1.
Sulfite
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Sulfites or sulphites are compounds that contain the sulfite ion, SO2−3. The sulfite ion is the base of bisulfite. Although its acid is elusive, its salts are widely used, Sulfites are substances that naturally occur in some foods and the human body. They are also used as regulated food additives, the structure of the sulfite anion can be described with three equivalent resonance structures. There is also a lone pair on the sulfur, so the structure predicted by VSEPR theory is trigonal pyramidal. In the hybrid resonance structure, the S-O bonds are equivalently of bond order one, evidence from 17O NMR spectroscopic data suggests that protonation of the sulfite ion gives a mixture of isomers, Sulfites are used as a food preservative or enhancer. Sulfites are commonly introduced to arrest fermentation at a desired time, sulfur dioxide protects wine from not only oxidation, but also from bacteria. Without sulfites, grape juice would turn to vinegar. Organic wines are not necessarily sulfite-free, but generally have lower amounts, in general, white wines contain more sulfites than red wines and sweeter wines contain more sulfites than drier ones. In the United States, wines bottled after mid-1987 must have a label stating that they contain sulfites if they contain more than 10 parts per million, in the European Union an equivalent regulation came into force in November 2005. In 2012, a new regulation for organic wines came into force, Sulfites are often used as preservatives in dried fruits, preserved radish, and dried potato products. Most beers no longer contain sulfites, although some alcoholic ciders contain them, although shrimp are sometimes treated with sulfites on fishing vessels, the chemical may not appear on the label. In 1986, the Food and Drug Administration in the United States banned the addition of sulfites to all fresh fruit, E numbers for sulfites as food additives are, Sulfites are counted among the top nine food allergens, but a reaction to sulfite is not a true allergy. Some people have positive skin allergy tests to sulfites indicating true allergy and it may cause breathing difficulty within minutes after eating a food containing it. Asthmatics and possibly people with sensitivity are at an elevated risk for reaction to sulfites. Anaphylaxis and life-threatening reactions are rare, other potential symptoms include sneezing, swelling of the throat, hives, and migraine. In 1986, the U. S. Food and Drug Administration banned the use of sulfites as preservatives on foods intended to be eaten fresh and this has contributed to the increased use of erythorbic acid and its salts as preservatives. In the U. S. Sulfites used in food processing, the products most likely to contain sulfites do not require ingredients labels, so the presence of sulfites usually is undisclosed
2.
Jmol
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Jmol is computer software for molecular modelling chemical structures in 3-dimensions. Jmol returns a 3D representation of a molecule that may be used as a teaching tool and it is written in the programming language Java, so it can run on the operating systems Windows, macOS, Linux, and Unix, if Java is installed. It is free and open-source software released under a GNU Lesser General Public License version 2.0, a standalone application and a software development kit exist that can be integrated into other Java applications, such as Bioclipse and Taverna. A popular feature is an applet that can be integrated into web pages to display molecules in a variety of ways, for example, molecules can be displayed as ball-and-stick models, space-filling models, ribbon diagrams, etc. Jmol supports a range of chemical file formats, including Protein Data Bank, Crystallographic Information File, MDL Molfile. There is also a JavaScript-only version, JSmol, that can be used on computers with no Java, the Jmol applet, among other abilities, offers an alternative to the Chime plug-in, which is no longer under active development. While Jmol has many features that Chime lacks, it does not claim to reproduce all Chime functions, most notably, Chime requires plug-in installation and Internet Explorer 6.0 or Firefox 2.0 on Microsoft Windows, or Netscape Communicator 4.8 on Mac OS9. Jmol requires Java installation and operates on a variety of platforms. For example, Jmol is fully functional in Mozilla Firefox, Internet Explorer, Opera, Google Chrome, fast and Scriptable Molecular Graphics in Web Browsers without Java3D
3.
ChemSpider
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ChemSpider is a database of chemicals. ChemSpider is owned by the Royal Society of Chemistry, the database contains information on more than 50 million molecules from over 500 data sources including, Each chemical is given a unique identifier, which forms part of a corresponding URL. This is an approach to develop an online chemistry database. The search can be used to widen or restrict already found results, structure searching on mobile devices can be done using free apps for iOS and for the Android. The ChemSpider database has been used in combination with text mining as the basis of document markup. The result is a system between chemistry documents and information look-up via ChemSpider into over 150 data sources. ChemSpider was acquired by the Royal Society of Chemistry in May,2009, prior to the acquisition by RSC, ChemSpider was controlled by a private corporation, ChemZoo Inc. The system was first launched in March 2007 in a release form. ChemSpider has expanded the generic support of a database to include support of the Wikipedia chemical structure collection via their WiChempedia implementation. A number of services are available online. SyntheticPages is an interactive database of synthetic chemistry procedures operated by the Royal Society of Chemistry. Users submit synthetic procedures which they have conducted themselves for publication on the site and these procedures may be original works, but they are more often based on literature reactions. Citations to the published procedure are made where appropriate. They are checked by an editor before posting. The pages do not undergo formal peer-review like a journal article. The comments are moderated by scientific editors. The intention is to collect practical experience of how to conduct useful chemical synthesis in the lab, while experimental methods published in an ordinary academic journal are listed formally and concisely, the procedures in ChemSpider SyntheticPages are given with more practical detail. Comments by submitters are included as well, other publications with comparable amounts of detail include Organic Syntheses and Inorganic Syntheses
4.
European Chemicals Agency
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ECHA is the driving force among regulatory authorities in implementing the EUs chemicals legislation. ECHA helps companies to comply with the legislation, advances the safe use of chemicals, provides information on chemicals and it is located in Helsinki, Finland. The Agency, headed by Executive Director Geert Dancet, started working on 1 June 2007, the REACH Regulation requires companies to provide information on the hazards, risks and safe use of chemical substances that they manufacture or import. Companies register this information with ECHA and it is freely available on their website. So far, thousands of the most hazardous and the most commonly used substances have been registered, the information is technical but gives detail on the impact of each chemical on people and the environment. This also gives European consumers the right to ask whether the goods they buy contain dangerous substances. The Classification, Labelling and Packaging Regulation introduces a globally harmonised system for classifying and labelling chemicals into the EU. This worldwide system makes it easier for workers and consumers to know the effects of chemicals, companies need to notify ECHA of the classification and labelling of their chemicals. So far, ECHA has received over 5 million notifications for more than 100000 substances, the information is freely available on their website. Consumers can check chemicals in the products they use, Biocidal products include, for example, insect repellents and disinfectants used in hospitals. The Biocidal Products Regulation ensures that there is information about these products so that consumers can use them safely. ECHA is responsible for implementing the regulation, the law on Prior Informed Consent sets guidelines for the export and import of hazardous chemicals. Through this mechanism, countries due to hazardous chemicals are informed in advance and have the possibility of rejecting their import. Substances that may have effects on human health and the environment are identified as Substances of Very High Concern 1. These are mainly substances which cause cancer, mutation or are toxic to reproduction as well as substances which persist in the body or the environment, other substances considered as SVHCs include, for example, endocrine disrupting chemicals. Companies manufacturing or importing articles containing these substances in a concentration above 0 and they are required to inform users about the presence of the substance and therefore how to use it safely. Consumers have the right to ask the retailer whether these substances are present in the products they buy, once a substance has been officially identified in the EU as being of very high concern, it will be added to a list. This list is available on ECHA’s website and shows consumers and industry which chemicals are identified as SVHCs, Substances placed on the Candidate List can then move to another list
5.
