Uranium
Uranium is a chemical element with symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 electrons, of which 6 are valence electrons. Uranium is weakly radioactive because all isotopes of uranium are unstable, with half-lives varying between 159,200 years and 4.5 billion years. The most common isotopes in natural uranium are uranium-238 and uranium-235. Uranium has the highest atomic weight of the primordially occurring elements, its density is about 70% higher than that of lead, lower than that of gold or tungsten. It occurs in low concentrations of a few parts per million in soil and water, is commercially extracted from uranium-bearing minerals such as uraninite. In nature, uranium is found as uranium-238, uranium-235, a small amount of uranium-234. Uranium decays by emitting an alpha particle; the half-life of uranium-238 is about 4.47 billion years and that of uranium-235 is 704 million years, making them useful in dating the age of the Earth.
Many contemporary uses of uranium exploit its unique nuclear properties. Uranium-235 is the only occurring fissile isotope, which makes it used in nuclear power plants and nuclear weapons. However, because of the tiny amounts found in nature, uranium needs to undergo enrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons, is fertile, meaning it can be transmuted to fissile plutonium-239 in a nuclear reactor. Another fissile isotope, uranium-233, can be produced from natural thorium and is important in nuclear technology. Uranium-238 has a small probability for spontaneous fission or induced fission with fast neutrons. In sufficient concentration, these isotopes maintain a sustained nuclear chain reaction; this generates the heat in nuclear power reactors, produces the fissile material for nuclear weapons. Depleted uranium is used in kinetic energy penetrators and armor plating. Uranium is used as a colorant in uranium glass. Uranium glass fluoresces green in ultraviolet light.
It was used for tinting and shading in early photography. The 1789 discovery of uranium in the mineral pitchblende is credited to Martin Heinrich Klaproth, who named the new element after the discovered planet Uranus. Eugène-Melchior Péligot was the first person to isolate the metal and its radioactive properties were discovered in 1896 by Henri Becquerel. Research by Otto Hahn, Lise Meitner, Enrico Fermi and others, such as J. Robert Oppenheimer starting in 1934 led to its use as a fuel in the nuclear power industry and in Little Boy, the first nuclear weapon used in war. An ensuing arms race during the Cold War between the United States and the Soviet Union produced tens of thousands of nuclear weapons that used uranium metal and uranium-derived plutonium-239; the security of those weapons and their fissile material following the breakup of the Soviet Union in 1991 is an ongoing concern for public health and safety. See Nuclear proliferation; when refined, uranium is a weakly radioactive metal.
It has a Mohs hardness of 6, sufficient to scratch glass and equal to that of titanium, rhodium and niobium. It is malleable, ductile paramagnetic electropositive and a poor electrical conductor. Uranium metal has a high density of 19.1 g/cm3, denser than lead, but less dense than tungsten and gold. Uranium metal reacts with all non-metal elements and their compounds, with reactivity increasing with temperature. Hydrochloric and nitric acids dissolve uranium, but non-oxidizing acids other than hydrochloric acid attack the element slowly; when finely divided, it can react with cold water. Uranium in ores is extracted chemically and converted into uranium dioxide or other chemical forms usable in industry. Uranium-235 was the first isotope, found to be fissile. Other occurring isotopes are fissionable, but not fissile. On bombardment with slow neutrons, its uranium-235 isotope will most of the time divide into two smaller nuclei, releasing nuclear binding energy and more neutrons. If too many of these neutrons are absorbed by other uranium-235 nuclei, a nuclear chain reaction occurs that results in a burst of heat or an explosion.
In a nuclear reactor, such a chain reaction is slowed and controlled by a neutron poison, absorbing some of the free neutrons. Such neutron absorbent materials are part of reactor control rods; as little as 15 lb of uranium-235 can be used to make an atomic bomb. The first nuclear bomb used in war, Little Boy, relied on uranium fission, but the first nuclear explosive and the bomb that destroyed Nagasaki were both plutonium bombs. Uranium metal has three allotropic forms: α stable up to 668 °C. Orthorhombic, space group No. 63, lattice parameters a = 285.4 pm, b = 587 pm, c = 495.5 pm. Β stable from 668 °C to 775 °C. Tetragonal, space group P42/mnm, P42nm, or P4n2, lattice parameters a = 565.6 pm, b = c = 1075.9 pm. Γ from 775 °C to melting point—this is the most malleable and ductile state. Body-centered cubic, lattice parameter a = 352.4 pm. The major application of uranium in the military sector is
Mineral
A mineral is, broadly speaking, a solid chemical compound that occurs in pure form. A rock may consist of a single mineral, or may be an aggregate of two or more different minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are excluded, but some minerals are biogenic and/or are organic compounds in the sense of chemistry. Moreover, living beings synthesize inorganic minerals that occur in rocks. In geology and mineralogy, the term "mineral" is reserved for mineral species: crystalline compounds with a well-defined chemical composition and a specific crystal structure. Minerals without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound may occur with different crystal structures, each structure is considered different mineral species. Thus, for example and stishovite are two different minerals consisting of the same compound, silicon dioxide; the International Mineralogical Association is the world's premier standard body for the definition and nomenclature of mineral species.
