1.
Siwa Oasis
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The Siwa Oasis is an oasis in Egypt, between the Qattara Depression and the Egyptian Sand Sea in the Western Desert, nearly 50 km east of the Libyan border, and 560 km from Cairo. Its fame lies primarily in its ancient role as the home to an oracle of Ammon, historically, it is part of Ancient Libya. The Siwa oasis is in a depression that reaches below sea level. To the west the Jaghbub oasis lies in a similar depression, the oasis was called Ammonium in ancient times. Early Arab geographers termed it Santariyyah and its modern name Siwa, first attested in the 15th century, is of uncertain origin. Although the oasis is known to have settled since at least the 10th millennium BC, the earliest evidence of connection with ancient Egypt is the 26th Dynasty. During the Ptolemaid period of Egypt its ancient Egyptian name was sḫ. t-ỉm3w, Greek settlers at Cyrene made contact with the oasis around the same time, and the oracle temple of Amun, who, Herodotus was told, took the image here of a ram. Herodotus knew of a fountain of the Sun that ran coldest in the noontide heat, during his campaign to conquer the Persian Empire, Alexander the Great reached the oasis, supposedly by following birds across the desert. Evidence of Christianity at Siwa is uncertain, but in 708 the Siwans resisted an Islamic army, a local manuscript mentions only seven families totaling 40 men living at the oasis in 1203. In the 12th century Al-Idrisi mentions it as being inhabited mainly by Berbers, with an Arab minority, the Egyptian historian Al-Maqrizi travelled to Siwa in the 15th century and described how the language spoken there is similar to the language of the Zenata. The first European to visit since Roman times was the English traveler William George Browne, the oasis was officially added to Egypt by Muhammad Ali of Egypt in 1819. In the Spring of 1893, German explorer and photographer, Hermann Burchardt, took photographs of the architecture of the town of Siwa, consequently, Arab rule from distant Cairo was at first tenuous and marked by several revolts. Siwa was also the site of fighting during World War I. The British Armys Long Range Desert Group was based here, German soldiers went skinny dipping in the lake of the oracle, contrary to local customs which prohibit public nudity. In 1942 while the Italian 136th Infantry Division Giovani Fascisti occupied the oasis, the oasis makes a brief appearance as a base of the LRDG in the 1958 war film Ice Cold in Alex. The ancient fortress of Siwa, known as the Shali Ghadi, was built on rock and made of kershif. Only one building in the Shali complex has been repaired and is in use, gradually eroded by infrequent rains and slowly collapsing, the Shali remains a prominent feature, towering five stories above the modern town and lit at night by floodlights. It is most easily approached from its southwest side, south of the end of the road which curves around from the north side of the Shali
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.
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
5.
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
6.
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
7.
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
8.
Acid dissociation constant
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An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the constant for a chemical reaction known as dissociation in the context of acid–base reactions. In the example shown in the figure, HA represents acetic acid, and A− represents the acetate ion, the chemical species HA, A− and H3O+ are said to be in equilibrium when their concentrations do not change with the passing of time. The definition can then be more simply H A ⇌ A − + H +, K a = This is the definition in common usage. A weak acid has a pKa value in the approximate range −2 to 12 in water, pKa values for strong acids can, however, be estimated by theoretical means. The definition can be extended to non-aqueous solvents, such as acetonitrile and dimethylsulfoxide. Denoting a solvent molecule by S H A + S ⇌ A − + S H +, K a = When the concentration of solvent molecules can be taken to be constant, K a =, as before. The value of pKa also depends on structure of the acid in many ways. For example, Pauling proposed two rules, one for successive pKa of polyprotic acids, and one to estimate the pKa of oxyacids based on the number of =O and −OH groups. Other structural factors that influence the magnitude of the dissociation constant include inductive effects, mesomeric effects. Hammett type equations have frequently applied to the estimation of pKa. The quantitative behaviour of acids and bases in solution can be only if their pKa values are known. These calculations find application in different areas of chemistry, biology, medicine. Acid dissociation constants are essential in aquatic chemistry and chemical oceanography. In living organisms, acid–base homeostasis and enzyme kinetics are dependent on the pKa values of the acids and bases present in the cell. According to Arrheniuss original definition, an acid is a substance that dissociates in solution, releasing the hydrogen ion H+. The equilibrium constant for this reaction is known as a dissociation constant. Brønsted and Lowry generalised this further to an exchange reaction
9.
Conjugate acid
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A conjugate acid, within the Brønsted–Lowry acid–base theory, is a species formed by the reception of a proton by a base—in other words, it is a base with a hydrogen ion added to it. On the other hand, a base is merely what is left after an acid has donated a proton in a chemical reaction. Hence, a base is a species formed by the removal of a proton from an acid. A proton is a particle with a unit positive electrical charge, it is represented by the symbol H+ because it constitutes the nucleus of a hydrogen atom, that is. A cation can be an acid, and an anion can be a conjugate base, depending on which substance is involved. In an acid-base reaction, an acid plus a base reacts to form a conjugate base plus a conjugate acid, refer to the following figure, We say that the water molecule is the conjugate acid of the hydroxide ion after the latter received the hydrogen proton donated by ammonium. On the other hand, ammonia is the base for the acid ammonium after ammonium has donated a hydrogen ion towards the production of the water molecule. The strength of an acid is directly proportional to its dissociation constant. If a conjugate acid is strong, its dissociation will have an equilibrium constant. The strength of a base can be seen as the tendency of the species to pull hydrogen protons towards itself. If a conjugate base is classified as strong, it will hold on to the hydrogen proton when in solution, if a chemical species is classified as a weak acid, its conjugate base will be strong in nature. This can be observed in ammonias reaction with water, the reaction proceeds until most of the ammonia has been transformed to ammonium. This shift to the right in the equilibrium of the reaction means that ammonium does not dissociate easily in water. On the other hand, if a species is classified as a strong acid, an example of this case would be the dissociation of Hydrochloric acid HCl in water. Since HCl is an acid, its conjugate base will be a weak conjugate base. Therefore, in system, most H+ will be in the form of a Hydronium ion H 3O+ instead of attached to a Cl anion. To summarize, the stronger the acid or base, the weaker the conjugate, the acid and conjugate base as well as the base and conjugate acid are known as conjugate pairs. When finding a conjugate acid or base, it is important to look at the reactants of the chemical equation, to identify the conjugate acid, look for the pair of compounds that are related
10.
