiation is not transferred with as much efficacy from one substance to another as thermal or electrical energy.

The existence of characteristic energy levels for different chemical substances is useful for their identification by the analysis of spectral lines. Different kinds of spectra are often used in chemical spectroscopy, e.g. IR, microwave, NMR, ESR, etc. Spectroscopy is also used to identify the composition of remote objects - like stars and distant galaxies - by analyzing their radiation spectra.

Emission spectrum of iron

The term chemical energy is often used to indicate the potential of a chemical substance to undergo a transformation through a chemical reaction or to transform other chemical substances.

### Chemical laws

Chemical reactions are governed by certain laws, which have become fundamental concepts in chemistry. Some of them are:

## Practice

### Subdisciplines

Chemistry is typically divided into several major sub-disciplines. There are also several main cross-disciplinary and more specialized fields of chemistry.[51]

• Analytical chemistry is the analysis of material samples to gain an understanding of their chemical composition and structure. Analytical chemistry incorporates standardized experimental methods in chemistry. These methods may be used in all subdisciplines of chemistry, excluding purely theoretical chemistry.
• Inorganic chemistry is the study of the properties and reactions of inorganic compounds. The distinction between organic and inorganic disciplines is not absolute and there is much overlap, most importantly in the sub-discipline of organometallic chemistry.
• Materials chemistry is the preparation, characterization, and understanding of substances with a useful function. The field is a new breadth of study in graduate programs, and it integrates elements from all classical areas of chemistry with a focus on fundamental issues that are unique to materials. Primary systems of study include the chemistry of condensed phases (solids, liquids, polymers) and interfaces between different phases.
• Neurochemistry is the study of neurochemicals; including transmitters, peptides, proteins, lipids, sugars, and nucleic acids; their interactions, and the roles they play in forming, maintaining, and modifying the nervous system.

Other disciplines within chemistry are traditionally grouped by the type of matter being studied or the kind of study. These include inorganic chemistry, the study of inorganic matter; organic chemistry, the study of organic (carbon based) matter; biochemistry, the study of substances found in biological organisms; physical chemistry, the study of chemical processes using physical concepts such as thermodynamics and quantum mechanics; and analytical chemistry, the analysis of material samples to gain an understanding of their chemical composition and structure. Many more specialized disciplines have emerged in recent years, e.g. neurochemistry the chemical study of the nervous system (see subdisciplines).

### Chemical industry

The chemical industry represents an important economic activity. The global top 50 chemical producers in 2004 had sales of 587 billion US dollars with a profit margin of 8.1% and research and development spending of 2.1% of total chemical sales.[53]

## References

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4. ^ Chemistry is seen as occupying an intermediate position in a hierarchy of the sciences by "reductive level" between physics and biology. See Carsten Reinhardt. Chemical Sciences in the 20th Century: Bridging Boundaries. Wiley-VCH, 2001. ISBN 3-527-30271-9. Pages 1-2.
5. ^ Is chemistry a branch of physics? a paper by Mario Bunge
6. ^ See: Chemistry (etymology) for possible origins of this word.
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• Atkins, P.W. Physical Chemistry (Oxford University Press) ISBN 0-19-879285-9
• Atkins, P.W. et al. Molecular Quantum Mechanics (Oxford University Press)
• McWeeny, R. Coulson's Valence (Oxford Science Publications) ISBN 0-19-855144-4
• Pauling, L. The Nature of the chemical bond (Cornell University Press) ISBN 0-8014-0333-2
• Pauling, L., and Wilson, E. B. Introduction to Quantum Mechanics with Applications to Chemistry (Dover Publications) ISBN 0-486-64871-0
• Smart and Moore Solid State Chemistry: An Introduction (Chapman and Hall) ISBN 0-412-40040-5
• Stephenson, G. Mathematical Methods for Science Students (Longman) ISBN 0-582-44416-0

Misspellings:

## chemistry

Top

Common misspelling(s) of chemistry

• chemestry

Translations:

## Chemistry

Top

Dansk (Danish)
n. - kemi

Nederlands (Dutch)
scheikunde, chemische samenstelling, complex proces, aantrekking of interactie tussen mensen

Français (French)
n. - chimie, propriétés chimiques, (fig) affinités

Deutsch (German)
n. - Chemie

Ελληνική (Greek)
n. - χημεία

Italiano (Italian)
chimica

Português (Portuguese)
n. - química (f)

Русский (Russian)
химия

Español (Spanish)
n. - química

Svenska (Swedish)
n. - kemi

n. - 化學, 化學作用

한국어 (Korean)
n. - 화학, 화학작용, 마음이 통함

n. - 化学, 化学的性質, 化学現象

العربيه (Arabic)
‏(الاسم) كيمياء, علم الكيمياء‏

עברית (Hebrew)
n. - ‮כימיה‬