PubChem
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PubChem is a database of chemical molecules and their activities against biological assays. The system is maintained by the National Center for Biotechnology Information, a component of the National Library of Medicine, PubChem can be accessed for free through a web user interface. Millions of compound structures and descriptive datasets can be downloaded via FTP. PubChem contains substance descriptions and small molecules with fewer than 1000 atoms and 1000 bonds, more than 80 database vendors contribute to the growing PubChem database. PubChem consists of three dynamically growing primary databases, as of 28 January 2016, Compounds,82.6 million entries, contains pure and characterized chemical compounds. Substances,198 million entries, contains also mixtures, extracts, complexes, bioAssay, bioactivity results from 1.1 million high-throughput screening programs with several million values. PubChem contains its own online molecule editor with SMILES/SMARTS and InChI support that allows the import and export of all common chemical file formats to search for structures and fragments. In the text search form the database fields can be searched by adding the name in square brackets to the search term. A numeric range is represented by two separated by a colon. The search terms and field names are case-insensitive, parentheses and the logical operators AND, OR, and NOT can be used. AND is assumed if no operator is used, example,0,5000,50,10 -5,5 PubChem was released in 2004. The American Chemical Society has raised concerns about the publicly supported PubChem database and they have a strong interest in the issue since the Chemical Abstracts Service generates a large percentage of the societys revenue. To advocate their position against the PubChem database, ACS has actively lobbied the US Congress, soon after PubChems creation, the American Chemical Society lobbied U. S. Congress to restrict the operation of PubChem, which they asserted competes with their Chemical Abstracts Service
6.
International Chemical Identifier
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Initially developed by IUPAC and NIST from 2000 to 2005, the format and algorithms are non-proprietary. The continuing development of the standard has supported since 2010 by the not-for-profit InChI Trust. The current version is 1.04 and was released in September 2011, prior to 1.04, the software was freely available under the open source LGPL license, but it now uses a custom license called IUPAC-InChI Trust License. Not all layers have to be provided, for instance, the layer can be omitted if that type of information is not relevant to the particular application. InChIs can thus be seen as akin to a general and extremely formalized version of IUPAC names and they can express more information than the simpler SMILES notation and differ in that every structure has a unique InChI string, which is important in database applications. Information about the 3-dimensional coordinates of atoms is not represented in InChI, the InChI algorithm converts input structural information into a unique InChI identifier in a three-step process, normalization, canonicalization, and serialization. The InChIKey, sometimes referred to as a hashed InChI, is a fixed length condensed digital representation of the InChI that is not human-understandable. The InChIKey specification was released in September 2007 in order to facilitate web searches for chemical compounds and it should be noted that, unlike the InChI, the InChIKey is not unique, though collisions can be calculated to be very rare, they happen. In January 2009 the final 1.02 version of the InChI software was released and this provided a means to generate so called standard InChI, which does not allow for user selectable options in dealing with the stereochemistry and tautomeric layers of the InChI string. The standard InChIKey is then the hashed version of the standard InChI string, the standard InChI will simplify comparison of InChI strings and keys generated by different groups, and subsequently accessed via diverse sources such as databases and web resources. Every InChI starts with the string InChI= followed by the version number and this is followed by the letter S for standard InChIs. The remaining information is structured as a sequence of layers and sub-layers, the layers and sub-layers are separated by the delimiter / and start with a characteristic prefix letter. The six layers with important sublayers are, Main layer Chemical formula and this is the only sublayer that must occur in every InChI. The atoms in the formula are numbered in sequence, this sublayer describes which atoms are connected by bonds to which other ones. Describes how many hydrogen atoms are connected to each of the other atoms, the condensed,27 character standard InChIKey is a hashed version of the full standard InChI, designed to allow for easy web searches of chemical compounds. Most chemical structures on the Web up to 2007 have been represented as GIF files, the full InChI turned out to be too lengthy for easy searching, and therefore the InChIKey was developed. With all databases currently having below 50 million structures, such duplication appears unlikely at present, a recent study more extensively studies the collision rate finding that the experimental collision rate is in agreement with the theoretical expectations. Example, Morphine has the structure shown on the right, as the InChI cannot be reconstructed from the InChIKey, an InChIKey always needs to be linked to the original InChI to get back to the original structure
7.
Simplified molecular-input line-entry system
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The simplified molecular-input line-entry system is a specification in form of a line notation for describing the structure of chemical species using short ASCII strings. SMILES strings can be imported by most molecule editors for conversion back into two-dimensional drawings or three-dimensional models of the molecules, the original SMILES specification was initiated in the 1980s. It has since modified and extended. In 2007, a standard called OpenSMILES was developed in the open-source chemistry community. Other linear notations include the Wiswesser Line Notation, ROSDAL and SLN, the original SMILES specification was initiated by David Weininger at the USEPA Mid-Continent Ecology Division Laboratory in Duluth in the 1980s. The Environmental Protection Agency funded the project to develop SMILES. It has since modified and extended by others, most notably by Daylight Chemical Information Systems. In 2007, a standard called OpenSMILES was developed by the Blue Obelisk open-source chemistry community. Other linear notations include the Wiswesser Line Notation, ROSDAL and SLN, in July 2006, the IUPAC introduced the InChI as a standard for formula representation. SMILES is generally considered to have the advantage of being slightly more human-readable than InChI, the term SMILES refers to a line notation for encoding molecular structures and specific instances should strictly be called SMILES strings. However, the term SMILES is also used to refer to both a single SMILES string and a number of SMILES strings, the exact meaning is usually apparent from the context. The terms canonical and isomeric can lead to confusion when applied to SMILES. The terms describe different attributes of SMILES strings and are not mutually exclusive, typically, a number of equally valid SMILES strings can be written for a molecule. For example, CCO, OCC and CC all specify the structure of ethanol, algorithms have been developed to generate the same SMILES string for a given molecule, of the many possible strings, these algorithms choose only one of them. This SMILES is unique for each structure, although dependent on the algorithm used to generate it. These algorithms first convert the SMILES to a representation of the molecular structure. A common application of canonical SMILES is indexing and ensuring uniqueness of molecules in a database, there is currently no systematic comparison across commercial software to test if such flaws exist in those packages. SMILES notation allows the specification of configuration at tetrahedral centers, and these are structural features that cannot be specified by connectivity alone and SMILES which encode this information are termed isomeric SMILES
8.