Molecular geometry
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Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism and biological activity. The angles between bonds that an atom forms depend only weakly on the rest of molecule, i. e. they can be understood as approximately local, the molecular geometry can be determined by various spectroscopic methods and diffraction methods. IR, microwave and Raman spectroscopy can give information about the molecule geometry from the details of the vibrational and rotational absorbance detected by these techniques. X-ray crystallography, neutron diffraction and electron diffraction can give molecular structure for crystalline solids based on the distance between nuclei and concentration of electron density, gas electron diffraction can be used for small molecules in the gas phase. NMR and FRET methods can be used to determine complementary information including relative distances, dihedral angles, angles, molecular geometries are best determined at low temperature because at higher temperatures the molecular structure is averaged over more accessible geometries. Larger molecules often exist in multiple stable geometries that are close in energy on the energy surface. Geometries can also be computed by ab initio quantum chemistry methods to high accuracy, the molecular geometry can be different as a solid, in solution, and as a gas. The position of each atom is determined by the nature of the bonds by which it is connected to its neighboring atoms. Since the motions of the atoms in a molecule are determined by quantum mechanics, the overall quantum mechanical motions translation and rotation hardly change the geometry of the molecule. In addition to translation and rotation, a type of motion is molecular vibration. The molecular vibrations are harmonic, and the atoms oscillate about their equilibrium positions, at higher temperatures the vibrational modes may be thermally excited, but they oscillate still around the recognizable geometry of the molecule. At 298 K, typical values for the Boltzmann factor β are, β =0.089 for ΔE =500 cm−1, β =0.008 for ΔE =1000 cm−1, β = 7×10−4 for ΔE =1500 cm−1. When an excitation energy is 500 cm−1, then about 8.9 percent of the molecules are excited at room temperature. To put this in perspective, the lowest excitation vibrational energy in water is the bending mode, thus, at room temperature less than 0.07 percent of all the molecules of a given amount of water will vibrate faster than at absolute zero. As stated above, rotation hardly influences the molecular geometry, but, as a quantum mechanical motion, it is thermally excited at relatively low temperatures. From a classical point of view it can be stated that at temperatures more molecules will rotate faster. In quantum mechanical language, more eigenstates of higher angular momentum become thermally populated with rising temperatures, typical rotational excitation energies are on the order of a few cm−1
11.
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
12.
Electric charge
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Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of charges, positive and negative. Like charges repel and unlike attract, an absence of net charge is referred to as neutral. An object is charged if it has an excess of electrons. The SI derived unit of charge is the coulomb. In electrical engineering, it is common to use the ampere-hour. The symbol Q often denotes charge, early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that dont require consideration of quantum effects. The electric charge is a conserved property of some subatomic particles. Electrically charged matter is influenced by, and produces, electromagnetic fields, the interaction between a moving charge and an electromagnetic field is the source of the electromagnetic force, which is one of the four fundamental forces. 602×10−19 coulombs. The proton has a charge of +e, and the electron has a charge of −e, the study of charged particles, and how their interactions are mediated by photons, is called quantum electrodynamics. Charge is the property of forms of matter that exhibit electrostatic attraction or repulsion in the presence of other matter. Electric charge is a property of many subatomic particles. The charges of free-standing particles are integer multiples of the charge e. Michael Faraday, in his electrolysis experiments, was the first to note the discrete nature of electric charge, robert Millikans oil drop experiment demonstrated this fact directly, and measured the elementary charge. By convention, the charge of an electron is −1, while that of a proton is +1, charged particles whose charges have the same sign repel one another, and particles whose charges have different signs attract. The charge of an antiparticle equals that of the corresponding particle, quarks have fractional charges of either −1/3 or +2/3, but free-standing quarks have never been observed. The electric charge of an object is the sum of the electric charges of the particles that make it up. An ion is an atom that has lost one or more electrons, giving it a net charge, or that has gained one or more electrons
13.
Polyatomic ion
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The prefix poly- means many, in Greek, but even ions of two atoms are commonly referred to as polyatomic. In older literature, an ion is also referred to as a radical. In contemporary usage, the term refers to free radicals that are species with an unpaired electron. An example of an ion is the hydroxide ion, consisting of one oxygen atom and one hydrogen atom. An ammonium ion is made up of one atom and four hydrogen atoms, it has a charge of +1. Polyatomic ions are often useful in the context of chemistry or in the formation of salts. A polyatomic ion can often be considered as the conjugate acid/base of a neutral molecule, for example, the conjugate base of sulfuric acid is the polyatomic hydrogen sulfate anion. The removal of hydrogen ion yields the sulfate anion. There are two rules that can be used for learning the nomenclature of polyatomic anions. First, when the prefix bi is added to a name, a hydrogen is added to the formula and its charge is increased by 1. An alternative to the bi- prefix is to use the hydrogen in its place. Note that many of the common polyatomic anions are conjugate bases of acids derived from the oxides of non-metallic elements, for example, the sulfate anion, SO42−, is derived from H 2SO4, which can be regarded as SO3 + H 2O. The second rule looks at the number of oxygens in an ion, consider the chlorine oxoanion family, First, think of the -ate ion as being the base name, in which case the addition of a per- prefix adds an oxygen. Changing the -ate suffix to -ite will reduce the oxygens by one, in all situations, the charge is not affected. The naming pattern follows within many different oxyanion series based on a root for that particular series. The -ite has one less oxygen than the -ate, but different -ate anions might have different numbers of oxygen atoms and these rules will not work with all polyatomic anions, but they do work with the most common ones. The following tables give examples of commonly encountered polyatomic ions, only a few representatives are given, as the number of polyatomic ions encountered in practice is very large. Monatomic ions Salt Mass spectrometry Hydrogen peroxide Molecule Hydrogen molecular ion List of polyatomic ions A Beginners Guide To Polyatomic Ions, tables of Common Polyatomic Ions, including PDB files
14.
