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Corundum

Corundum

Corundum is the crystalline form of aluminium oxide and one of the rock-forming minerals. Corundum is naturally clear, but can have different colors when impurities are added. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire. The word corundum comes from the Tamil kurundam. The oxygen atoms in corundum are arranged in a hexagonal close-packing, with the smaller aluminium atoms occupying 2/3 of the octahedral gaps. The coordination of the atoms are thus 6:4, compared to 4:2 for quartz, which accounts for its greater hardness despite the Al-O bonds being less covalent. Due to corundum's hardness (typically 9.0), it is commonly used as an abrasive in machining, from huge machines to sandpaper. Emery is an impure and less abrasive variety, with a Mohs hardness of 9.0. Diamond is harder at 10.0, but typically much more expensive. Category:Oxide minerals Category:Superhard materials ja:コランダム

Crystal

:This article is about the form of solid matter. For other uses of this word, see Crystal (disambiguation). Crystal (disambiguation) A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. Generally, fluid substances form crystals when they undergo a process of solidification. Under ideal conditions, the result may be a single crystal, where all of the atoms in the solid fit into the same lattice or crystal structure but, generally, many crystals form simultaneously during solidification, leading to a polycrystalline solid. For example, most metals encountered in everyday life are polycrystals. Crystals are often symmetrically intergrown to form crystal twins. Which crystal structure the fluid will form depends on the chemistry of the fluid, the conditions under which it is being solidified, and also on the ambient pressure. The process of forming a crystalline structure is often referred to as crystallization. pressure While the process of cooling usually results in the generation of a crystalline material, under certain conditions the fluid may be frozen in a noncrystalline state. In most cases, this involves cooling the fluid so rapidly that atoms cannot travel to their lattice sites before they lose mobility. A noncrystalline material, which has no long-range order, is called an amorphous, vitreous, or glassy material. It is also often referred to as an amorphous solid, although there are distinct differences between solids and glasses: most notably, the process of forming a glass does not release the latent heat of fusion. For this reason, many scientists consider glassy materials to be viscous liquids rather than solids, although this is a controversial topic; see the entry on glass for more details. glass Crystalline structures occur in all classes of materials, with all types of chemical bonds. Almost all metal exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. Ionically bonded crystals can form upon solidification of salts, either from a molten fluid or when it condenses from a solution. Covalently bonded crystals are also very common, notable examples being diamond, silica, and graphite. Polymer materials generally will form crystalline regions, but the lengths of the molecules usually prevents complete crystallization. Weak Van der Waals forces can also play a role in a crystal structure; for example, this type of bonding loosely holds together the hexagonal-patterned sheets in graphite. Most crystalline materials have a variety of crystallographic defects. The types and structures of these defects can have a profound effect on the properties of the materials. crystallographic defect crystallographic defect.]] While the term "crystal" has a precise meaning within materials science and solid-state physics, colloquially "crystal" refers to solid objects that exhibit well-defined and often pleasing geometric shapes. Various shapes of such crystals are found in nature. The shape of these crystals is dependent on the types of molecular bonds between the atoms to determine the structure, as well as on the conditions under which they formed. Snowflakes, diamonds, and common salt are common examples of crystals. Some crystalline materials may exhibit special electrical properties such as the ferroelectric effect or the piezoelectric effect. The behaviour of light in crystals is described by crystal optics. In periodic dielectric structures a range of unique optical properties can be expected as described in photonic crystals. Crystallography is the scientific study of crystals and crystal formation.

See also


- Crystal habit
- Crystal structure
- Crystallite
- Crystallization processes
- Liquid crystal
- Quasicrystal
- Seed crystal
- Single crystal

External links


- [http://www.rockhounds.com/rockshop/xtal/index.html Introduction to Crystallography and Mineral Crystal Systems]
- [http://www.iucr.ac.uk/iucr-top/comm/cteach/pamphlets.html Crystallographic Teaching Pamphlets]
- [http://cst-www.nrl.navy.mil/lattice/spcgrp/ Crystal Lattice Structures] ja:結晶

Rock (geology)

, plutonic, metamorphic rock types of North America. ]] Rock is a naturally occurring aggregate of minerals and/or mineraloids. Rocks are classified by mineral and chemical composition; the texture of the constituent particles; and also by the processes that formed them. These indicators separate rocks into igneous, sedimentary, and metamorphic. Igneous rocks are formed from molten magma, and are divided into two main categories: Plutonic rock and Volcanic rock. Plutonic rocks result when the magma cools and crystallises slowly within the Earth's crust, while Volcanic rocks result from the magma reaching the surface either as lava or fragmental ejecta. Sedimentary rocks are formed by deposition of either detrital or organic matter, or chemical precipitates (evaporites), followed by compaction of the particulate matter and cementation. The latter can occur at or near the earth's surface, especially in the case of carbonate-rich sediments. Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the earth's surface, and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals (e.g. by recrystallisation). The transformation of one rock type to another is described by the geological model called the rock cycle. The Earth's crust (including the lithosphere) and mantle are formed of rock.

