Condition in the course of a reversible chemical reaction in which no net change in the amounts of reactants and products occurs: Products are reverting to reactants at the same rate as reactants are forming products. For practical purposes, the reaction under those conditions is completed. Expressed in terms of the law of mass action, the reaction rate to form products is equal to the reaction rate to re-form reactants. The ratio of the reaction rate constants (i.e., of the amounts of reactants and products, each raised to the proper power), defines the equilibrium constant. Changing the conditions of temperature or pressure changes the reaction's equilibrium; a high temperature or pressure may be used to “push” a reaction that at ordinary conditions makes little product. See also H.-L. Le Châtelier.
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One of the 117 presently known kinds of substances that constitute all matter at and above the level of atoms (the smallest units of any element). All atoms of an element are identical in nuclear charge (number of protons) and number of electrons (see atomic number), but their mass (atomic weight) may differ if they have different numbers of neutrons (see isotope). Each permanently named element has a one- or two-letter chemical symbol. Elements combine to form a wide variety of compounds. All elements with atomic numbers greater than 83 (bismuth), and some isotopes of lighter elements, are unstable and radioactive (see radioactivity). The transuranium elements, with atomic numbers greater than 92 (see uranium), artificially created by bombardment of other elements with neutrons or other particles, were discovered beginning in 1940. The most common elements (by weight) in Earth's crust are oxygen, 49percnt; silicon, 26percnt; aluminum, 8percnt; and iron, 5percnt. Of the known elements, 11 (hydrogen, nitrogen, oxygen, fluorine, chlorine, and the six noble gases) are gases under ordinary conditions, two (bromine and mercury) are liquids (two more, cesium and gallium, melt at about or just above room temperature), and the rest are solids. Seealso periodic table.
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Use of lethal or incapacitating chemical weapons in war, and the methods of combating such agents. Chemical weapons include choking agents such as the chlorine and phosgene gas employed first by the Germans and later by the Allies in World War I; blood agents such as hydrogen cyanide or cyanogen gas, which block red blood cells from taking up oxygen; blister agents such as sulfur gas and Lewisite, also dispensed as a gas, which burn and blister the skin; and nerve agents such as Tabun, Sarin, Soman, and VX, which block the transmission of nerve impulses to the muscles, heart, and diaphragm. The horrific casualties suffered in World War I led to the 1925 Geneva Protocol, which made it illegal to employ chemical weapons but did not ban their production. Chemical weapons were used a number of times afterward, most notably by Italy in Ethiopia (1935–36), by Japan in China (1938–42), by Egypt in Yemen (1966–67), and by Iran and Iraq against each other (1984–88). During the Cold War the Soviet Union and U.S. built up enormous chemical arsenals; these were dismantled under the terms of the 1993 Chemical Weapons Convention, which prohibits all development, production, acquisition, stockpiling, or transfer of such weapons. Not all countries have signed the convention, and many are suspected of pursuing clandestine chemical programs. Many military forces have adopted various defensive measures, including chemical sensors, protective garments and gas masks, decontaminants, and injectable antidotes, and some have reserved the option of retaliating in kind to any chemical attack. In 1995 a religious cult killed 12 civilians and injured thousands more with Sarin gas in Tokyo; this pointed out the power of chemical agents as weapons of terror as well as the difficulty of protecting civilian populations. Seealso biological warfare.
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Notation of one or two letters derived from the scientific names of the chemical elements (e.g., S for sulfur, Cl for chlorine, Zn for zinc). Some hark back to Latin names: Au (aurum) for gold, Pb (plumbum) for lead. Others are named for people or places (e.g. einsteinium, Es, for Einstein). The present symbols express the system set out by the atomic theory of matter. John Dalton first used symbols to designate single atoms of elements, not indefinite amounts, and Jons Jacob Berzelius gave many of the current names. Chemical formulas of compounds are written as combinations of the elements' symbols, with numbers indicating their atomic proportions, using various conventions for ordering and grouping. Thus, sodium chloride is written as NaCl and sulfuric acid as H2SO4.
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Any chemical process in which substances are changed into different ones, with different properties, as distinct from changing position or form (phase). Chemical reactions involve the rupture or rearrangement of the bonds holding atoms together (see bonding), never atomic nuclei. The total mass and number of atoms of all reactants equals those of all products, and energy is almost always consumed or liberated (see heat of reaction). The speed of reactions varies (see reaction rate). Understanding their mechanisms lets chemists alter reaction conditions to optimize the rate or the amount of a given product; the reversibility of the reaction and the presence of competing reactions and intermediate products complicate these studies. Reactions can be syntheses, decompositions, or rearrangements, or they can be additions, eliminations, or substitutions. Examples include oxidation-reduction, polymerization, ionization (see ion), combustion (burning), hydrolysis, and acid-base reactions.