Substitution reaction
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Substitution reaction is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions are of importance in organic chemistry. Substitution reactions in chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved. There are other classifications as well that are mentioned below, detailed understanding of a reaction type helps to predict the product outcome in a reaction. It also is helpful for optimizing a reaction with regard to such as temperature. A good example of a reaction is halogenation. When chlorine gas is irradiated, some of the molecules are split into two chlorine radicals whose free electrons are strongly nucleophilic, one of them breaks a weak C-H covalent bond and grabs the liberated proton to form the electrically neutral H-Cl. The other radical reforms a covalent bond with the CH3. to form CH3Cl, as it does so, it replaces a weaker nucleophile which then becomes a leaving group, The remaining positive or partially positive atom becomes an electrophile. The whole molecular entity of which the electrophile and the group are part is usually called the substrate. The most general form for the reaction may be given as where R-LG indicates the substrate, nuc, + R-LG → R-Nuc + LG, The electron pair from the nucleophile attacks the substrate forming a new covalent bond Nuc-R-LG. The prior state of charge is restored when the group departs with an electron pair. The principal product in case is R-Nuc. In such reactions, the nucleophile is usually neutral or negatively charged. An example of substitution is the hydrolysis of an alkyl bromide, R-Br, under basic conditions, where the attacking nucleophile is the base OH−. These substitutions can be produced by two different mechanisms categorized at, unimolecular nucleophilic substitution and bimolecular nucleophilic substitution, the SN1 mechanism has two steps. In the first step, the group departs, forming a carbocation C+. In the second step, the nucleophilic reagent attaches to the carbocation, if the substrate has a chiral carbon, this mechanism can result in either inversion of the stereochemistry or retention of configuration. For an example with diagrams see the reaction of tert-butyl bromide with water shown at SN1 reaction, the SN2 mechanism have just one step
15.
Amine
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In organic chemistry, amines are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group, important amines include amino acids, biogenic amines, trimethylamine, and aniline, see Category, Amines for a list of amines. Inorganic derivatives of ammonia are also called amines, such as chloramine, see Category, compounds with a nitrogen atom attached to a carbonyl group, thus having the structure R–CO–NR′R″, are called amides and have different chemical properties from amines. An aliphatic amine has no aromatic ring attached directly to the nitrogen atom, aromatic amines have the nitrogen atom connected to an aromatic ring as in the various anilines. The aromatic ring decreases the alkalinity of the amine, depending on its substituents, the presence of an amine group strongly increases the reactivity of the aromatic ring, due to an electron-donating effect. Amines are organized into four subcategories, Primary amines — Primary amines arise when one of three atoms in ammonia is replaced by an alkyl or aromatic. Important primary alkyl amines include, methylamine, most amino acids, Secondary amines — Secondary amines have two organic substituents bound to the nitrogen together with one hydrogen. Important representatives include dimethylamine, while an example of an aromatic amine would be diphenylamine, tertiary amines — In tertiary amines, nitrogen has three organic substituents. Examples include trimethylamine, which has a fishy smell. Cyclic amines — Cyclic amines are either secondary or tertiary amines, examples of cyclic amines include the 3-membered ring aziridine and the six-membered ring piperidine. N-methylpiperidine and N-phenylpiperidine are examples of tertiary amines. It is also possible to have four organic substituents on the nitrogen and these species are not amines but are quaternary ammonium cations and have a charged nitrogen center. Quaternary ammonium salts exist with many kinds of anions, Amines are named in several ways. Typically, the compound is given the prefix amino- or the suffix, the prefix N- shows substitution on the nitrogen atom. An organic compound with multiple amino groups is called a diamine, triamine, tetraamine, systematic names for some common amines, Hydrogen bonding significantly influences the properties of primary and secondary amines. Thus the melting point and boiling point of amines is higher than those of the corresponding phosphines, for example, methyl and ethyl amines are gases under standard conditions, whereas the corresponding methyl and ethyl alcohols are liquids. Amines possess a characteristic smell, liquid amines have a distinctive fishy smell. The nitrogen atom features a lone pair that can bind H+ to form an ammonium ion R3NH+
16.
Quaternary ammonium cation
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Quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure NR+4, R being an alkyl group or an aryl group. Unlike the ammonium ion and the primary, secondary, or tertiary ammonium cations, Quaternary ammonium salts or quaternary ammonium compounds are salts of quaternary ammonium cations. Quaternary ammonium compounds are prepared by the alkylation of amines with a halocarbon. In older literature this is called a Menshutkin reaction, however modern chemists usually refer to it simply as quaternization. The reaction can be used to produce a compound with unequal alkyl chain lengths and they also are stable toward most nucleophiles. The latter is indicated by the stability of the hydroxide salts such as tetramethylammonium hydroxide, because of their resilience, many unusual anions have been isolated as the quaternary ammonium salts. Examples include tetramethylammonium pentafluoroxenate, containing the highly reactive pentafluoroxenate ion, permanganate can be solubilized in organic solvents, when deployed as its NBu+4 salt. With exceptionally strong bases, quat cations degrade and they undergo Sommelet–Hauser rearrangement and Stevens rearrangement, as well as dealkylation under harsh conditions. Quaternary ammonium cations containing N–C–C–H units can also undergo the Hofmann elimination, Quaternary ammonium salts are used as disinfectants, surfactants, fabric softeners, and as antistatic agents. In liquid fabric softeners, the salts are often used. In dryer anticling strips, the salts are often used. Spermicidal jellies also contain quaternary ammonium salts, Quaternary ammonium compounds have also been shown to have antimicrobial activity. Certain quaternary ammonium compounds, especially those containing long alkyl chains, are used as antimicrobials, also good against fungi, amoebas, and enveloped viruses, quaternary ammonium compounds are believed to act by disrupting the cell membrane. Quaternary ammonium compounds are lethal to a variety of organisms except endospores, Mycobacterium tuberculosis. Quaternary ammonium compounds are cationic detergents, as well as disinfectants and they are very effective in combination with phenols. Quaternary ammonium compounds are deactivated by anionic detergents, also, they work best in soft waters. Effective levels are at 200 ppm and they are effective at temperatures up to 100 °C. Quaternary ammonium salts are used in the foodservice industry as sanitizing agents
17.