As of November 2018, the IMA recognizes 5,413 official mineral species. Out of more than 5,500 proposed or traditional ones; the chemical composition of a named mineral species may vary somewhat by the inclusion of small amounts of impurities. Specific varieties of a species sometimes have official names of their own. For example, amethyst is a purple variety of the mineral species quartz; some mineral species can have variable proportions of two or more chemical elements that occupy equivalent positions in the mineral's structure. Sometimes a mineral with variable composition is split into separate species, more or less arbitrarily, forming a mineral group. Besides the essential chemical composition and crystal structure, the description of a mineral species includes its common physical properties such as habit, lustre, colour, tenacity, fracture, specific gravity, fluorescence, radioactivity, as well as its taste or smell and its reaction to acid. Minerals are classified by key chemical constituents.
Silicate minerals comprise 90% of the Earth's crust. Other important mineral groups include the native elements, oxides, carbonates and phosphates. One definition of a mineral encompasses the following criteria: Formed by a natural process. Stable or metastable at room temperature. In the simplest sense, this means. Classical examples of exceptions to this rule include native mercury, which crystallizes at −39 °C, water ice, solid only below 0 °C. Modern advances have included extensive study of liquid crystals, which extensively involve mineralogy. Represented by a chemical formula. Minerals are chemical compounds, as such they can be described by fixed or a variable formula. Many mineral groups and species are composed of a solid solution. For example, the olivine group is described by the variable formula 2SiO4, a solid solution of two end-member species, magnesium-rich forsterite and iron-rich fayalite, which are described by a fixed chemical formula. Mineral species themselves could have a variable composition, such as the sulfide mackinawite, 9S8, a ferrous sulfide, but has a significant nickel impurity, reflected in its formula.
Ordered atomic arrangement. This means crystalline. An ordered atomic arrangement gives rise to a variety of macroscopic physical properties, such as crystal form and cleavage. There have been several recent proposals to classify amorphous substances as minerals; the formal definition of a mineral approved by the IMA in 1995: "A mineral is an element or chemical compound, crystalline and, formed as a result of geological processes." Abiogenic. Biogenic substances are explicitly excluded by the IMA: "Biogenic substances are chemical compounds produced by biological processes without a geological component and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound the product can be accepted as a mineral."The first three general characteristics are less debated than the last two. Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. Recent changes have included the addition of an organic class, in both the new Dana and the Strunz classification schemes.
The organic class includes a rare group of minerals with hydrocarbons. The IMA Commission on New Minerals and Mineral Names adopted in 2009 a hierarchical scheme for the naming and classification of mineral groups and group names and established seven commissions and four working groups to review and classify minerals into an official listing of their published names. According to these new r
Fracture (mineralogy)
In the field of mineralogy, fracture is the texture and shape of a rock's surface formed when a mineral is fractured. Minerals have a distinctive fracture, making it a principal feature used in their identification. Fracture differs from cleavage in that the latter involves clean splitting along the cleavage planes of the mineral's crystal structure, as opposed to more general breakage. All minerals exhibit fracture, but when strong cleavage is present, it can be difficult to see. Conchoidal fracture breakage that resembles the concentric ripples of a mussel shell, it occurs in amorphous or fine-grained minerals such as flint, opal or obsidian, but may occur in crystalline minerals such as quartz. Subconchoidal fracture is similar to with less significant curvature. Earthy fracture is reminiscent of freshly broken soil, it is seen in soft, loosely bound minerals, such as limonite and aluminite. Hackly fracture is jagged and not even, it occurs when metals are torn, so is encountered in native metals such as copper and silver.