See also


- Geology
- Petrology
- List of minerals
- List of rocks
- List of stone
- Quarrying
- Rock formations
- Megalith
- Riprap

External links


- [http://www.geol.lsu.edu/henry/Geology3041/2IgneousClassify/IgneousClassFlow.htm Classification of Igneous Rocks] Category:Geology Category:Rocks ja:岩石 ms:Batu th:หิน

Transparency (optics)

In optics, transparency is the property of allowing light to pass. The opposite property is opacity. Though transparency usually refers to visible light in common usage, it can actually refer to any type of radiation. For example, flesh is transparent to X-rays, while bone is not, allowing the use of medical X-ray machines. Examples of transparent materials are air and some other gases, liquids such as water, most glasses, and plastics such as Perspex. Where the degree of transparency varies according to the wavelength of the light, the image seen through the material is tinted. This may for instance be due to certain metallic oxide molecules in glass, or larger colored particles, as in a thin smoke. If many such particles are present the material may become opaque, as in a thick smoke. Transparent materials can be seen through; that is, they allow clear images to pass. Translucent materials allow light to pass through them only diffusely, and hence cannot be clearly seen through. Examples of translucent materials are frosted glass, paper, and some kinds of amber. Opacity is based either on absorption or on reflection of the light falling onto the material. From electro-dynamics it results that only a vacuum is really transparent in the strict meaning, any matter has a certain absorption for electro-magnetic waves. There are transparent glass walls that can be made opaque by the press of a button, a technology known as electrochromics. Certain crystals are transparent because there are straight lines through the crystal structure. Light passes unobstructed along these lines. The zebrafish is a special animal to biologists because its body is transparent, and it is easy to observe the animal's development or trace the metabolism of a particular substance, without interfering with the animal. For this reason, a tremendous amount of genetics, developmental biology and other biological research has been done on this animal. Certain other animals are transparent, notably some jellyfish. Transparent or semi-transparent clothing enables play with the boundaries of dress-codes regarding modesty; one example is the wet T-shirt contest. For derived uses of the idea of transparency. Category:Optics

Ruby

Ruby is a red gemstone, a variety of the mineral corundum (aluminium oxide) in which the color is caused mainly by chromium. Its name originates from ruber, Latin for red. Natural rubies are exceptionally rare, though artificial ones (sometimes called created ruby) can be manufactured which are comparatively inexpensive. Rubies are mined in Africa, Asia, Australia and Greenland. They are most often found in Myanmar, Sri Lanka and Thailand, though they have also been found in Montana and South Carolina. Sometimes spinels are found along with rubies in the same geological formations and are mistaken for the more valuable gem. However, fine red spinels may approach the average ruby in value. Rubies have a hardness of 9 on the Mohs scale of mineral hardness, and among the natural gems are only surpassed by diamonds in hardness. Other varieties of corundum are called sapphires. sapphireRuby gemstones are valued according to several characteristics including size, color, clarity and cut. All natural rubies have imperfections in them. On the other hand, artificial rubies may have no imperfections. The fewer the number and the less obvious the imperfections, the more valuable the ruby is—unless there are no imperfections (i.e., a "perfect" ruby), in which case it is suspected of being artificially made and its status as a priceless gem is therefore not assured. Some manufactured rubies have dopants added to them so that they can be identified as artificial, but most require gemological testing to determine their origin. A synthetic ruby crystal was used to create the first laser. Ruby is the birthstone associated with July. The world's biggest star ruby is the Rajaratna Ruby, which weighs 2,475 carats (495 g). The world's biggest double-star ruby (with a 12-pointed star) is the Neelanjali Ruby, weighing 1,370 carats (274 g). Both rubies currently belong to G. Vidyaraj from Bangalore in India.

See also


- List of minerals Category:Gemstones ko:루비 ja:ルビー

Sapphire

Sapphire is the single-crystal form of aluminium oxide (Al2O3), a mineral known as corundum. It can be found naturally as gemstones or manufactured in large crystal boules for a variety of applications.