Chemical formula
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These are limited to a single typographic line of symbols, which may include subscripts and superscripts. A chemical formula is not a name, and it contains no words. Although a chemical formula may imply certain simple chemical structures, it is not the same as a full chemical structural formula. Chemical formulas can fully specify the structure of only the simplest of molecules and chemical substances, the simplest types of chemical formulas are called empirical formulas, which use letters and numbers indicating the numerical proportions of atoms of each type. Molecular formulas indicate the numbers of each type of atom in a molecule. For example, the formula for glucose is CH2O, while its molecular formula is C6H12O6. This is possible if the relevant bonding is easy to show in one dimension, an example is the condensed molecular/chemical formula for ethanol, which is CH3-CH2-OH or CH3CH2OH. For reasons of structural complexity, there is no condensed chemical formula that specifies glucose, chemical formulas may be used in chemical equations to describe chemical reactions and other chemical transformations, such as the dissolving of ionic compounds into solution. A chemical formula identifies each constituent element by its chemical symbol, in empirical formulas, these proportions begin with a key element and then assign numbers of atoms of the other elements in the compound, as ratios to the key element. For molecular compounds, these numbers can all be expressed as whole numbers. For example, the formula of ethanol may be written C2H6O because the molecules of ethanol all contain two carbon atoms, six hydrogen atoms, and one oxygen atom. Some types of compounds, however, cannot be written with entirely whole-number empirical formulas. An example is boron carbide, whose formula of CBn is a variable non-whole number ratio with n ranging from over 4 to more than 6.5. When the chemical compound of the consists of simple molecules. These types of formulas are known as molecular formulas and condensed formulas. A molecular formula enumerates the number of atoms to reflect those in the molecule, so that the formula for glucose is C6H12O6 rather than the glucose empirical formula. However, except for very simple substances, molecular chemical formulas lack needed structural information, for simple molecules, a condensed formula is a type of chemical formula that may fully imply a correct structural formula. For example, ethanol may be represented by the chemical formula CH3CH2OH
9.
Odor
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An odor or odour or fragrance is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also commonly called scents, which can refer to both pleasant and unpleasant odors, the terms fragrance and aroma are used primarily by the food and cosmetic industry to describe a pleasant odor, and are sometimes used to refer to perfumes, and to describe floral scent. In contrast, malodor, stench, reek, and stink are used specifically to describe unpleasant odor, the term smell is used for both pleasant and unpleasant odors. In the United Kingdom, odour refers to scents in general, the sense of smell gives rise to the perception of odors, mediated by the olfactory nerve. The olfactory receptor cells are present in the olfactory epithelium. There are millions of olfactory receptor neurons that act as sensory signaling cells, each neuron has cilia in direct contact with air. The olfactory nerve is considered the smell mediator, the axon connects the brain to the external air, odorous molecules act as a chemical stimulus. Molecules bind to receptor proteins extended from cilia, initiating an electric signal, thus, by using a chemical that binds to copper in the mouse nose, so that copper wasn’t available to the receptors, the authors showed that the mice couldnt detect the thiols. However, these also found that MOR244-3 lacks the specific metal ion binding site suggested by Suslick. When the signal reaches a threshold, the fires, sending a signal traveling along the axon to the olfactory bulb. Interpretation of the begins, relating the smell to past experiences. The olfactory bulb acts as a station connecting the nose to the olfactory cortex in the brain. Olfactory information is processed and projected through a pathway to the central nervous system. Odor sensation usually depends on the concentration available to the olfactory receptors, the olfactory system does not interpret a single compound, but instead the whole odorous mix, not necessarily corresponding to concentration or intensity of any single constituent. The widest range of odors consists of compounds, although some simple compounds not containing carbon, such as hydrogen sulfide. The perception of an effect is a two-step process. First, there is the part, the detection of stimuli by receptors in the nose. The stimuli are processed by the region of the brain which is responsible for olfaction
10.
Density
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The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is ρ, although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume, ρ = m V, where ρ is the density, m is the mass, and V is the volume. In some cases, density is defined as its weight per unit volume. For a pure substance the density has the numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity, osmium and iridium are the densest known elements at standard conditions for temperature and pressure but certain chemical compounds may be denser. Thus a relative density less than one means that the floats in water. The density of a material varies with temperature and pressure and this variation is typically small for solids and liquids but much greater for gases. Increasing the pressure on an object decreases the volume of the object, increasing the temperature of a substance decreases its density by increasing its volume. In most materials, heating the bottom of a results in convection of the heat from the bottom to the top. This causes it to rise relative to more dense unheated material, the reciprocal of the density of a substance is occasionally called its specific volume, a term sometimes used in thermodynamics. Density is a property in that increasing the amount of a substance does not increase its density. Archimedes knew that the irregularly shaped wreath could be crushed into a cube whose volume could be calculated easily and compared with the mass, upon this discovery, he leapt from his bath and ran naked through the streets shouting, Eureka. As a result, the term eureka entered common parlance and is used today to indicate a moment of enlightenment, the story first appeared in written form in Vitruvius books of architecture, two centuries after it supposedly took place. Some scholars have doubted the accuracy of this tale, saying among other things that the method would have required precise measurements that would have been difficult to make at the time, from the equation for density, mass density has units of mass divided by volume. As there are units of mass and volume covering many different magnitudes there are a large number of units for mass density in use. The SI unit of kilogram per metre and the cgs unit of gram per cubic centimetre are probably the most commonly used units for density.1,000 kg/m3 equals 1 g/cm3. In industry, other larger or smaller units of mass and or volume are often more practical, see below for a list of some of the most common units of density
11.