Splintery fracture comprises sharp elongated points. It is seen in fibrous minerals such as chrysotile, but may occur in non-fibrous minerals such as kyanite. Uneven fracture is a rough one with random irregularities, it occurs in a wide range of minerals including arsenopyrite and magnetite. Rudolf Duda and Lubos Rejl: Minerals of the World http://www.galleries.com/minerals/property/fracture.htm
Saxony
Saxony the Free State of Saxony, is a landlocked federal state of Germany, bordering the federal states of Brandenburg, Saxony Anhalt and Bavaria, as well as the countries of Poland and the Czech Republic. Its capital is Dresden, its largest city is Leipzig. Saxony is the tenth largest of Germany's sixteen states, with an area of 18,413 square kilometres, the sixth most populous, with 4 million people; the history of the state of Saxony spans more than a millennium. It has been a medieval duchy, an electorate of the Holy Roman Empire, a kingdom, twice a republic; the area of the modern state of Saxony should not be confused with Old Saxony, the area inhabited by Saxons. Old Saxony corresponds to the modern German states of Lower Saxony, Saxony-Anhalt, the Westphalian part of North Rhine-Westphalia. Saxony is divided into 10 districts: 1. Bautzen 2. Erzgebirgskreis 3. Görlitz 4. Leipzig 5. Meißen 6. Mittelsachsen 7. Nordsachsen 8. Sächsische Schweiz-Osterzgebirge 9. Vogtlandkreis 10. Zwickau In addition, three cities have the status of an urban district: Chemnitz Dresden Leipzig Between 1990 and 2008, Saxony was divided into the three regions of Chemnitz and Leipzig.
After a reform in 2008, these regions - with some alterations of their respective areas - were called Direktionsbezirke. In 2012, the authorities of these regions were merged into one central authority, the Landesdirektion Sachsen; the Erzgebirgskreis district includes the Ore Mountains, the Sächsische Schweiz-Osterzgebirge district includes Saxon Switzerland and the Eastern Ore Mountains. There are numerous rivers in Saxony; the Elbe is the most dominant one. Oder and Neiße define the border between Poland. Other rivers include the Weiße Elster; the largest cities in Saxony according to the 31 December 2015 estimate are listed below. To this can be added that Leipzig forms a metropolitan-like region with Halle, known as Ballungsraum Leipzig/Halle; the latter city is located just across the border of Saxony-Anhalt. Leipzig shares, for an S-train system and an airport with Halle. Saxony has, after the most vibrant economy of the states of the former East Germany, its economy grew by 1.9% in 2010. Nonetheless, unemployment remains above the German average.
The eastern part of Germany, excluding Berlin, qualifies as an "Objective 1" development-region within the European Union, was eligible to receive investment subsidies up to 30% until 2013. FutureSAX, a business plan competition and entrepreneurial support organisation, has been in operation since 2002. Microchip-makers near Dresden have given the region the nickname "Silicon Saxony"; the publishing and porcelain industries of the region are well known, although their contributions to the regional economy are no longer significant. Today, the automobile industry, machinery production, services contribute to the economic development of the region. Saxony is one of the most renowned tourist destinations in Germany - the cities of Leipzig and Dresden and their surroundings. New tourist destinations are developing, notably in the lake district of Lausitz. Saxony reported an average unemployment of 6.2% in 2017. By comparison, the average in the former GDR was 6.8% and 5.5% for Germany overall. The unemployment rate stood at 5.5% in October 2018.
The Leipzig area, which until was among the regions with the highest unemployment rate, could benefit from investments by Porsche and BMW. With the VW Phaeton factory in Dresden, many parts suppliers, the automobile industry has again become one of the pillars of Saxon industry, as it was in the early 20th century. Zwickau is another major Volkswagen location. Freiberg, a former mining town, has emerged as a foremost location for solar technology. Dresden and some other regions of Saxony play a leading role in some areas of international biotechnology, such as electronic bioengineering. While these high-technology sectors do not yet offer a large number of jobs, they have stopped or reversed the brain drain, occurring until the early 2000s in many parts of Saxony. Regional universities have strengthened their positions by partnering with local industries. Unlike smaller towns and Leipzig in the past experienced significant population growth; the population of Saxony began declining around the middle of the 20th century, a process which accelerated after German reunification in 1990.