Sapphire gems

Sapphire is any gemstone-quality corundum. (The red variety of corundum is also known as ruby.) When color is not specified, sapphire refers to the blue variety. Pink, yellow, green, white, and parti-color (multi-colored) sapphires are often valued less than the blue variety of the same quality and size. However a pink-orange sapphire, called a padparadsha, is highly prized. They were found in many countries especially in Asia such as India, Sri Lanka, Thailand, Myanmar, and Cambodia. It is the impurities in the aluminium oxide crystal that give the color variations, with different impurity chemical elements giving the different colors that can be found. Pure sapphire is transparent. Traces of iron and titanium give sapphires a blue color. The crystals are exceptionally hard, with only diamond being harder among natural gems. They have a hardness of 9 on the Mohs hardness scale (Diamond is 10). Gem quality sapphires and rubies occur naturally and can be easily and cheaply produced in the laboratory. The chemical compositions and physical properties are identical to the natural sapphires. The tell-tale sign of synthetic sapphires is the crystalline growth lines which are usually curved due to the pulling during the accelerated crystal growth process. A version which shows an asterism is called a star sapphire (see picture above). Although natural sapphires can show an asterism, the shape of the star is usually somewhat irregular and sometimes indistinct. A manufactured star sapphire called the Linde Star shows a very regularly-shaped and distinct asterism because the formation process is more tightly controlled than the natural version. The Logan sapphire is one of the largest blue sapphire gems known. It weighs 423 carats (84.6 g). Lady Diana Spencer's engagement ring from Charles, Prince of Wales was a sapphire ring. Cornflower blue is one of the most popular colors for sapphires (the other choice color a deep royal blue), though there is little objective consensus about which shade of blue is the most cornflower or the most desirable. Sapphire is also the birthstone associated with September.

Synthetic sapphire for non-gemstone applications

Synthetic sapphire crystals can be grown in cylindrical crystal ingots of large size, up to many inches in diameter. As well as gemstone applications there are many other uses: The first ever laser produced was based on the ruby chromium impurity in sapphire. While this laser has few commercial applications, the Ti-sapphire laser is popular due the relatively rare ability to tune the laser wavelength in the red-to-near infrared region of the electromagnetic spectrum. It can also be easily modelocked. In these lasers, a synthetically produced sapphire crystal with chromium or titanium impurities is irradiated with intense light from a special lamp, or another laser, to create stimulated emission. Pure sapphire ingots can be sliced into wafers and polished to form transparent crystal slices. Such slices are used as watch faces in high quality watches, as the material's exceptional hardness makes the face almost impossible to scratch. Wafers of single crystal sapphire are also used in the semiconductor industry as a substrate for the growth of gallium nitride based blue and green light emitting diodes. The word sapphire is probably Phoenician in origin, coming to English from the Ancient Greek word σάπφειρος, through the Latin sapphirus. It refers to a "blue gem," either the sapphire proper or possibly lapis lazuli. The major deposits are: Sri Lanka, Thailand, Burma, Cambodia, Nigeria, Madagascar and Australia.

See also


- List of minerals Category:Gemstones Category:Oxide minerals ja:サファイア

Tamil

Tamil may refer to:
- The Tamil language, which is one of the Dravidian languages primarily spoken in the Indian subcontinent.
- A member of the Tamil people (Tamils) who speak Tamil and live mostly in the state of Tamil Nadu in south India, in northern and eastern parts of Sri Lanka, in Malaysia and Singapore.
- The Tamil script, primarily used to write the Tamil language.
- Tamil Tigers, Liberation Tigers of Tamil Eazham The phrase Tamil country is used sometimes to refer to the regions with a significant population of Tamil-speaking people. The extent of this region varies with time. ko:타밀어 ja:タミル語 th:ภาษาทมิฬ

Oxygen

Oxygen is a chemical element in the periodic table. It has the symbol O and atomic number 8. The element is very common, found not only on Earth but throughout the universe, usually covalently bonded with other elements. Unbound oxygen (usually called molecular oxygen, O2, a diatomic molecule) first appeared on Earth during the Paleoproterozoic era (between 2500 million years ago and 1600 million years ago) and as a product of the metabolic action of early anaerobes (archaea and bacteria). The presence of free oxygen drove most of the organisms then living to extinction. The atmospheric abundance of free oxygen in later geological epochs and up to the present has been largely driven by photosynthetic organisms, roughly three quarters by phytoplankton and algae in the oceans and one quarter from terrestrial plants.

Characteristics

At standard temperature and pressure, oxygen is mostly found as a gas consisting of a diatomic molecule with the chemical formula O2. O2 has two energetic forms:
- The low-energy predominant single-bonded diradical triplet oxygen. This native diradical quality of oxygen contributes to its destructive chemical nature. This form is stabilized by the degeneracy effect.
- The high-energy double-bonded molecule singlet oxygen. Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals. The word oxygen derives from two words in Greek, οξυς (oxys) (acid, sharp) and γεινομαι (geinomai) (engender). The name "oxygen" was chosen because, at the time it was discovered in the late 18th century, it was believed that all acids contained oxygen. The definition of acid has since been revised to not require oxygen in the molecular structure. Liquid O2 and solid O2 have a light blue color and both are highly paramagnetic. Liquid O2 is usually obtained by the fractional distillation of liquid air. Liquid and solid O3 (ozone) have a deeper color of blue. A recently discovered allotrope of oxygen, tetraoxygen (O4), is a deep red solid that is created by pressurizing O2 to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O2 or O3.