Melting point
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The melting point of a solid is the temperature at which it changes state from solid to liquid at atmospheric pressure. At the melting point the solid and liquid phase exist in equilibrium, the melting point of a substance depends on pressure and is usually specified at standard pressure. When considered as the temperature of the change from liquid to solid. Because of the ability of some substances to supercool, the point is not considered as a characteristic property of a substance. For most substances, melting and freezing points are approximately equal, for example, the melting point and freezing point of mercury is 234.32 kelvins. However, certain substances possess differing solid-liquid transition temperatures, for example, agar melts at 85 °C and solidifies from 31 °C to 40 °C, such direction dependence is known as hysteresis. The melting point of ice at 1 atmosphere of pressure is close to 0 °C. In the presence of nucleating substances the freezing point of water is the same as the melting point, the chemical element with the highest melting point is tungsten, at 3687 K, this property makes tungsten excellent for use as filaments in light bulbs. Many laboratory techniques exist for the determination of melting points, a Kofler bench is a metal strip with a temperature gradient. Any substance can be placed on a section of the strip revealing its thermal behaviour at the temperature at that point, differential scanning calorimetry gives information on melting point together with its enthalpy of fusion. A basic melting point apparatus for the analysis of crystalline solids consists of an oil bath with a transparent window, the several grains of a solid are placed in a thin glass tube and partially immersed in the oil bath. The oil bath is heated and with the aid of the melting of the individual crystals at a certain temperature can be observed. In large/small devices, the sample is placed in a heating block, the measurement can also be made continuously with an operating process. For instance, oil refineries measure the point of diesel fuel online, meaning that the sample is taken from the process. This allows for more frequent measurements as the sample does not have to be manually collected, for refractory materials the extremely high melting point may be determined by heating the material in a black body furnace and measuring the black-body temperature with an optical pyrometer. For the highest melting materials, this may require extrapolation by several hundred degrees, the spectral radiance from an incandescent body is known to be a function of its temperature. An optical pyrometer matches the radiance of a body under study to the radiance of a source that has been previously calibrated as a function of temperature, in this way, the measurement of the absolute magnitude of the intensity of radiation is unnecessary. However, known temperatures must be used to determine the calibration of the pyrometer, for temperatures above the calibration range of the source, an extrapolation technique must be employed
12.
Boiling point
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The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the environmental pressure. A liquid in a vacuum has a lower boiling point than when that liquid is at atmospheric pressure. A liquid at high pressure has a boiling point than when that liquid is at atmospheric pressure. For a given pressure, different liquids boil at different temperatures, for example, water boils at 100 °C at sea level, but at 93.4 °C at 2,000 metres altitude. The normal boiling point of a liquid is the case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level,1 atmosphere. At that temperature, the pressure of the liquid becomes sufficient to overcome atmospheric pressure. The standard boiling point has been defined by IUPAC since 1982 as the temperature at which boiling occurs under a pressure of 1 bar, the heat of vaporization is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure. Liquids may change to a vapor at temperatures below their boiling points through the process of evaporation, evaporation is a surface phenomenon in which molecules located near the liquids edge, not contained by enough liquid pressure on that side, escape into the surroundings as vapor. On the other hand, boiling is a process in which molecules anywhere in the liquid escape, a saturated liquid contains as much thermal energy as it can without boiling. The saturation temperature is the temperature for a corresponding saturation pressure at which a liquid boils into its vapor phase, the liquid can be said to be saturated with thermal energy. Any addition of energy results in a phase transition. If the pressure in a system remains constant, a vapor at saturation temperature will begin to condense into its liquid phase as thermal energy is removed, similarly, a liquid at saturation temperature and pressure will boil into its vapor phase as additional thermal energy is applied. The boiling point corresponds to the temperature at which the pressure of the liquid equals the surrounding environmental pressure. Thus, the point is dependent on the pressure. Boiling points may be published with respect to the NIST, USA standard pressure of 101.325 kPa, at higher elevations, where the atmospheric pressure is much lower, the boiling point is also lower. The boiling point increases with increased pressure up to the critical point, the boiling point cannot be increased beyond the critical point. Likewise, the point decreases with decreasing pressure until the triple point is reached
13.
Aqueous solution
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An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending to the relevant chemical formula, for example, a solution of table salt, or sodium chloride, in water would be represented as Na+ + Cl−. The word aqueous means pertaining to, related to, similar to, as water is an excellent solvent and is also naturally abundant, it is a ubiquitous solvent in chemistry. Substances that are hydrophobic often do not dissolve well in water, an example of a hydrophilic substance is sodium chloride. Acids and bases are aqueous solutions, as part of their Arrhenius definitions, the ability of a substance to dissolve in water is determined by whether the substance can match or exceed the strong attractive forces that water molecules generate between themselves. If the substance lacks the ability to dissolve in water the molecules form a precipitate, reactions in aqueous solutions are usually metathesis reactions. Metathesis reactions are another term for double-displacement, that is, when a cation displaces to form a bond with the other anion. The cation bonded with the latter anion will dissociate and bond with the other anion, aqueous solutions that conduct electric current efficiently contain strong electrolytes, while ones that conduct poorly are considered to have weak electrolytes. Those strong electrolytes are substances that are ionized in water. Nonelectrolytes are substances that dissolve in water yet maintain their molecular integrity, examples include sugar, urea, glycerol, and methylsulfonylmethane. When writing the equations of reactions, it is essential to determine the precipitate. To determine the precipitate, one must consult a chart of solubility, soluble compounds are aqueous, while insoluble compounds are the precipitate. Remember that there may not always be a precipitate, when performing calculations regarding the reacting of one or more aqueous solutions, in general one must know the concentration, or molarity, of the aqueous solutions. Solution concentration is given in terms of the form of the prior to it dissolving. Metal ions in aqueous solution Solubility Dissociation Acid-base reaction theories Properties of water Zumdahl S.1997, 4th ed. Boston, Houghton Mifflin Company
14.
Sulfuric acid
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Sulfuric acid is a highly corrosive strong mineral acid with the molecular formula H2SO4 and molecular weight 98.079 g/mol. It is a pungent-ethereal, colorless to slightly yellow liquid that is soluble in water at all concentrations. Sometimes, it is dyed dark brown during production to alert people to its hazards, the historical name of this acid is oil of vitriol. Sulfuric acid is an acid and shows different properties depending upon its concentration. Its corrosiveness on other materials, like metals, living tissues or even stones, can be ascribed to its strong acidic nature and, if concentrated. It is also hygroscopic, readily absorbing water vapour from the air, Sulfuric acid at a high concentration can cause very serious damage upon contact, since not only does it cause chemical burns via hydrolysis, but also secondary thermal burns through dehydration. It can lead to permanent blindness if splashed onto eyes and irreversible damage if swallowed, Sulfuric acid has a wide range of applications including in domestic acidic drain cleaners, as an electrolyte in lead-acid batteries and in various cleaning agents. It is also a central substance in the chemical industry, principal uses include mineral processing, fertilizer manufacturing, oil refining, wastewater processing, and chemical synthesis. It is widely produced with different methods, such as process, wet sulfuric acid process, lead chamber process. The study of vitriol, a category of glassy minerals from which the acid can be derived, sumerians had a list of types of vitriol that they classified according to the substances color. Some of the earliest discussions on the origin and properties of vitriol is in the works of the Greek physician Dioscorides, galen also discussed its medical use. Ibn Sina focused on its uses and different varieties of vitriol. Sulfuric acid was called oil of vitriol by medieval European alchemists because it was prepared by roasting green vitriol in an iron retort, there are references to it in the works of Vincent of Beauvais and in the Compositum de Compositis ascribed to Saint Albertus Magnus. A passage from Pseudo-Geber´s Summa Perfectionis was long considered to be the first recipe for sulfuric acid, in the seventeenth century, the German-Dutch chemist Johann Glauber prepared sulfuric acid by burning sulfur together with saltpeter, in the presence of steam. As saltpeter decomposes, it oxidizes the sulfur to SO3, in 1736, Joshua Ward, a London pharmacist, used this method to begin the first large-scale production of sulfuric acid. This process allowed the effective industrialization of sulfuric acid production, after several refinements, this method, called the lead chamber process or chamber process, remained the standard for sulfuric acid production for almost two centuries. Sulfuric acid created by John Roebucks process approached a 65% concentration, later refinements to the lead chamber process by French chemist Joseph Louis Gay-Lussac and British chemist John Glover improved concentration to 78%. However, the manufacture of dyes and other chemical processes require a more concentrated product
15.