The second decade of the 21st century has seen demographic decline stabilize through immigration. In recent years the cities of Dresden and Leipzig, some towns in their hinterlands, have had population increases; the following table illustrates the population of Saxony since 1905: The average number of children per woman in Saxony was 1.49 in 2010, the highest of all German states. In 2016, the value reached 1.59. Within Saxony, the highest is the Bautzen district with 1.77, while Leipzig is the lowest with 1.49. Dresden's birth rate of 1.58 is the highest of all German cities with more than 500,000 inhabitants. Births from January–September 2016 = 28,714 Births from January–September 2017 = 28,129 Deaths from January–September 2016 = 39,386 Deaths from January–September 2017 = 41,284 Natural growth from January–September 2016 = -10,672 Natural growth from January–September 2017 = -13,155 Saxony has a long history as a duchy, an electorate of the Holy
Lacquer
The term lacquer is used for a number of hard and shiny finishes applied to materials such as wood. These fall into a number of different groups; the term lacquer originates from the Sanskrit word lākshā representing the number 100,000, used for both the lac insect and the scarlet resinous secretion, rich in shellac, that it produces, used as wood finish in ancient India and neighbouring areas. Asian lacquerware, which may be called "true lacquer", are objects coated with the treated and dried sap of Toxicodendron vernicifluum or related trees, applied in several coats to a base, wood; this dries to a hard and smooth surface layer, durable and attractive to feel and look at. Asian lacquer is sometimes painted with pictures, inlaid with shell and other materials, or carved, as well as dusted with gold and given other further decorative treatments. In modern techniques, lacquer means a range of clear or coloured wood finishes that dry by solvent evaporation or a curing process that produces a hard, durable finish.
The finish can be of any sheen level from ultra matte to high gloss, it can be further polished as required. It is used for "lacquer paint", a paint that dries better on a hard and smooth surface. In terms of modern products for coating finishes, lac-based finishes are to be referred to as shellac, while lacquer refers to other polymers dissolved in volatile organic compounds, such as nitrocellulose, acrylic compounds dissolved in lacquer thinner, a mixture of several solvents containing butyl acetate and xylene or toluene. Lacquer is more durable than shellac; the English lacquer is from the archaic French word lacre "a kind of sealing wax", from Portuguese lacre, itself an unexplained variant of Medieval Latin lacca "resinous substance" from Arabic lakk, from Persian lak, from Hindi lakh. These derive from Sanskrit lākshā, used for both the Lac insect and the scarlet resinous secretion it produces, used as wood finish. Lac resin was once imported in sizeable quantity into Europe from India along with Eastern woods.
Lacquer sheen is a measurement of the shine for a given lacquer. Different manufacturers have their own standards for their sheen; the most common names from least shiny to most shiny are: flat, egg shell, semi-gloss, gloss. In India the insect lac, or shellac was used since ancient times. Shellac is the secretion of the lac bug, it is used for the production of a red dye and pigment, for the production of different grades of shellac, used in surface coating. Urushiol-based lacquers differ from most others, being slow-drying, set by oxidation and polymerization, rather than by evaporation alone. In order for it to set properly it requires a warm environment; the phenols oxidize and polymerize under the action of an enzyme laccase, yielding a substrate that, upon proper evaporation of its water content, is hard. These lacquers produce hard, durable finishes that are both beautiful and resistant to damage by water, alkali or abrasion; the active ingredient of the resin is urushiol, a mixture of various phenols suspended in water, plus a few proteins.
The resin is derived from trees indigenous to East Asia, like lacquer tree Toxicodendron vernicifluum, wax tree Toxicodendron succedaneum. The fresh resin from the T. vernicifluum trees causes urushiol-induced contact dermatitis and great care is required in its use. The Chinese treated the allergic reaction with crushed shellfish, which prevents lacquer from drying properly. Lacquer skills became highly developed in Asia, many decorated pieces were produced. During the Shang Dynasty, the sophisticated techniques used in the lacquer process were first developed and it became a artistic craft, although various prehistoric lacquerwares have been unearthed in China dating back to the Neolithic period and objects with lacquer coating in Japan from the late Jōmon period; the earliest extant lacquer object, a red wooden bowl, was unearthed at a Hemudu culture site in China. By the Han Dynasty, many centres of lacquer production became established; the knowledge of the Chinese methods of the lacquer process spread from China during the Han and Song dynasties.