Applications

Liquid oxygen finds use as an oxidizer in rocket propulsion. Oxygen is essential to respiration, so oxygen supplementation has found use in medicine (as oxygen therapy). People who climb mountains or fly in airplanes sometimes have supplemental oxygen supplies (as air). Oxygen is used in welding (such as the oxyacetylene torch), and in the making of steel and methanol. Oxygen presents two absorption bands centered in the wavelengths 687 and 760 nanometers. Some scientists have proposed to use the measurement of the radiance coming from vegetation canopies in those oxygen bands to characterize plant health status from a satellite platform. This is because in those bands, it is possible to discriminate the vegetation's reflectance from the vegetation's fluorescence, which is much weaker. The measurement presents several technical difficulties due to the low signal to noise ratio and due to the vegetation's architecture, but it has been proposed as possibility to monitor the carbon cycle from satellite, thus in a global scale. Oxygen, as a mild euphoric, has a history of recreational use that extends into modern times. Oxygen bars can be seen at parties to this day. In the 19th century, oxygen was often mixed with nitrous oxide to promote an analgesic effect; indeed, such a mixture (Entonox) is commonly used in medicine today.

History

Oxygen was first discovered by Michał Sędziwój, Polish alchemist and philosopher in late 16th century. Sędziwój assumed the existence of oxygen by warming nitre (saltpeter). He thought of the gas given off as "the elixir of life". Oxygen was again discovered by the Swedish pharmacist Carl Wilhelm Scheele sometime before 1773, but the discovery was not published until after the independent discovery by Joseph Priestley on August 1, 1774, who called the gas dephlogisticated air (see phlogiston theory). Priestley published his discoveries in 1775 and Scheele in 1777; consequently Priestley is usually given the credit. It was named by Antoine Laurent Lavoisier after Priestley's publication in 1775.

Occurrence

Oxygen is the second most common component of the earth's atmosphere (20.947% by volume).

Compounds

Due to its electronegativity, oxygen forms chemical bonds with almost all other elements (which is the origin of the original definition of oxidation). The only elements to escape the possibility of oxidation are a few of the noble gases. The most famous of these oxides is dihydrogen monoxide, or water (H2O). Other well known examples include compounds of carbon and oxygen, such as carbon dioxide (CO2), alcohols (R-OH), aldehydes, (R-CHO), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO3−), perchlorates (ClO4−), chromates (CrO42−), dichromates (Cr2O72−), permanganates (MnO4−), and nitrates (NO3−) are strong oxidizing agents in and of themselves. Many metals such as iron bond with oxygen atoms, iron (III) oxide (Fe2O3). Ozone (O3) is formed by electrostatic discharge in the presence of molecular oxygen. A double oxygen molecule (O2)2 is known and is found as a minor component of liquid oxygen. Epoxides are ethers in which the oxygen atom is part of a ring of three atoms.

Isotopes

Oxygen has fifteen known isotopes with atomic masses ranging from 12 to 26. Three of them are stable and twelve are radioactive. The radioisotopes all have half lives of less than three minutes. The stable isotopes have mass numbers of 16, 17 and 18, of which oxygen-16 is the most common (over 99%).

Precautions

Oxygen can be toxic at elevated partial pressures (i.e. high relative concentrations). This is important in some forms of scuba diving, such as with a rebreather. Certain derivatives of oxygen, such as ozone (O3), singlet oxygen, hydrogen peroxide, hydroxyl radicals and superoxide, are also highly toxic. The body has developed mechanisms to protect against these toxic species. For instance, the naturally-occurring glutathione can act as an antioxidant, as can bilirubin which is normally a breakdown product of hemoglobin. Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels. This is true as well of compounds of oxygen such as chlorates, perchlorates, dichromates, etc. Compounds with a high oxidative potential can often cause chemical burns. The fire that killed the Apollo 1 crew on a test launchpad spread so rapidly because the pure oxygen atmosphere was at normal atmospheric pressure instead of the one third pressure that would be used during an actual launch. (See partial pressure.) Oxygen derivatives are prone to form free radicals, especially in metabolic processes. Because they can cause severe damage to cells and their DNA, they are thought to be related to cancer and aging.

See also


- Winkler test for dissolved oxygen for instructions on how to determine the amount of oxygen dissolved in fresh water.
- Combustion
- Oxidation
- Oxygen Catastrophe in geology
- The role of oxygen as a diving breathing gas
- Oxygen depletion aquatic ecology
- Ozone layer

References


- [http://periodic.lanl.gov/elements/8.html Los Alamos National Laboratory – Oxygen]
- [http://physics.nist.gov/cgi-bin/AtData/main_asd Nist atomic spectra database]
- [http://chartofthenuclides.com/default.html Nuclides and Isotopes Fourteenth Edition]: Chart of the Nuclides, General Electric Company, 1989