Standard enthalpy of formation
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Its symbol is ΔHo f or ΔfHo. The superscript Plimsoll on this symbol indicates that the process has occurred under standard conditions at the specified temperature. One exception is phosphorus, for which the most stable form at 1 atm is black phosphorus and this is true for all enthalpies of formation. In physics the energy per particle is expressed in electronvolts. All elements in their states have a standard enthalpy of formation of zero. The formation reaction is a constant pressure and constant temperature process, since the pressure of the standard formation reaction is fixed at 1 atm, the standard formation enthalpy or reaction heat is a function of temperature. For tabulation purposes, standard formation enthalpies are all given at a temperature,298 K. The standard enthalpy of formation is equivalent to the sum of separate processes included in the Born–Haber cycle of synthesis reactions. This is because enthalpy is a state function, in the example above the standard enthalpy change of formation for sodium chloride is equal to the sum of the standard enthalpy change of formation for each of the steps involved in the process. This is especially useful for very long reactions with many intermediate steps, chemists may use standard enthalpies of formation for a reaction that is hypothetical. That it is shows that the reaction, if it were to proceed, would be exothermic. It is possible to heat of formations for simple unstrained organic compounds with the Heat of formation group additivity method. Standard enthalpies of formation are used in thermochemistry to find the enthalpy change of any reaction. This implies that the reaction is exothermic, the converse is also true, the standard enthalpy of reaction will be positive for an endothermic reaction. When a reaction is reversed, the magnitude of ΔH stays the same, when the balanced equation for a reaction is multiplied by an integer, the corresponding value of ΔH must be multiplied by that integer as well. Allotropes of an element other than the state generally have non-zero standard enthalpies of formation. Standard enthalpy of sublimation, or heat of sublimation, is defined as the required to sublime one mole of the substance under standard conditions. Standard enthalpy of solution is the change associated with the dissolution of a substance in a solvent at constant pressure under standard conditions
16.
Occupational safety and health
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These terms of course also refer to the goals of this field, so their use in the sense of this article was originally an abbreviation of occupational safety and health program/department etc. The goals of occupational safety and health programs include to foster a safe, OSH may also protect co-workers, family members, employers, customers, and many others who might be affected by the workplace environment. In the United States, the occupational health and safety is referred to as occupational health and occupational and non-occupational safety. In common-law jurisdictions, employers have a common law duty to take care of the safety of their employees. As defined by the World Health Organization occupational health deals with all aspects of health, Health has been defined as a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity. Occupational health is a field of healthcare concerned with enabling an individual to undertake their occupation. Health has been defined as It contrasts, for example, with the promotion of health and safety at work, since 1950, the International Labour Organization and the World Health Organization have shared a common definition of occupational health. It was adopted by the Joint ILO/WHO Committee on Occupational Health at its first session in 1950, the concept of working culture is intended in this context to mean a reflection of the essential value systems adopted by the undertaking concerned. Such a culture is reflected in practice in the systems, personnel policy, principles for participation, training policies. Professionals advise on a range of occupational health matters. The research and regulation of safety and health are a relatively recent phenomenon. As labor movements arose in response to concerns in the wake of the industrial revolution. The initial remit of the Inspectorate was to police restrictions on the hours in the textile industry of children. The commission sparked public outrage resulted in the Mines Act of 1842. Otto von Bismarck inaugurated the first social insurance legislation in 1883, similar acts followed in other countries, partly in response to labor unrest. Although work provides many economic and other benefits, an array of workplace hazards also present risks to the health. Personal protective equipment can protect against many of these hazards. Physical hazards affect many people in the workplace, Falls are also a common cause of occupational injuries and fatalities, especially in construction, extraction, transportation, healthcare, and building cleaning and maintenance
17.
Safety data sheet
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A safety data sheet, material safety data sheet, or product safety data sheet is an important component of product stewardship, occupational safety and health, and spill-handling procedures. SDS formats can vary from source to source within a country depending on national requirements, SDSs are a widely used system for cataloging information on chemicals, chemical compounds, and chemical mixtures. SDS information may include instructions for the use and potential hazards associated with a particular material or product. The SDS should be available for reference in the area where the chemicals are being stored or in use, there is also a duty to properly label substances on the basis of physico-chemical, health and/or environmental risk. Labels can include hazard symbols such as the European Union standard symbols, a SDS for a substance is not primarily intended for use by the general consumer, focusing instead on the hazards of working with the material in an occupational setting. It is important to use an SDS specific to country and supplier, as the same product can have different formulations in different countries. The formulation and hazard of a product using a name may vary between manufacturers in the same country. Safety data sheets have made an integral part of the system of Regulation No 1907/2006. The SDS must be supplied in a language of the Member State where the substance or mixture is placed on the market. The 16 sections are, SECTION1, Identification of the substance/mixture, relevant identified uses of the substance or mixture and uses advised against 1.3. Details of the supplier of the safety data sheet 1.4, Emergency telephone number SECTION2, Hazards identification 2.1. Classification of the substance or mixture 2.2, Other hazards SECTION3, Composition/information on ingredients 3.1. Mixtures SECTION4, First aid measures 4.1, Description of first aid measures 4.2. Most important symptoms and effects, both acute and delayed 4.3, indication of any immediate medical attention and special treatment needed SECTION5, Firefighting measures 5.1. Special hazards arising from the substance or mixture 5.3, advice for firefighters SECTION6, Accidental release measure 6.1. Personal precautions, protective equipment and emergency procedures 6.2, methods and material for containment and cleaning up 6.4. Reference to other sections SECTION7, Handling and storage 7.1, conditions for safe storage, including any incompatibilities 7.3. Specific end use SECTION8, Exposure controls/personal protection 8.1, Exposure controls SECTION9, Physical and chemical properties 9.1
18.