Hydrogen
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Hydrogen is a chemical element with chemical symbol H and atomic number 1. With a standard weight of circa 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form is the most abundant chemical substance in the Universe, non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton, the universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays an important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a charge when it is known as a hydride. The hydrogen cation is written as though composed of a bare proton, Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. Industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production, mostly for the fertilizer market, Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks. Hydrogen gas is flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol,2 H2 + O2 →2 H2O +572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%, the explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C, the detection of a burning hydrogen leak may require a flame detector, such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames, the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element, the ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 91 nm wavelength. The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, however, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. The most complicated treatments allow for the effects of special relativity
18.
Atom
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An atom is the smallest constituent unit of ordinary matter that has the properties of a chemical element. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms, Atoms are very small, typical sizes are around 100 picometers. Atoms are small enough that attempting to predict their behavior using classical physics - as if they were billiard balls, through the development of physics, atomic models have incorporated quantum principles to better explain and predict the behavior. Every atom is composed of a nucleus and one or more bound to the nucleus. The nucleus is made of one or more protons and typically a number of neutrons. Protons and neutrons are called nucleons, more than 99. 94% of an atoms mass is in the nucleus. The protons have an electric charge, the electrons have a negative electric charge. If the number of protons and electrons are equal, that atom is electrically neutral, if an atom has more or fewer electrons than protons, then it has an overall negative or positive charge, respectively, and it is called an ion. The electrons of an atom are attracted to the protons in a nucleus by this electromagnetic force. The number of protons in the nucleus defines to what chemical element the atom belongs, for example, the number of neutrons defines the isotope of the element. The number of influences the magnetic properties of an atom. Atoms can attach to one or more other atoms by chemical bonds to form compounds such as molecules. The ability of atoms to associate and dissociate is responsible for most of the changes observed in nature. The idea that matter is made up of units is a very old idea, appearing in many ancient cultures such as Greece. The word atom was coined by ancient Greek philosophers, however, these ideas were founded in philosophical and theological reasoning rather than evidence and experimentation. As a result, their views on what look like. They also could not convince everybody, so atomism was but one of a number of competing theories on the nature of matter. It was not until the 19th century that the idea was embraced and refined by scientists, in the early 1800s, John Dalton used the concept of atoms to explain why elements always react in ratios of small whole numbers
19.
Hydrochloric acid
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Hydrochloric acid is a corrosive, strong mineral acid with many industrial uses. A colorless, highly pungent solution of chloride in water. Free hydrochloric acid was first formally described in the 16th century by Libavius, later, it was used by chemists such as Glauber, Priestley, and Davy in their scientific research. It has numerous applications, including household cleaning, production of gelatin and other food additives, descaling. About 20 million tonnes of acid are produced worldwide annually. It is also found naturally in gastric acid, Hydrochloric acid was known to European alchemists as spirits of salt or acidum salis. Both names are used, especially in other languages, such as German, Salzsäure, Dutch, Zoutzuur, Swedish, Saltsyra, Turkish, Tuz Ruhu, Polish, kwas solny and Chinese. Gaseous HCl was called marine acid air, the old name muriatic acid has the same origin, and this name is still sometimes used. The name hydrochloric acid was coined by the French chemist Joseph Louis Gay-Lussac in 1814, aqua regia, a mixture consisting of hydrochloric and nitric acids, prepared by dissolving sal ammoniac in nitric acid, was described in the works of Pseudo-Geber, a 13th-century European alchemist. Other references suggest that the first mention of aqua regia is in Byzantine manuscripts dating to the end of the 13th century, free hydrochloric acid was first formally described in the 16th century by Libavius, who prepared it by heating salt in clay crucibles. Joseph Priestley of Leeds, England prepared pure hydrogen chloride in 1772, during the Industrial Revolution in Europe, demand for alkaline substances increased. A new industrial process developed by Nicolas Leblanc of Issoundun, France enabled cheap large-scale production of sodium carbonate, in this Leblanc process, common salt is converted to soda ash, using sulfuric acid, limestone, and coal, releasing hydrogen chloride as a by-product. Until the British Alkali Act 1863 and similar legislation in other countries, after the passage of the act, soda ash producers were obliged to absorb the waste gas in water, producing hydrochloric acid on an industrial scale. In the 20th century, the Leblanc process was replaced by the Solvay process without a hydrochloric acid by-product. Since hydrochloric acid was already settled as an important chemical in numerous applications. After the year 2000, hydrochloric acid is made by absorbing by-product hydrogen chloride from industrial organic compounds production. Hydrochloric acid is the salt of hydronium ion, H3O+ and chloride and it is usually prepared by treating HCl with water. H C l + H2 O ⟶ H3 O + + C l − Hydrochloric acid can therefore be used to prepare salts called chlorides, Hydrochloric acid is a strong acid, since it is completely dissociated in water
20.