It was introduced to Korea, Japan and South Asia. Trade of lacquer objects travelled through various routes to the Middle East. Known applications of lacquer in China included coffins, music instruments and various household items. Lacquer mixed with powdered cinnabar is used to produce the traditional red lacquerware from China; the trees must be at least ten years old before cutting to bleed the resin. It sets by a process called absorbing oxygen to set. Lacquer-yielding trees in Thailand, Vietnam and Taiwan, called Thitsi, are different; the end result is similar but softer than the Japanese lacquer. Burmese lacquer sets slower, is painted by craftsmen's hands without using brushes. Raw lacquer can be "coloured" by the addition of small amounts of iron oxides, giving red or black depending on the oxide. There is some evidence that its use is older than 8,000 years from archaeological digs in China. Pigments were added to make colours, it is used not only as a finish, but mixed with ground fired and unfired clays applied to a mould
Refractive index
In optics, the refractive index or index of refraction of a material is a dimensionless number that describes how fast light propagates through the material. It is defined as n = c v, where c is the speed of light in vacuum and v is the phase velocity of light in the medium. For example, the refractive index of water is 1.333, meaning that light travels 1.333 times as fast in vacuum as in water. The refractive index determines how much the path of light is bent, or refracted, when entering a material; this is described by Snell's law of refraction, n1 sinθ1 = n2 sinθ2, where θ1 and θ2 are the angles of incidence and refraction of a ray crossing the interface between two media with refractive indices n1 and n2. The refractive indices determine the amount of light, reflected when reaching the interface, as well as the critical angle for total internal reflection and Brewster's angle; the refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is v = c/n, the wavelength in that medium is λ = λ0/n, where λ0 is the wavelength of that light in vacuum.
This implies that vacuum has a refractive index of 1, that the frequency of the wave is not affected by the refractive index. As a result, the energy of the photon, therefore the perceived color of the refracted light to a human eye which depends on photon energy, is not affected by the refraction or the refractive index of the medium. While the refractive index affects wavelength, it depends on photon frequency and energy so the resulting difference in the bending angle causes white light to split into its constituent colors; this is called dispersion. It can be observed in prisms and rainbows, chromatic aberration in lenses. Light propagation in absorbing materials can be described using a complex-valued refractive index; the imaginary part handles the attenuation, while the real part accounts for refraction. The concept of refractive index applies within the full electromagnetic spectrum, from X-rays to radio waves, it can be applied to wave phenomena such as sound. In this case the speed of sound is used instead of that of light, a reference medium other than vacuum must be chosen.
The refractive index n of an optical medium is defined as the ratio of the speed of light in vacuum, c = 299792458 m/s, the phase velocity v of light in the medium, n = c v. The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves; the definition above is sometimes referred to as the absolute refractive index or the absolute index of refraction to distinguish it from definitions where the speed of light in other reference media than vacuum is used. Air at a standardized pressure and temperature has been common as a reference medium. Thomas Young was the person who first used, invented, the name "index of refraction", in 1807. At the same time he changed this value of refractive power into a single number, instead of the traditional ratio of two numbers; the ratio had the disadvantage of different appearances. Newton, who called it the "proportion of the sines of incidence and refraction", wrote it as a ratio of two numbers, like "529 to 396".
Hauksbee, who called it the "ratio of refraction", wrote it as a ratio with a fixed numerator, like "10000 to 7451.9". Hutton wrote it as a ratio with a fixed denominator, like 1.3358 to 1. Young did not use a symbol for the index of refraction, in 1807. In the next years, others started using different symbols: n, m, µ; the symbol n prevailed. For visible light most transparent media have refractive indices between 1 and 2. A few examples are given in the adjacent table; these values are measured at the yellow doublet D-line of sodium, with a wavelength of 589 nanometers, as is conventionally done. Gases at atmospheric pressure have refractive indices close to 1 because of their low density. All solids and liquids have refractive indices above 1.3, with aerogel as the clear exception. Aerogel is a low density solid that can be produced with refractive index in the range from 1.002 to 1.265. Moissanite lies at the other end of the range with a refractive index as high as 2.65. Most plastics have refractive indices in the range from 1.3 to 1.7, but some high-refractive-index polymers can have values as high as 1.76.