GHS hazard pictograms
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Hazard pictograms form part of the international Globally Harmonized System of Classification and Labelling of Chemicals. Two sets of pictograms are included within the GHS, one for the labelling of containers and for workplace hazard warnings, either one or the other is chosen, depending on the target audience, but the two are not used together. The two sets of use the same symbols for the same hazards, although certain symbols are not required for transport pictograms. Transport pictograms come in variety of colors and may contain additional information such as a subcategory number. It has still to be implemented by the European Union in 2009, the following pictograms are included in the Worldwide Model Using but have not been incorporated into the GHS, ICZ and Catwallsh Hazcom Labelling because of the nature of the hazards. Globally Harmonized System of Classification and Labelling of Chemicals, New York and Geneva, United Nations,2007, ISBN 978-92-1-116957-7, ST/SG/AC. 10/30/Rev. Model Regulations, New York and Geneva, United Nations,2007, ISBN 978-92-1-139120-6, manual of Tests and Criteria, New York and Geneva, United Nations,2002, ISBN 92-1-139087-7, ST/SG/AC. 10/11/Rev.4 GHS pictogram gallery from the United Nations Economic Commission for Europe
19.
Flash point
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The flash point is the lowest temperature at which vapours of a volatile material will ignite, when given an ignition source. The flash point may sometimes be confused with the autoignition temperature, the fire point is the lowest temperature at which the vapor will keep burning after being ignited and the ignition source removed. The fire point is higher than the point, because at the flash point the vapor may be reliably expected to cease burning when the ignition source is removed. The flash point is a characteristic that is used to distinguish between flammable liquids, such as petrol, and combustible liquids, such as diesel. It is also used to characterize the fire hazards of liquids, all liquids have a specific vapor pressure, which is a function of that liquids temperature and is subject to Boyles Law. As temperature increases, vapor pressure increases, as vapor pressure increases, the concentration of vapor of a flammable or combustible liquid in the air increases. Hence, temperature determines the concentration of vapor of the liquid in the air. The flash point is the lowest temperature at which there will be enough flammable vapor to induce ignition when a source is applied. There are two types of flash point measurement, open cup and closed cup. In open cup devices, the sample is contained in a cup which is heated and, at intervals. The measured flash point will vary with the height of the flame above the liquid surface and, at sufficient height. The best-known example is the Cleveland open cup, in both these types, the cups are sealed with a lid through which the ignition source can be introduced. Closed cup testers normally give lower values for the point than open cup and are a better approximation to the temperature at which the vapour pressure reaches the lower flammable limit. The flash point is an empirical measurement rather than a physical parameter. The measured value will vary with equipment and test protocol variations, including temperature ramp rate, time allowed for the sample to equilibrate, sample volume, methods for determining the flash point of a liquid are specified in many standards. For example, testing by the Pensky-Martens closed cup method is detailed in ASTM D93, IP34, ISO2719, DIN51758, JIS K2265 and AFNOR M07-019. Determination of flash point by the Small Scale closed cup method is detailed in ASTM D3828 and D3278, EN ISO3679 and 3680, cEN/TR15138 Guide to Flash Point Testing and ISO TR29662 Guidance for Flash Point Testing cover the key aspects of flash point testing. Gasoline is a used in a spark-ignition engine
20.
Ion
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An ion is an atom or a molecule in which the total number of electrons is not equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge. Ions can be created, by chemical or physical means. In chemical terms, if an atom loses one or more electrons. If an atom gains electrons, it has a net charge and is known as an anion. Ions consisting of only a single atom are atomic or monatomic ions, because of their electric charges, cations and anions attract each other and readily form ionic compounds, such as salts. In the case of ionization of a medium, such as a gas, which are known as ion pairs are created by ion impact, and each pair consists of a free electron. The word ion comes from the Greek word ἰόν, ion, going and this term was introduced by English physicist and chemist Michael Faraday in 1834 for the then-unknown species that goes from one electrode to the other through an aqueous medium. Faraday also introduced the words anion for a charged ion. In Faradays nomenclature, cations were named because they were attracted to the cathode in a galvanic device, arrhenius explanation was that in forming a solution, the salt dissociates into Faradays ions. Arrhenius proposed that ions formed even in the absence of an electric current, ions in their gas-like state are highly reactive, and do not occur in large amounts on Earth, except in flames, lightning, electrical sparks, and other plasmas. These gas-like ions rapidly interact with ions of charge to give neutral molecules or ionic salts. These stabilized species are commonly found in the environment at low temperatures. A common example is the present in seawater, which are derived from the dissolved salts. Electrons, due to their mass and thus larger space-filling properties as matter waves, determine the size of atoms. Thus, anions are larger than the parent molecule or atom, as the excess electron repel each other, as such, in general, cations are smaller than the corresponding parent atom or molecule due to the smaller size of its electron cloud. One particular cation contains no electrons, and thus consists of a single proton - very much smaller than the parent hydrogen atom. Since the electric charge on a proton is equal in magnitude to the charge on an electron, an anion, from the Greek word ἄνω, meaning up, is an ion with more electrons than protons, giving it a net negative charge. A cation, from the Greek word κατά, meaning down, is an ion with fewer electrons than protons, there are additional names used for ions with multiple charges
21.
Selenium trioxide
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Selenium trioxide is the inorganic compound with the formula SeO3. It is also an agent and a Lewis acid. It is of academic interest as a precursor to Se compounds, one method entails dehydration of anhydrous selenic acid with phosphorus pentoxide at 150-160 °C. The reaction of sulfur trioxide with potassium selenate SO3 + K2SeO4 → K2SO4 + SeO3 In its chemistry SeO3 generally resembles sulfur trioxide, SO3, rather than tellurium trioxide. With Na2O it gives Na4SeO5, containing the square pyramidal SeVIO54−, with Se-O bond lengths ranging from range 1.729 →1.815 pm, SeVIO66− is the conjugate base of the unknown orthoselenic acid. In the solid phase SeO3 consists of cyclic tetramers, with an 8 membered 4 ring, selenium atoms are 4-coordinate, bond lengths being Se-O bridging are 175 pm and 181pm, non-bridging 156 and 154 pm. SeO3 in the gas phase consists of tetramers and monomeric SeO3 which is planar with an Se-O bond length of 168.78 pm. Schmidt, Prof. Dr. Max, Dr. P. Bornmann, angewandte Chemie International Edition in English
22.