Ammonium chloride
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Ammonium chloride is an inorganic compound with the formula NH4Cl and a white crystalline salt that is highly soluble in water. Solutions of ammonium chloride are mildly acidic, sal ammoniac is a name of the natural, mineralogical form of ammonium chloride. The mineral is formed on burning coal dumps from condensation of coal-derived gases. It is also found some types of volcanic vents. It is mainly used as fertilizer and an agent in some types of liquorice. It is the product from the reaction of acid and ammonia. The crystals deposit directly from the state and tend to be short-lived. Ammonium chloride appears to sublime upon heating but actually decomposes into ammonia, some of ammonium chlorides reactions with other chemicals are endothermic like its reaction with barium hydroxide and its dissolving in water. The main application of ammonium chloride is as a source in fertilizers such as chloroammonium phosphate. The main crops are rice and wheat in Asia, Ammonium chloride was used in pyrotechnics in the 18th century but was superseded by safer and less hygroscopic chemicals. Its purpose was to provide a donor to enhance the green. It had a use to provide white smoke, but its ready double decomposition reaction with potassium chlorate producing the highly unstable ammonium chlorate made its use very suspect. Ammonium chloride is used as a flux in preparing metals to be tin coated, galvanized or soldered and it works as a flux by cleaning the surface of workpieces by reacting with the metal oxides at the surface to form a volatile metal chloride. For that purpose, it is sold in blocks at hardware stores for use in cleaning the tip of a soldering iron, Ammonium chloride is used as an expectorant in cough medicine. Its expectorant action is caused by action on the bronchial mucosa, which causes the production of excess respiratory tract fluid. Ammonium salts are an irritant to the mucosa and may induce nausea. In several countries, ammonium chloride, under the name sal ammoniac or colloquially salmiakki is used as food additive under the E number E510 and it is a feed supplement for cattle and an ingredient in nutritive media for yeasts and many microorganisms. Ammonium chloride is used to spice up dark sweets called salty liquorice, in baking to give cookies a very crisp texture, in India and Pakistan, it is called Noshader and is used to improve the crispness of snacks such as samosas and jalebi
21.
PH
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In chemistry, pH is a numeric scale used to specify the acidity or basicity of an aqueous solution. It is approximately the negative of the base 10 logarithm of the concentration, measured in units of moles per liter. More precisely it is the negative of the logarithm to base 10 of the activity of the hydrogen ion, solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. Pure water is neutral, at pH7, being neither an acid nor a base, contrary to popular belief, the pH value can be less than 0 or greater than 14 for very strong acids and bases respectively. The pH scale is traceable to a set of standard solutions whose pH is established by international agreement, the pH of aqueous solutions can be measured with a glass electrode and a pH meter, or an indicator. In the first papers, the notation had the H as a subscript to the p, as so. The exact meaning of the p in pH is disputed, but according to the Carlsberg Foundation and it has also been suggested that the p stands for the German Potenz, others refer to French puissance. Another suggestion is that the p stands for the Latin terms pondus hydrogenii, potentia hydrogenii and it is also suggested that Sørensen used the letters p and q simply to label the test solution and the reference solution. Currently in chemistry, the p stands for decimal cologarithm of, PH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution. P H = − log 10 = log 10 For example and this definition was adopted because ion-selective electrodes, which are used to measure pH, respond to activity. For H+ number of electrons transferred is one and it follows that electrode potential is proportional to pH when pH is defined in terms of activity. The reference electrode may be a silver chloride electrode or a calomel electrode, the hydrogen-ion selective electrode is a standard hydrogen electrode. Reference electrode | concentrated solution of KCl || test solution | H2 | Pt Firstly, the cell is filled with a solution of hydrogen ion activity. Then the emf, EX, of the cell containing the solution of unknown pH is measured. PH = pH + E S − E X z The difference between the two measured emf values is proportional to pH and this method of calibration avoids the need to know the standard electrode potential. The proportionality constant, 1/z is ideally equal to 12.303 R T / F the Nernstian slope, to apply this process in practice, a glass electrode is used rather than the cumbersome hydrogen electrode. A combined glass electrode has a reference electrode. It is calibrated against buffer solutions of hydrogen ion activity
22.
Hydrogen ion
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A hydrogen ion is created when a hydrogen atom loses or gains an electron. A lone positively charged ion can readily combine with other particles. Due to its high charge density of approximately 2×1010 times that of a sodium ion. The hydrogen ion is recommended by IUPAC as a term for all ions of hydrogen. Depending on the charge of the ion, two different classes can be distinguished, positively charged ions and negatively charged ions, a hydrogen atom is made up of a nucleus with charge +1, and a single electron. Therefore, the positively charged ion possible has charge +1. In connection with acids, hydrogen ions typically refers to hydrons, Hydrogen atom contains a single proton and a single electron. Removal of the electron gives a cation, whereas addition of a gives a anion. The hydrogen anion, with its loosely held two-electron cloud, has a larger radius than the neutral atom, which in turn is much larger than the bare proton of the cation. Hydrogen forms the cation that has no electrons, but even cations that still retain one or more electrons are still smaller than the neutral atoms or molecules from which they are derived. This happens when hydrogen ions get pushed across the membrane creating a high concentration inside the thylakoid membrane, however, because of osmosis the H+ will force itself out of the membrane through ATP synthase. Utilizing their kinetic energy to escape, the protons will spin the ATP synthase which in turn will create ATP and this happens in cellular respiration as well though the concentrated membrane will instead be the inner membrane of the mitochondria. Hydrogen ions are also important in pH as they are responsible for if a compound is acidic or basic, water detaches to form H+ and hydroxides. This process is referred to as the self-ionization of water Acid Protonation Dihydrogen cation Trihydrogen cation Britannica Molecular Hydrogen Foundation
23.