External links


- [http://www.priestleysociety.net Priestley Society, Dedicated to Joseph Priestley the man who discovered oxygen]
- [http://www.best-home-remedies.com/minerals/oxygen.htm Oxygen - Benefits, Deficiency Symptoms And Food Sources]
- [http://www.josephpriestley.info Joseph Priestley Information Website, about the man who discovered oxygen]
- [http://periodic.lanl.gov/elements/8.html Los Alamos National Laboratory – Oxygen]
- [http://www.webelements.com/webelements/elements/text/O/index.html WebElements.com – Oxygen]
- [http://education.jlab.org/itselemental/ele008.html It's Elemental – Oxygen]
- [http://members.tripod.com/tjaartdb0/html/oxygen_toxicity.html Oxygen Toxicity]
- [http://www.uigi.com/oxygen.html Oxygen (O2) Properties, Uses, Applications]
- [http://www.compchemwiki.org/index.php?title=Oxygen Computational Chemistry Wiki]
- [http://koti.mbnet.fi/antitz/dime/en Tests with liquid oxygen :-)] Category:Nonmetals Category:Chalcogens als:Sauerstoff ko:산소 ms:Oksigen ja:酸素 simple:Oxygen th:ออกซิเจน

Octahedron

An octahedron (plural: octahedra) is a polyhedron with eight faces. A regular octahedron is a Platonic solid composed of eight faces each of which is an equilateral triangle four of which meet at each vertex. The regular octahedron is a special kind of triangular antiprism and of square bipyramid, and is dual to the cube. __TOC__ Polygonal net

The octahedron in general

Some octahedra are:
- hexagonal prism
- 7-sided pyramid
- 4-sided bipyramid; a special case is the regular octahedron In the general meaning the term octahedron it is not much used because of these different types which have not much in common except having the same number of faces.

Canonical coordinates

Canonical coordinates for the vertices of a regular octahedron centered at the origin are (±1,0,0), (0,±1,0), (0,0,±1).

Area and volume

The area A and the volume V of a regular octahedron of edge length a are: :A=2\sqrta^2 :V=\begin\end\sqrta^3 Thus the volume is four times that of a regular tetrahedron with the same edge length, while the surface area is twice (because we have 8 vs. 4 triangles).

Geometric relations

The interior of the compound of two dual tetrahedra is an octahedron, and this compound, called the stella octangula, is its first and only stellation. Correspondingly, a regular octahedron is the result of cutting off from a regular tetrahedron, four regular tetrahedra of half the linear size (i.e. rectifying the tetrahedron). The vertices of the octahedron lie at the midpoints of the edges of the tetrahedron, and in this sense it relates to the tetrahedron in the same way that the cuboctahedron and icosidodecahedron relate to the other Platonic solids. One can also divide the edges of an octahedron in the ratio of the golden mean to define the vertices of an icosahedron. There are five octahedra that define any given icosahedron in this fashion, and together they define a regular compound. Octahedra and tetrahedra can be mixed together to form a vertex, edge, and face-uniform tiling of space, called the octet truss by Buckminster Fuller. This is the only such tiling save the regular tessellation of cubes, and is one of the 28 Andreini tessellations. Another is a tessellation of octahedra and cuboctahedra. The octahedron is unique among the Platonic solids in having an even number of faces meeting at each vertex. Consequently, it is the only member of that group to possess mirror planes that do not pass through any of the faces. Using the standard nomenclature for Johnson solids, an octahedron would be called a square bipyramid.

Uses

Especially in roleplaying, this solid is known as a d8, one of the more common Polyhedral dice. If each edge of an octahedron is replaced by a one ohm resistor, the resistance between opposite vertices is 0.5 ohms, and that between adjacent vertices 5/12 ohms.

See also


- Triakis octahedron
- Hexakis octahedron
- Truncated octahedron

External links


- [http://mathworld.wolfram.com/Octahedron.html Octahedron] - Mathworld.com
- [http://www.mathsisfun.com/geometry/octahedron.html Spinning Octahedron] - MathsIsFun.com
- [http://www.mathconsult.ch/showroom/unipoly/ The Uniform Polyhedra]
- [http://www.georgehart.com/virtual-polyhedra/vp.html Virtual Reality Polyhedra] The Encyclopedia of Polyhedra
- [http://www.korthalsaltes.com/ Paper Models of Polyhedra] Many links Category:Deltahedra Category:Platonic solids Category:Uniform polyhedra Category:Prismatoid polyhedra ja:正八面体 ko:%EC%A0%95%ED%8C%94%EB%A9%B4%EC%B2%B4

Covalent bond

.]] Covalent bonding is an intramolecular form of chemical bonding characterized by the sharing of one or more pairs of electrons between two species, producing a mutual attraction that holds the resultant molecule together. Atoms tend to share electrons in such a way that their outer electron shells are filled. Such bonds are always stronger than the intermolecular hydrogen bond and similar in strength to or stronger than the ionic bond. Covalent bonding most frequently occurs between atoms with similar electronegativities. For this reason, non-metals tend to engage in covalent bonding more readily since metals have access to metallic bonding, where the easily-removed electrons are more free to roam about. For non-metals, liberating an electron is more difficult, so sharing is the only option when confronted with another species of similar electronegativity. However, covalent bonding involving metals is particularly important, especially in industrial catalysis and process chemistry. Many polymerization techniques require catalysis involving metal-organic covalent bonds. In their more useful applications, metals often engage in more exotic covalent bonding, such as those between a metal and the σ bond of molecular hydrogen, or between a metal and the π bond of an alkane or alkene.