Sulfur
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Sulfur or sulphur is a chemical element with symbol S and atomic number 16. It is abundant, multivalent, and nonmetallic, under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a yellow crystalline solid at room temperature. Chemically, sulfur reacts with all elements except for gold, platinum, iridium, tellurium, though sometimes found in pure, native form, sulfur usually occurs as sulfide and sulfate minerals. Being abundant in native form, sulfur was known in ancient times, being mentioned for its uses in ancient India, ancient Greece, China, in the Bible, sulfur is called brimstone. Today, almost all elemental sulfur is produced as a byproduct of removing sulfur-containing contaminants from natural gas, the greatest commercial use of the element is the production of sulfuric acid for sulfate and phosphate fertilizers, and other chemical processes. The element sulfur is used in matches, insecticides, and fungicides, many sulfur compounds are odoriferous, and the smells of odorized natural gas, skunk scent, grapefruit, and garlic are due to organosulfur compounds. Hydrogen sulfide gives the characteristic odor to rotting eggs and other biological processes, sulfur is an essential element for all life, but almost always in the form of organosulfur compounds or metal sulfides. Three amino acids and two vitamins are organosulfur compounds, many cofactors also contain sulfur including glutathione and thioredoxin and iron–sulfur proteins. Disulfides, S–S bonds, confer mechanical strength and insolubility of the keratin, found in outer skin, hair. Sulfur is one of the chemical elements needed for biochemical functioning and is an elemental macronutrient for all organisms. Sulfur is derived from the Latin word sulpur, which was Hellenized to sulphur, the spelling sulfur appears toward the end of the Classical period. In 12th-century Anglo-French, it was sulfre, in the 14th century the Latin ph was restored, for sulphre, the parallel f~ph spellings continued in Britain until the 19th century, when the word was standardized as sulphur. Sulfur was the form chosen in the United States, whereas Canada uses both, the IUPAC adopted the spelling sulfur in 1990, as did the Nomenclature Committee of the Royal Society of Chemistry in 1992, restoring the spelling sulfur to Britain. Sulfur forms polyatomic molecules with different chemical formulas, the best-known allotrope being octasulfur, cyclo-S8. The point group of cyclo-S8 is D4d and its dipole moment is 0 D. Octasulfur is a soft, bright-yellow solid that is odorless and it melts at 115.21 °C, boils at 444.6 °C and sublimes easily. At 95.2 °C, below its melting temperature, cyclo-octasulfur changes from α-octasulfur to the β-polymorph, the structure of the S8 ring is virtually unchanged by this phase change, which affects the intermolecular interactions. At higher temperatures, the viscosity decreases as depolymerization occurs, molten sulfur assumes a dark red color above 200 °C
23.
Oxide
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An oxide /ˈɒksaɪd/ is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. Oxide itself is the dianion of oxygen, an O2– atom, Metal oxides thus typically contain an anion of oxygen in the oxidation state of −2. Most of the Earths crust consists of oxides, the result of elements being oxidized by the oxygen in air or in water. Hydrocarbon combustion affords the two principal carbon oxides, carbon monoxide and carbon dioxide, even materials considered pure elements often develop an oxide coating. For example, aluminium foil develops a skin of Al2O3 that protects the foil from further corrosion. Individual elements can form multiple oxides, each containing different amounts of the element. Certain elements can form many different oxides, such those of nitrogen, due to its electronegativity, oxygen forms stable chemical bonds with almost all elements to give the corresponding oxides. Noble metals are prized because they resist direct chemical combination with oxygen, two independent pathways for corrosion of elements are hydrolysis and oxidation by oxygen. The combination of water and oxygen is even more corrosive, virtually all elements burn in an atmosphere of oxygen, or an oxygen rich environment. In the presence of water and oxygen, some elements— sodium—react rapidly, even dangerously, in part for this reason, alkali and alkaline earth metals are not found in nature in their metallic, i. e. native, form. Caesium is so reactive with oxygen that it is used as a getter in vacuum tubes, the surface of most metals consists of oxides and hydroxides in the presence of air. A well-known example is aluminium foil, which is coated with a film of aluminium oxide that passivates the metal. The aluminium oxide layer can be built to greater thickness by the process of electrolytic anodising, though solid magnesium and aluminium react slowly with oxygen at STP—they, like most metals, burn in air, generating very high temperatures. Finely grained powders of most metals can be explosive in air. Consequently, they are used in Solid-fuel rockets. In dry oxygen, iron forms iron oxide, but the formation of the hydrated ferric oxides, Fe2O3−x2x. Free oxygen production by photosynthetic bacteria some 3.5 billion years ago precipitated iron out of solution in the oceans as Fe2O3 in the important iron ore hematite. Oxides have a range of different structures, from molecules to polymeric
24.
Acid rain
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Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions. It can have effects on plants, aquatic animals and infrastructure. Acid rain is caused by emissions of sulfur dioxide and nitrogen oxide, some governments have made efforts since the 1970s to reduce the release of sulfur dioxide and nitrogen oxide into the atmosphere with positive results. Nitrogen oxides can also be produced naturally by lightning strikes, Acid rain is a popular term referring to the deposition of a mixture from wet and dry acidic components. Distilled water, once carbon dioxide is removed, has a neutral pH of 7, liquids with a pH less than 7 are acidic, and those with a pH greater than 7 are alkaline. Clean or unpolluted rain has an acidic pH, but usually no lower than 5, a common example is nitric acid produced by electric discharge in the atmosphere such as lightning. Acid deposition as an issue would include additional acids other than H2CO3. The corrosive effect of polluted, acidic city air on limestone and marble was noted in the 17th century by John Evelyn, since the Industrial Revolution, emissions of sulfur dioxide and nitrogen oxides into the atmosphere have increased. In 1852, Robert Angus Smith was the first to show the relationship between rain and atmospheric pollution in Manchester, England. Though acidic rain was discovered in 1853, it was not until the late 1960s that scientists began widely observing and studying the phenomenon, the term acid rain was coined in 1872 by Robert Angus Smith. Canadian Harold Harvey was among the first to research a dead lake, occasional pH readings in rain and fog water of well below 2.4 have been reported in industrialized areas. Industrial acid rain is a problem in China and Russia. These areas all burn sulfur-containing coal to heat and electricity. The problem of acid rain has not only increased with population and industrial growth, the use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation. Often deposition occurs a considerable distance downwind of the emissions, with mountainous regions tending to receive the greatest deposition, an example of this effect is the low pH of rain which falls in Scandinavia. The earliest report about acid rain in the United States was from the evidence from Hubbard Brook Valley. In 1972, a group of scientists including Gene Likens discovered the rain that was deposited at White Mountains of New Hampshire was acidic, the pH of the sample was measured to be 4.03 at Hubbard Brook. The Hubbard Brook Ecosystem Study followed up with a series of research that analyzed the effects of acid rain
25.
Trigonal planar molecular geometry
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In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of an equilateral triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all angles are 120°. Such species belong to the point group D3h, molecules where the three ligands are not identical, such as H2CO, deviate from this idealized geometry. Examples of molecules with trigonal planar geometry include boron trifluoride, formaldehyde, phosgene, some ions with trigonal planar geometry include nitrate, carbonate, and guanidinium. In organic chemistry, planar, three-connected carbon centers that are trigonal planar are often described as having sp2 hybridization, nitrogen inversion is the distortion of pyramidal amines through a transition state that is trigonal planar. Pyramidalization is a distortion of this molecular shape towards a tetrahedral molecular geometry, one way to observe this distortion is in pyramidal alkenes
26.