Hydroxide
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Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a covalent bond and it is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, the hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical, a hydroxide attached to a strongly electropositive center may itself ionize, liberating a hydrogen cation, making the parent compound an acid. The corresponding electrically neutral compound •HO is the hydroxyl radical, the corresponding covalently-bound group –OH of atoms is the hydroxyl group. Hydroxide ion and hydroxyl group are nucleophiles and can act as a catalyst in organic chemistry, many inorganic substances which bear the word hydroxide in their names are not ionic compounds of the hydroxide ion, but covalent compounds which contain hydroxyl groups. The pH of a solution is equal to the decimal cologarithm of the cation concentration. The concentration of ions can be expressed in terms of pOH. Addition of a base to water will reduce the hydrogen cation concentration, POH can be kept at a nearly constant value with various buffer solutions. In aqueous solution the hydroxide ion is a base in the Brønsted–Lowry sense as it can accept a proton from a Brønsted–Lowry acid to form a water molecule and it can also act as a Lewis base by donating a pair of electrons to a Lewis acid. In aqueous solution both hydrogen and hydroxide ions are solvated, with hydrogen bonds between oxygen and hydrogen atoms. Indeed, the bihydroxide ion H 3O−2 has been characterized in the solid state and this compound is centrosymmetric and has a very short hydrogen bond that is similar to the length in the bifluoride ion HF−2. In aqueous solution the hydroxide ion forms strong bonds with water molecules. A consequence of this is that concentrated solutions of sodium hydroxide have high viscosity due to the formation of a network of hydrogen bonds as in hydrogen fluoride solutions. In solution, exposed to air, the ion reacts rapidly with atmospheric carbon dioxide, acting as an acid, to form, initially. OH− + CO2 ⇌ HCO−3 The equilibrium constant for this reaction can be specified either as a reaction with dissolved carbon dioxide or as a reaction with carbon dioxide gas. At neutral or acid pH, the reaction is slow, but is catalyzed by the carbonic anhydrase. Solutions containing the hydroxide ion attack glass, in this case, the silicates in glass are acting as acids
24.
Vapor
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A vapor is different from an aerosol. An aerosol is a suspension of particles of liquid, solid. For example, water has a temperature of 647 K. In the atmosphere at ordinary temperatures, therefore, gaseous water will condense into a liquid if its pressure is increased sufficiently. A vapor may co-exist with a liquid, when this is true, the two phases will be in equilibrium, and the gas-partial pressure will be equal to the equilibrium vapor pressure of the liquid. Vapor refers to a gas phase at a temperature where the substance can also exist in the liquid or solid state. If the vapor is in contact with a liquid or solid phase, the term gas refers to a compressible fluid phase. Fixed gases are gases for which no liquid or solid can form at the temperature of the gas, a liquid or solid does not have to boil to release a vapor. Vapor is responsible for the processes of cloud formation and condensation. It is commonly employed to carry out the processes of distillation. The constituent molecules of a vapor possess vibrational, rotational, and these motions are considered in the kinetic theory of gases. The vapor pressure is the pressure from a liquid or a solid at a specific temperature. The equilibrium vapor pressure of a liquid or solid is not affected by the amount of contact with the liquid or solid interface, the normal boiling point of a liquid is the temperature at which the vapor pressure is equal to normal atmospheric pressure. For two-phase systems, the pressure of the individual phases are equal. The total vapor pressure is the sum of the component partial pressures, perfumes contain chemicals that vaporize at different temperatures and at different rate in scent accords known as notes. Atmospheric water vapor is found near the surface, and may condense into small liquid droplets and form meteorological phenomena such as fog, mist. Mercury-vapor lamps and sodium vapor lamps produce light from atoms in excited states, flammable liquids do not burn when ignited. It is the cloud above the liquid that will burn if the vapors concentration is between the lower flammable limit and upper flammable limit of the flammable liquid
25.
Solid
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Solid is one of the four fundamental states of matter. It is characterized by structural rigidity and resistance to changes of shape or volume, unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to other, either in a regular geometric lattice or irregularly. The branch of physics deals with solids is called solid-state physics. Materials science is concerned with the physical and chemical properties of solids. Solid-state chemistry is concerned with the synthesis of novel materials, as well as the science of identification. The atoms, molecules or ions which make up solids may be arranged in a repeating pattern. Materials whose constituents are arranged in a regular pattern are known as crystals, in some cases, the regular ordering can continue unbroken over a large scale, for example diamonds, where each diamond is a single crystal. Almost all common metals, and many ceramics, are polycrystalline, in other materials, there is no long-range order in the position of the atoms. These solids are known as amorphous solids, examples include polystyrene, whether a solid is crystalline or amorphous depends on the material involved, and the conditions in which it was formed. Solids which are formed by slow cooling will tend to be crystalline, likewise, the specific crystal structure adopted by a crystalline solid depends on the material involved and on how it was formed. While many common objects, such as an ice cube or a coin, are chemically identical throughout, for example, a typical rock is an aggregate of several different minerals and mineraloids, with no specific chemical composition. Wood is an organic material consisting primarily of cellulose fibers embedded in a matrix of organic lignin. In materials science, composites of more than one constituent material can be designed to have desired properties, the forces between the atoms in a solid can take a variety of forms. For example, a crystal of sodium chloride is made up of sodium and chlorine. In diamond or silicon, the atoms share electrons and form covalent bonds, in metals, electrons are shared in metallic bonding. Some solids, particularly most organic compounds, are together with van der Waals forces resulting from the polarization of the electronic charge cloud on each molecule. The dissimilarities between the types of solid result from the differences between their bonding, metals typically are strong, dense, and good conductors of both electricity and heat
26.