For infrared light refractive indices can be higher. Germanium is transparent in the wavelength region from 2 to 14 µm and has a refractive index of about 4. A type of new materials, called topological insulator, was found holding higher refractive index of up to 6 in near to mid infrared frequency range. Moreover, topological insulator material are transparent; these excellent properties make them a type of significant materials for infrared optics. According to the theory of relativity, no information can travel faster than the speed of light in vacuum, but this does not mean that the refractive index cannot be lower than 1; the refractive index measures the phase velocity of light. The phase velocity is the speed at which the crests of the wave move and can be faster than the speed of light in vacuum, thereby give a refractive index below 1; this can occur close to resonance frequencies, for absorbing media, in plasmas, for X-rays. In the X-ray regime the refractive indices are
Redox
Redox is a chemical reaction in which the oxidation states of atoms are changed. Any such reaction involves both a reduction process and a complementary oxidation process, two key concepts involved with electron transfer processes. Redox reactions include all chemical reactions; the chemical species from which the electron is stripped is said to have been oxidized, while the chemical species to which the electron is added is said to have been reduced. It can be explained in simple terms: Oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction is a decrease in oxidation state by a molecule, atom, or ion; as an example, during the combustion of wood, oxygen from the air is reduced, gaining electrons from carbon, oxidized. Although oxidation reactions are associated with the formation of oxides from oxygen molecules, oxygen is not included in such reactions, as other chemical species can serve the same function; the reaction can occur slowly, as with the formation of rust, or more in the case of fire.
There are simple redox processes, such as the oxidation of carbon to yield carbon dioxide or the reduction of carbon by hydrogen to yield methane, more complex processes such as the oxidation of glucose in the human body. "Redox" is a portmanteau of the words "reduction" and "oxidation". The word oxidation implied reaction with oxygen to form an oxide, since dioxygen was the first recognized oxidizing agent; the term was expanded to encompass oxygen-like substances that accomplished parallel chemical reactions. The meaning was generalized to include all processes involving loss of electrons; the word reduction referred to the loss in weight upon heating a metallic ore such as a metal oxide to extract the metal. In other words, ore was "reduced" to metal. Antoine Lavoisier showed. Scientists realized that the metal atom gains electrons in this process; the meaning of reduction became generalized to include all processes involving a gain of electrons. Though "reduction" seems counter-intuitive when speaking of the gain of electrons, it might help to think of reduction as the loss of oxygen, its historical meaning.
Since electrons are negatively charged, it is helpful to think of this as reduction in electrical charge. The electrochemist John Bockris has used the words electronation and deelectronation to describe reduction and oxidation processes when they occur at electrodes; these words are analogous to protonation and deprotonation, but they have not been adopted by chemists worldwide. The term "hydrogenation" could be used instead of reduction, since hydrogen is the reducing agent in a large number of reactions in organic chemistry and biochemistry. But, unlike oxidation, generalized beyond its root element, hydrogenation has maintained its specific connection to reactions that add hydrogen to another substance; the word "redox" was first used in 1928. The processes of oxidation and reduction occur and cannot happen independently of one another, similar to the acid–base reaction; the oxidation alone and the reduction alone are each called a half-reaction, because two half-reactions always occur together to form a whole reaction.
When writing half-reactions, the gained or lost electrons are included explicitly in order that the half-reaction be balanced with respect to electric charge. Though sufficient for many purposes, these general descriptions are not correct. Although oxidation and reduction properly refer to a change in oxidation state — the actual transfer of electrons may never occur; the oxidation state of an atom is the fictitious charge that an atom would have if all bonds between atoms of different elements were 100% ionic. Thus, oxidation is best defined as an increase in oxidation state, reduction as a decrease in oxidation state. In practice, the transfer of electrons will always cause a change in oxidation state, but there are many reactions that are classed as "redox" though no electron transfer occurs. In redox processes, the reductant transfers electrons to the oxidant. Thus, in the reaction, the reductant or reducing agent loses electrons and is oxidized, the oxidant or oxidizing agent gains electrons and is reduced.
The pair of an oxidizing and reducing agent that are involved in a particular reaction is called a redox pair. A redox couple is a reducing species and its corresponding oxidizing form, e.g. Fe2+/Fe3+ Substances that have the ability to oxidize other substances are said to be oxidative or oxidizing and are known as oxidizing agents, oxidants, or oxidizers; that is, the oxidant removes electrons from another substance, is thus itself reduced. And, because it "accepts" electrons, the oxidizing agent is called an electron acceptor. Oxygen is the quintessential oxidizer. Oxidants are chemical substances with elements in high oxidation states, or else electronegative elements that can gain extra electrons by oxidizing another substance. Substances that have the ability to reduce other substances are said to be reductive or reducing and are known as
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