VSEPR theory
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Valence shell electron pair repulsion theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. It is also named the Gillespie-Nyholm theory after its two main developers, the acronym VSEPR is pronounced either ves-pur or vuh-seh-per. Gillespie has emphasized that the repulsion due to the Pauli exclusion principle is more important in determining molecular geometry than the electrostatic repulsion. VSEPR theory is based on electron density rather than mathematical wave functions and hence unrelated to orbital hybridisation. While it is qualitative, VSEPR has a quantitative basis in quantum chemical topology methods such as the electron localization function. In 1957, Ronald Gillespie and Ronald Sydney Nyholm of University College London refined this concept into a detailed theory. In VSEPR theory, a bond or triple bond are treated as a single bonding group. The sum of the number of atoms bonded to a central atom, the number of electron pairs, therefore, determines the overall geometry that they will adopt. For example, when there are two electron pairs surrounding the atom, their mutual repulsion is minimal when they lie at opposite poles of the sphere. Therefore, the atom is predicted to adopt a linear geometry. If there are 3 electron pairs surrounding the atom, their repulsion is minimized by placing them at the vertices of an equilateral triangle centered on the atom. Therefore, the geometry is trigonal. Likewise, for 4 electron pairs, the arrangement is tetrahedral. The overall geometry is further refined by distinguishing between bonding and nonbonding electron pairs, the bonding electron pair shared in a sigma bond with an adjacent atom lies further from the central atom than a nonbonding pair of that atom, which is held close to its positively charged nucleus. VSEPR theory therefore views repulsion by the pair to be greater than the repulsion by a bonding pair. As such, when a molecule has 2 interactions with different degrees of repulsion, for instance, when 5 valence electron pairs surround a central atom, they adopt a trigonal bipyramidal molecular geometry with two collinear axial positions and three equatorial positions. The difference between pairs and bonding pairs may also be used to rationalize deviations from idealized geometries. For example, the H2O molecule has four pairs in its valence shell
27.
Molecular symmetry
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Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a concept in chemistry, as it can predict or explain many of a molecules chemical properties, such as its dipole moment. Many university level textbooks on chemistry, quantum chemistry. While various frameworks for the study of symmetry exist, group theory is the predominant one. This framework is useful in studying the symmetry of molecular orbitals, with applications such as the Hückel method, ligand field theory. Another framework on a scale is the use of crystal systems to describe crystallographic symmetry in bulk materials. Many techniques for the assessment of molecular symmetry exist, including X-ray crystallography and various forms of spectroscopy. Spectroscopic notation is based on symmetry considerations, the study of symmetry in molecules is an adaptation of mathematical group theory. The symmetry of a molecule can be described by 5 types of symmetry elements, symmetry axis, an axis around which a rotation by 360 ∘ n results in a molecule indistinguishable from the original. This is also called a rotational axis and abbreviated Cn. Examples are the C2 axis in water and the C3 axis in ammonia, a molecule can have more than one symmetry axis, the one with the highest n is called the principal axis, and by convention is aligned with the z-axis in a Cartesian coordinate system. Plane of symmetry, a plane of reflection through which a copy of the original molecule is generated. This is also called a plane and abbreviated σ. Water has two of them, one in the plane of the molecule itself and one perpendicular to it, a symmetry plane parallel with the principal axis is dubbed vertical and one perpendicular to it horizontal. A third type of symmetry plane exists, If a vertical symmetry plane additionally bisects the angle between two 2-fold rotation axes perpendicular to the axis, the plane is dubbed dihedral. A symmetry plane can also be identified by its Cartesian orientation, center of symmetry or inversion center, abbreviated i. A molecule has a center of symmetry when, for any atom in the molecule, in other words, a molecule has a center of symmetry when the points and correspond to identical objects. For example, if there is an atom in some point
28.
Oxidation state
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The oxidation state, often called the oxidation number, is an indicator of the degree of oxidation of an atom in a chemical compound. This is never exactly true for real bonds, the term oxidation was first used by Lavoisier to signify reaction of a substance with oxygen. Much later, it was realized that the substance, upon being oxidized, loses electrons, oxidation states are typically represented by integers. In some cases, the oxidation state of an element is a fraction. It is predicted that even a +10 oxidation state may be achieveable by platinum in the platinum tetroxide dication, the increase in oxidation state of an atom, through a chemical reaction, is known as an oxidation, a decrease in oxidation state is known as a reduction. Such reactions involve the transfer of electrons, a net gain in electrons being a reduction. For pure elements, the state is zero. There are various methods for determining oxidation states, in inorganic nomenclature, the oxidation state is determined and expressed as an oxidation number, and is represented by a Roman numeral placed after the element name. In coordination chemistry, oxidation number is defined differently from oxidation state, a “Comprehensive definition of oxidation state ” has been published with free access. This article is a distillation of an IUPAC technical report Toward a comprehensive definition of oxidation state. Exceptions to this are that hydrogen has a state of −1 in hydrides of active metals, e. g. LiH. There are two different methods for determining the state of elements in chemical compounds. First a method based on the rules in the IUPAC definition, second, a method based on the relative electronegativity of the elements in the compound, where the more electronegative element is assumed to take the negative charge. Any pure element—even if it forms diatomic molecules like chlorine —has an oxidation state of zero, examples of this are Cu or O2. For monatomic ions, the state is the same as the charge of the ion. For example, the anion has an oxidation state of −2. The sum of oxidation states for all atoms in a molecule or polyatomic ion is equal to the charge of the molecule or ion, thus, the oxidation state of one element can be calculated from the oxidation states of the other elements. An application of this rule is that the sum of the states of all atoms in a neutral molecule must be zero
29.
Formal charge
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In chemistry, a formal charge is the charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of relative electronegativity. When determining the best Lewis structure for a molecule, the structure is such that the formal charge on each of the atoms is as close to zero as possible. Example, CO2 is a molecule with 16 total valence electrons. The following is equivalent, Draw a circle around the atom for which the charge is requested Count up the number of electrons in the atoms circle. Since the circle cuts the covalent bond in half, each covalent bond counts as one instead of two. Subtract the number of electrons in the circle from the number of the element to determine the formal charge. The formal charges computed for the atoms in this Lewis structure of carbon dioxide are shown below. It is important to keep in mind that formal charges are just that – formal, the formal charge system is just a method to keep track of all of the valence electrons that each atom brings with it when the molecule is formed. Formal charge is a tool for estimating the distribution of charge within a molecule. The concept of oxidation states constitutes a competing method to assess the distribution of electrons in molecules, with formal charge, the electrons in each covalent bond are assumed to be split exactly evenly between the two atoms in the bond. This can be most effectively visualized in an electrostatic potential map, with the oxidation state formalism, the electrons in the bonds are awarded to the atom with the greater electronegativity. In reality, the distribution of electrons in the molecule lies somewhere between two extremes
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