History

alkene The idea of covalent bonding can be traced to Gilbert N. Lewis, who in 1916 described the sharing of electron pairs between atoms. He introduced the so called Lewis Notation or Electron Dot Notation in which valence electrons (those in the outer shell) are represented as dots around the atomic symbols. Pairs of electrons located between atoms represent covalent bonds. Multiple pairs represent multiple bonds, such as double and triple bonds. Some examples of Electron Dot Notation are shown in the following figure. An alternative form, in which bond-forming electron pairs are represented as solid lines, is shown in blue. While the idea of shared electron pairs provides an effective qualitative picture of covalent bonding, quantum mechanics is needed to understand the nature of these bonds and predict the structures and properties of simple molecules. Heitler and London are credited with the first successful quantum mechanical explanation of a chemical bond, specifically that of molecular hydrogen, in 1927. Their work was based on the valence bond model, which assumes that a chemical bond is formed when there is good overlap between the atomic orbitals of participating atoms. These atomic orbitals are known to have specific angular relationships between each other, and thus the valence bond model can successfully predict the bond angles observed in simple molecules.

Bond Polarity

There are two types of covalent bonds: Polar covalent bonds, and non-polar (or pure) covalent bonds. The most widely accepted definition of polar covalent is when the atoms involved have an electronegativity difference that is less than 1.67 (though some texts read 1.7), but greater than zero. A pure covalent bond is a bond that occurs when the atoms involved have an electronegativity difference of zero (though some texts read less than 0.2). Pure covalent bonds (which are usually non-soluble, electrically non-conductive and tend to exist as individual molecules), and ionic bonds (which conversely are soluble, electrically conductive when molten or in solution and generally tend to exist in a crystalline form) are on two opposite ends of the figurative spectrum and have differing properties. Polar covalent bonds fall in the middle and have properties of both.

Bond order

Bond order is a term that describes the number of pairs of electrons shared between atoms forming a covalent bond. The most common type of covalent bond is the single bond, the sharing of only one pair of electrons between two individual atoms. All bonds with more than one shared pair are called multiple covalent bonds. The sharing of two pairs is called a double bond and the sharing of three pairs is called a triple bond. An example of a double bond is nitrous acid (between N and O), and an example of a triple bond is in hydrogen cyanide (between C and N). A single bond usually consists of one sigma bond, a double bond of one sigma and one pi bond, and a triple bond of one sigma and two pi bonds. Quadruple bonds, though rare, also exist. Both carbon and silicon can theoretically form these; however, the formed molecules are explosively unstable. Stable quadruple bonds are observed as transition metal-metal bonds, usually between two transition metal atoms in organometallic compounds. Molybdenum and Ruthenium are the elements most commonly observed with this bonding configuration. An example of a quadruple bond is also found in Di-tungsten tetra(hpp). Quintuple Bonds are found to exist in certain chromium dimers. Sextuple bonds of order 6 have also been observed in transition metals in the gaseous phase at very low temperatures and are extremely rare. Other more exotic bonds, such as three center bonds are known and defy the conventions of bond order. It is also important to note that bond order is an integer value only in the elementary sense and is often fractional in more advanced contexts.

Coordinate covalent bonds

A special case is called a dative covalent bond, also known as a coordinate covalent bond, which occurs when one atom gives both of the electrons in the bond.

Rigidity

Typically, two atoms can rotate about a single bond with relative ease. However, double and triple bonds are very difficult to rotate because they require p orbital overlap. p orbital overlaps are parallel.

Resonance

Some structures can have more than one valid Lewis Dot Structure (for example, ozone, O3). In an LDS diagram of O3, the center atom will have a single bond with one atom and a double bond with the other. The LDS diagram cannot tell us which atom has the double bond; the first and second adjoining atoms have equal chances of having the double bond. These two possible structures are called resonance structures. In reality, the structure of ozone is a resonance hybrid between its two possible resonance structures. Instead of having one double bond and one single bond, there are actually two 1.5 bonds with approximately three electrons in each at all times. A special resonance case is exhibited in aromatic rings of atoms (for example, benzene). Aromatic rings are composed of atoms arranged in a circle (held together by covalent bonds) that alternate between single and double bonds according to their LDS. In actuality, the electrons tend to be disambiguously and evenly spaced within the ring. Electron sharing in aromatic structures is often represented with a ring inside the circle of atoms.