Salt (chemistry)
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In chemistry, a salt is an ionic compound that results from the neutralization reaction of an acid and a base. Salts are composed of related numbers of cations and anions so that the product is electrically neutral and these component ions can be inorganic, such as chloride, or organic, such as acetate, and can be monatomic, such as fluoride, or polyatomic, such as sulfate. There are several varieties of salts, salts that hydrolyze to produce hydroxide ions when dissolved in water are alkali salts, whilst those that hydrolyze to produce hydronium ions in water are acidic salts. Neutral salts are those salts that are neither acidic nor basic, zwitterions contain an anionic centre and a cationic centre in the same molecule, but are not considered to be salts. Examples of zwitterions include amino acids, many metabolites, peptides, usually, non-dissolved salts at standard conditions for temperature and pressure are solid, but there are exceptions. Molten salts and solutions containing dissolved salts are called electrolytes, as they are able to conduct electricity. As observed in the cytoplasm of cells, in blood, urine, plant saps and mineral waters, therefore, their salt content is given for the respective ions. Salts can appear to be clear and transparent, opaque, and even metallic, in many cases, the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the boundaries, larger crystals tend to be transparent. The color of the salt is due to the electronic structure in the d-orbitals of transition elements or in the conjugated organic dye framework. Different salts can elicit all five basic tastes, e. g. salty, sweet, sour, bitter, and umami or savory. Salts of strong acids and strong bases are non-volatile and odorless and that slow, partial decomposition is usually accelerated by the presence of water, since hydrolysis is the other half of the reversible reaction equation of formation of weak salts. Many ionic compounds can be dissolved in water or other similar solvents, the exact combination of ions involved makes each compound have a unique solubility in any solvent. The solubility is dependent on how well each ion interacts with the solvent, for example, all salts of sodium, potassium and ammonium are soluble in water, as are all nitrates and many sulfates – barium sulfate, calcium sulfate and lead sulfate are examples of exceptions. However, ions that bind tightly to each other and form highly stable lattices are less soluble, for example, most carbonate salts are not soluble in water, such as lead carbonate and barium carbonate. Some soluble carbonate salts are, sodium carbonate, potassium carbonate, solid salts do not conduct electricity. Moreover, solutions of salts also conduct electricity, the name of a salt starts with the name of the cation followed by the name of the anion. Salts are often referred to only by the name of the cation or by the name of the anion. g
27.
Ammonium carbonate
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Ammonium carbonate is a salt with the chemical formula 2CO3. Since it readily degrades to ammonia and carbon dioxide upon heating, it is used as a leavening agent. It is also known as ammonia and was a predecessor to the more modern leavening agents baking soda. It is a component of what was known as sal volatile. Ammonium carbonate is produced by combining carbon dioxide and aqueous ammonia, about 7000 tons/year were produced as of 1997. Ammonium carbonate slowly decomposes at standard temperature and pressure through two pathways, thus any initially pure sample of ammonium carbonate will soon become a mixture including various byproducts. It also serves as an acidity regulator and has the E number E503 and it can be replaced with baking powder, but this may affect both the taste and texture of the finished product. Mr. Accum, in his Treatise on Culinary Poisons, has stigmatized this process as fraudulent, Ammonium carbonate is the main component of smelling salts, although the commercial scale of their production is small. Buckleys cough syrup from Canada today uses ammonium carbonate as an active ingredient intended to relieve symptoms of bronchitis. It is also used as an emetic and it is also found in smokeless tobacco products, such as Skoal. It is also used in solution as a photographic lens cleaning agent. Sal ammoniac, the form of ammonium chloride Ammonium nitrate Ammonium bicarbonate
28.
Ammonium nitrate
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Ammonium nitrate is a chemical compound, the nitrate salt of the ammonium cation. It has the chemical formula NH4NO3, simplified to N2H4O3 and it is a white crystal solid and is highly soluble in water. It is predominantly used in agriculture as a high-nitrogen fertilizer and its other major use is as a component of explosive mixtures used in mining, quarrying, and civil construction. It is the constituent of ANFO, a popular industrial explosive which accounts for 80% of explosives used in North America. Many countries are phasing out its use in consumer applications due to concerns over its potential for misuse, ammonium nitrate was mined there in the past, but virtually 100% of the chemical now used is synthetic. This reaction is violent owing to its exothermic nature. After the solution is formed, typically at about 83% concentration, the AN melt is then made into prills or small beads in a spray tower, or into granules by spraying and tumbling in a rotating drum. The prills or granules may be dried, cooled. These prills or granules are the typical AN products in commerce, the ammonia required for this process is obtained by the Haber process from nitrogen and hydrogen. Ammonia produced by the Haber process is oxidized to nitric acid by the Ostwald process, transformations of the crystal states due to changing conditions affect the physical properties of ammonium nitrate. Ammonium nitrate is an important fertilizer with the NPK rating 34-0-0 and it is less concentrated than urea, giving ammonium nitrate a slight transportation disadvantage. Ammonium nitrates advantage over urea is that it is more stable, during warm weather it is best to apply urea soon before rain is expected or to cover it with soil to minimize nitrogen loss. ANFO is a mixture of 94% ammonium nitrate and 6% fuel oil used as a bulk industrial explosive. Ammonium nitrate-based explosives were used in the Oklahoma City Bombing in 1995 and 2011 Delhi bombings, the 2013 Hyderabad blasts, due to these bans, Potassium chlorate — the stuff that makes matches catch fire — has surpassed fertilizer as the explosive of choice for insurgents. Ammonium nitrate is used in instant cold packs, as its dissolution in water is highly endothermic. Health and safety data are shown on the safety data available from suppliers. Heating or any ignition source may cause violent combustion or explosion, ammonium nitrate reacts with combustible and reducing materials as it is a strong oxidant. Although it is used for fertilizer, it can be used for explosives
29.
Solubility
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Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent. The solubility of a substance depends on the physical and chemical properties of the solute and solvent as well as on temperature, pressure. The solubility of a substance is a different property from the rate of solution. Most often, the solvent is a liquid, which can be a substance or a mixture. One may also speak of solid solution, but rarely of solution in a gas, the extent of solubility ranges widely, from infinitely soluble such as ethanol in water, to poorly soluble, such as silver chloride in water. The term insoluble is often applied to poorly or very poorly soluble compounds, a common threshold to describe something as insoluble is less than 0.1 g per 100 mL of solvent. Under certain conditions, the solubility can be exceeded to give a so-called supersaturated solution. Metastability of crystals can also lead to apparent differences in the amount of a chemical that dissolves depending on its form or particle size. A supersaturated solution generally crystallises when seed crystals are introduced and rapid equilibration occurs, phenylsalicylate is one such simple observable substance when fully melted and then cooled below its fusion point. Solubility is not to be confused with the ability to dissolve a substance, for example, zinc dissolves in hydrochloric acid as a result of a chemical reaction releasing hydrogen gas in a displacement reaction. The zinc ions are soluble in the acid, the smaller a particle is, the faster it dissolves although there are many factors to add to this generalization. Crucially solubility applies to all areas of chemistry, geochemistry, inorganic, physical, organic, in all cases it will depend on the physical conditions and the enthalpy and entropy directly relating to the solvents and solutes concerned. By far the most common solvent in chemistry is water which is a solvent for most ionic compounds as well as a range of organic substances. This is a factor in acidity/alkalinity and much environmental and geochemical work. According to the IUPAC definition, solubility is the composition of a saturated solution expressed as a proportion of a designated solute in a designated solvent. Solubility may be stated in units of concentration such as molarity, molality, mole fraction, mole ratio, mass per volume. Solubility occurs under dynamic equilibrium, which means that solubility results from the simultaneous and opposing processes of dissolution, the solubility equilibrium occurs when the two processes proceed at a constant rate. The term solubility is used in some fields where the solute is altered by solvolysis
30.