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Subdivision of hydrology that deals with the chemical characteristics of the water on and beneath the surface of the Earth. Water in all forms is affected chemically by the materials with which it comes into contact, and it can dissolve many elements in significant quantities. Chemical hydrology is concerned with the processes involved and thus includes study of phenomena such as the transport of salts from land to sea (by erosion of rocks and surface runoff) and from sea to land (by evaporation, cloud formation, and precipitation) and the age and origin of groundwater in desert regions and of ice sheets and glaciers.
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Expression of the composition or structure of a chemical compound. Formulas for molecules use chemical symbols with subscript numbers to show the number of atoms of each element: O2 for molecular oxygen, O3 for ozone, CH4 for methane, C6H6 for benzene. Parentheses may enclose atoms that act as a group. General formulas show the proportions of atoms in members of a class (e.g., Cmath.nH2math.n+ 2 for alkanes). If the substance does not exist as molecules (see ionic bond), empirical formulas show the relative proportions of the constituents (e.g., NaCl for sodium chloride). Structural formulas show bonds (see bonding) between atoms in a molecule as short lines between symbols; they are particularly useful for showing how isomers differ. A projection formula also indicates the three-dimensional arrangement of the atoms (see Fischer projection; stereochemistry).
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Academic discipline and industrial activity concerned with developing processes and designing and operating plants to change materials' physical or chemical states. With roots in the inorganic and coal-based chemical industries of western Europe and the oil-refining industry in North America, it was spurred by the need to supply chemicals and products during the two World Wars. The field includes research, design, construction, operation, sales, and management activities. Chemical engineers must master chemistry (including the nature of chemical reactions, the effects of temperature and pressure on equilibrium, and the effects of catalysts on reaction rates), physics, and mathematics. The engineering aspect, involving fluid flow (see deformation and flow) and heat and mass transfer, is broken down into “unit operations,” including vaporization, distillation, absorption, filtration, extraction, crystallization, agitation and mixing, drying, and size reduction; each is described mathematically, and its principles apply to any material. Chemical engineers work not only in the chemical and oil industries but also in such processing industries as foods, paper, textiles, plastics, nuclear, and biotechnology.
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Physical and/or psychological dependency on a psychoactive (mind-altering) substance (e.g., alcohol, narcotics, nicotine), defined as continued use despite knowing that the substance causes harm. Physical dependency results when the body builds up a tolerance to a drug, needing increasing doses to achieve the desired effects and to prevent withdrawal symptoms. Psychological dependency may have more to do with one's psychological makeup; some people may have a genetic tendency to addiction. The most common addictions are to alcohol (see alcoholism), barbiturates, tranquilizers, and amphetamines, as well as to the stimulants nicotine and caffeine. Initial treatment (detoxification) should be conducted with medical supervision. Individual and group psychotherapy are critical elements. Alcoholics Anonymous and similar support groups can increase the success rate of other efforts. The ability to admit addiction and the will to change are necessary first steps.
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Any of the interactions that account for the association of atoms into molecules, ions, crystals, metals, and other stable species. When atoms' nuclei and electrons interact, they tend to distribute themselves so that the total energy is lowest; if the energy of a group arrangement is lower than the sum of the components' energies, they bond. The physics and mathematics of bonding were developed as part of quantum mechanics. The number of bonds an atom can form—its valence—equals the number of electrons it contributes or receives. Covalent bonds form molecules; atoms bond to specific other atoms by sharing an electron pair between them. If the sharing is even, the molecule is not polar; if it is uneven, the molecule is an electric dipole. Ionic bonds are the extreme of uneven sharing; certain atoms give up electrons, becoming cations. Other atoms take up the electrons and become anions. All the ions are held together in a crystal by electrostatic forces. In crystalline metals, a diffuse electron sharing bonds the atoms (metallic bonding). Other types include hydrogen bonding; bonds in aromatic compounds; coordinate covalent bonds; multicentre bonds, exemplified by boranes (boron hydrides), in which more than two atoms share electron pairs; and the bonds in coordination complexes (see transition element), still poorly understood. Seealso van der Waals forces.
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The process uses an abrasive and corrosive chemical slurry (commonly a colloid) in conjunction with a polishing pad and retaining ring, typically of a greater diameter than the wafer. The pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. The dynamic polishing head is rotated with different axes of rotation (i.e., not concentric). This removes material and tends to even out any irregular topography, making the wafer flat or planar. This may be necessary in order to set up the wafer for the formation of additional circuit elements. For example, this might be necessary in order to bring the entire surface within the depth of field of a photolithography system, or to selectively remove material based on its position. Typical depth-of-field requirements are down to Angstrom levels for the latest 65 nm technology.
Another analogy is the act of brushing one's teeth. The toothbrush is the mechanical part and the toothpaste is the chemical part. Using either the toothbrush or the toothpaste alone will get one's teeth somewhat clean, but using the toothbrush and toothpaste together makes a superior process.
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