Current theory

Today the valence bond model has been supplemented with the molecular orbital model. In this model, as atoms are brought together, the atomic orbitals interact to form hybrid molecular orbitals. These molecular orbitals are a cross between the original atomic orbitals and generally extend between the two bonding atoms. Using quantum mechanics it is possible to calculate the electronic structure, energy levels, bond angles, bond distances, dipole moments, and frequency spectra of simple molecules with a high degree of accuracy. Currently, bond distances and angles can be calculated as accurately as they can be measured (distances to a few pm and bond angles to a few degrees). For small molecules, energy calculations are sufficiently accurate to be useful for determining thermodynamic heats of formation and kinetic activation energy barriers.

See also


- Chemical bond
- Ionic bond
- Linear combination of atomic orbitals
- Metallic bonding

External links


- [http://wps.prenhall.com/wps/media/objects/602/616516/Chapter_07.html Covalent Bonds and Molecular Structure] Category:Chemical bonding ja:共有結合



Abrasive

An abrasive is usually a material that is used to smooth or to machine another softer material through extensive rubbing. Some common examples of abrasive objects are: :
- Borazon or Cubic Boron Nitride (CBN) :
- Carborundum :
- Coated abrasives :
- Diamond dust :
- Emery (mineral) (impure corundum) :
- Grinding wheel :
- Powdered glass :
- Pumice dust :
- Sand :
- Sandpaper Category:Manufacturing Category:Metalworking

Sandpaper

Sandpaper is a form of paper where an abrasive material has been fixed to its surface; it is part of the "coated abrasives" family of abrasive products. It is used to remove small amounts of material from surfaces, either to make them smoother (painting and wood finishing), to remove a layer of material (e.g. old paint), or sometimes to make the surface rougher (e.g. as a preparation to gluing).

Types of sandpaper

There are countless varieties of sandpaper, with variations in the paper or backing, the material used for the grit, grit size, and the bond.

Backing

In addition to paper, backing for sandpaper includes cloth (cotton, polyester, rayon), polyester film (Mylar), and "Fibre". Cloth backing is used for sanding discs and belts, while mylar is used with extremely fine grits. Fibre or vulcanized fibre is a strong backing material consisting of many layers of impregnated paper made from rags. The weight of the backing is usually designated by a letter. For paper, the letters range from A to F, with A being the lightest and F the heaviest. Letter nomenclature is different for cloth, with the weight of the backing being, from lightest to heaviest: J, X, Y , T and M.

Material

Materials used for the abrading particles are:
- flint — no longer commonly used;
- garnet — commonly used in woodworking;
- emery — commonly used to abrade or polish metal;
- aluminium oxide — perhaps most common in widest variety of grits;
- silicon carbide — used in microgrits, especially for wet-and-dry sandpaper;
- Alumina-zirconia — (an aluminium oxide - Zirconium oxide alloy), used in very fine (microgrit) papers;
- chromium oxide — used in extremely fine micron grit (micrometre level) papers
- Ceramic Aluminum Oxide, used in high pressure applications, commonly known as CubitronTM a 3M Corp. Trademark who invented sol gel ceramic grains. Used in both coated abrasives, as well as in bonded abrasives. As well, sandpaper may be "stearated" where a dry lubricant is loaded to the abrasive. Stearated papers are useful in sanding coats of finish and paint as the stearate soap prevents clogging and increases the useful life of the sandpaper.

Bonds

Different adhesives are used to bond the abrasive to the paper. Hide glue is still used, but this paper is not waterproof. Waterproof or wet/dry sandpapers use a resin and a waterproof backing such as cloth. Sandpapers can also be open coat, where the particles are separated from each other and the sandpaper is more flexible. This helps prevent clogging of the sandpaper.

Shapes

Sandpaper comes in a number of different shapes and sizes.
- sheet — usually 9 by 11 inches, but other sizes may be available
- belt — usually cloth backed, comes in different sizes to fit different belt sanders.
- disk — made to fit different models of disc and random orbit sanders. May be perforated for some models of sanders. Attachment includes Pressure sensitive adhesive (PSA) and "hook-and-loop" (similar to velcro).
- rolls

Grit sizes

Grit size refers to the size of the particles of abrading materials embedded in the sandpaper. A number of different standards have been established for grit size. These standards establish not only the average grit size, but also the allowable variation from the average. The two most common are the United States CAMI (Coated Abrasive Manufacturers Institute, now part of the Unified Abrasives Manufacturers' Association) and the European FEPA (Federation of European Producers of Abrasives) "P" grade. The FEPA system is the same as the ISO 6344 standard. Other systems used in sandpaper include the Japan Industrial Standards Committee (JIS), the micron grade (generally used for very fine grits). The "ought" system was used in the past in the United States. Also, cheaper sandpapers sometimes are sold with nomenclature such as "Coarse", "Medium" and "Fine", but it is not clear what standards these names refer to.

Grit size table

The following table, compiled from the references at the bottom, compares the CAMI and "P" designations with the average grit size in micrometres (µm).
 

ISO/FEPA Grit designation

CAMI Grit designation

Average particle diameter (µm)

MACROGRITS

Extra Coarse (Very fast removal of material)

P12

 

1815

P16

 

1324

P20

 

1000

P24

 

764

 

24

708

P30

 

642

 

30

632

 

36

530

P36

 

538

Coarse (Rapid removal of material)

P40

40

425

 

50

348

P50

 

336

Medium (sanding bare wood in preparation for finishing)

 

60

265

P60

 

269

P80

 

201

 

80

190

Fine (sanding bare wood in preparation for finishing)

P100

 

162

 

100

140

P120

 

125

 

120

115

Very Fine (final sanding of bare wood)

P150

 

100

 

150

92

P180

180

82

P220

220

68

MICROGRITS

Very Fine (sanding finishes between coats)

P240

 

58.5

 

240

53.0

P280

 

52.2

P320

 

46.2

P360

 

40.5

Extra fine

 

320

36.0

P400

 

35.0

P500

 

30.2

 

360

28.0

P600

 

25.8

Super fine (final sanding of finishes)

 

400

23.0

P800

 

21.8

 

500

20.0

P1000

 

18.3

 

600

16.0

P1200

 

15.3

Ultra fine (final sanding of finishes)

P1500

800

12.6

P2000

1000

10.3

P2500

 

8.4

History

The first recorded instance of sandpaper was in 13th century China when crushed shells, seeds, and sand were bonded to parchment using natural gum. Shark skin was used as a sandpaper. Sandpaper was originally known as glass paper, as it used particles of glass. Sandpaper has occasionally been used as a surface for painting, as by Joan Miro. Sandpaper was even used as a musical instrument, in Leroy Anderson's Sandpaper Ballet. Sandpaper was patented in the United States on June 14 1834 by Isaac Fischer, Jr., of Springfield, Vermont. In 1916 3M invented the waterproof sandpaper, know as Wetordry(TM), and its first application was for automotive paint refinishing.

References


- Michael Dresdner (1992). The Woodfinishing Book. Taunton Press. ISBN 1-56158-037-6
- [http://www.fepa-abrasives.org Federation of European Producers of Abrasives]
- [http://www.klingspor.com/referencedesk.htm Klingspor reference pages]
- [http://www.sizes.com/tools/sandpaper.htm sizes.com on sandpaper]
- [http://www.3M.com/abrasives.htm 3M Abrasives] Category:Grinding and lapping Category:Woodworking Category:Paper

Mohs hardness

Mohs' scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. It was created, in 1812, by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. Mohs based the scale on ten minerals that are all readily available except the last one, diamond. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, and/or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on Mohs scale is 4.5. The table below shows comparison with absolute hardness measures by a sclerometer. Mohs' is a purely ordinal scale with, for example, corundum being twice as hard as topaz, but diamond, almost four times as hard as corundum. On the Mohs scale, fingernail has hardness 2; copper penny, about 3; a knife blade, 5; window glass, 5.5; steel file, 6.5. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale. Some mnemonics traditionally taught to geology students to remember this table are "The Girls Can Flirt And Other Queer Things Can Do" or "To Get Candy From Aunt Fanny, Quit Teasing Cousin Danny". An alternative table is shown below which has been modified to incorporate additional substances that may fall in between two levels. Source: [http://www.amfed.org/t_mohs.htm American Federation of Mineralogical Societies: Mohs Scale of Mineral Hardness] Category:Materials science Category:Mineralogy Category:Scales ja:モース硬度

Category:Oxide minerals

Minerals with oxygen as the anion. Category:Minerals

Category:Superhard materials

This category is for materials with hardness approaching, equalling, or possibly surpassing the hardness of diamond. Fictional "hardest materials", such as adamantium and scrith, are to be categorized in :Category:Fictional materials, not here. Category:Materials

Euctemon

For the crater, see Euctemon (crater). Euctemon (unknown-fl. 432 BC), Athenian astronomer. He was a contemporary of Meton and worked closely with this astronomer. Nothing is known of his work apart from his partnership with Meton and what is mentioned by Ptolemy. With Meton, he made a series of observations of the solstices (the points at which the sun is at greatest distance from the equator) in order to determine the length of the tropical year. Geminus and Ptolemy quote him as a source on the rising and setting of the stars, so it is possible he left some work on this subject. The lunar crater Euctemon is named after him.

External links


- [http://www.cosmovisions.com/Euctemon.htm Imago Mundi: Euctemon]
- [http://www.ancientlibrary.com/smith-bio/2177.html The Ancient Library]
- [http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Greek_astronomy.html Greek Astronomy] Category:Greek and Roman astronomers

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