Ammonium hexachloroplatinate
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Ammonium hexachloroplatinate, also known as ammonium chloroplatinate, is the inorganic compound with the formula 2. It is an example of a soluble platinum salt that is not hygroscopic. It forms intensely yellow solutions in water, in the presence of 1M NH4Cl, its solubility is only 0.0028 g/100 mL. The compound consists of separate tetrahedral ammonium cations and octahedral 2− anions and it is usually generated as a fine yellow precipitate by treating a solution of hexachloroplatinic acid with a solution of an ammonium salt. The complex is so poorly soluble that this step is employed in the isolation of platinum from ores, as analyzed by X-ray crystallography, the salt crystallizes in a cubic motif reminiscent of the fluorite structure. The NH4+ centers are bonded to the chloride ligands. Ammonium hexachloroplatinate is used in platinum plating, heating 2 under a stream of hydrogen at 200 °C produces platinum sponge. Treating this with chlorine gives H2PtCl6
31.
Amalgam (chemistry)
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An amalgam is an alloy of mercury with another metal. Almost all metals can form amalgams with mercury, the exceptions being iron, platinum, tungsten. Silver-mercury amalgams are important in dentistry, and gold-mercury amalgam is used in the extraction of gold from ore, zinc amalgam finds use in organic synthesis, e. g. for the Clemmensen Reduction. It is the agent in the Jones reductor, used in analytical chemistry. For the alkali metals, amalgamation is exothermic, and distinct chemical forms can be identified, such as KHg, KHg is a gold-coloured compound with a melting point of 178 °C, and KHg2 a silver-coloured compound with a melting point of 278 °C. These amalgams are very sensitive to air and water, but can be worked with under dry nitrogen, the Hg-Hg distance is around 300 picometres, Hg-K around 358 pm. Phases K5Hg7 and KHg11 are also known, rubidium, strontium and barium undecamercurides are known, sodium amalgam is produced as a by product of the chloralkali process and used as an important reducing agent in organic and inorganic chemistry. With water it decomposes into concentrated sodium hydroxide solution, hydrogen and mercury, if absolutely water-free alcohol is used instead of water, an alkoxide of sodium is produced instead of the alkali solution. Aluminium can form an amalgam through a reaction with mercury, aluminium amalgam may be prepared by either grinding aluminium pellets or wire in mercury, or by allowing aluminium wire or foil to react with a solution of mercuric chloride. This amalgam is used as a reagent to reduce compounds, such as the reduction of imines to amines, the aluminium is the ultimate electron donor, and the mercury serves to mediate the electron transfer. The reaction itself and the waste from it contain mercury, so safety precautions. As an environmentally friendlier alternative, hydrides or other reducing agents can often be used to accomplish the same synthetic result, another environmentally friendly alternative is an alloy of aluminium and gallium which similarly renders the aluminium more reactive by preventing it from forming an oxide layer. Tin amalgam was used in the middle of the 19th century as a mirror coating. A variety of amalgams are known that are of interest mainly in the research context, ammonium amalgam is a grey, soft, spongy mass discovered in 1808 by Humphry Davy and Jöns Jakob Berzelius. Dentistry has used alloys of mercury with metals such as silver, copper, indium, tin, Amalgam is an excellent and versatile restorative material and is used in dentistry for a number of reasons. It is inexpensive and relatively easy to use and manipulate during placement, it remains soft for a time so it can be packed to fill any irregular volume. Amalgam possesses greater longevity compared to other direct restorative materials. However, this difference has decreased with continual development of composite resins, Amalgam is typically compared to resin-based composites because many applications are similar and many physical properties and costs are comparable
32.
Electrolysis
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In chemistry and manufacturing, electrolysis is a technique that uses a direct electric current to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from naturally occurring sources such as using an electrolytic cell. The voltage that is needed for electrolysis to occur is called the decomposition potential, nevertheless, electrolysis, as a tool to study chemical reactions and obtain pure elements, precedes the coinage of the term and formal description by Faraday. 1785 – Martinus van Marums electrostatic generator was used to reduce tin, zinc,1800 – William Nicholson and Anthony Carlisle, decomposed water into hydrogen and oxygen. 1808 – Potassium, sodium, barium, calcium and magnesium were discovered by Sir Humphry Davy using electrolysis,1821 – Lithium was discovered by the English chemist William Thomas Brande, who obtained it by electrolysis of lithium oxide. 1833 – Michael Faraday develops his two laws of electrolysis, and provides an explanation of his laws. 1875 – Paul Émile Lecoq de Boisbaudran discovered gallium using electrolysis,1886 – Fluorine was discovered by Henri Moissan using electrolysis. The main components required to achieve electrolysis are, An electrolyte, a substance, frequently an ion-conducting polymer that contains free ions, if the ions are not mobile, as in a solid salt then electrolysis cannot occur. A direct current electrical supply, provides the necessary to create or discharge the ions in the electrolyte. Electric current is carried by electrons in the external circuit, two electrodes, electrical conductors that provide the physical interface between the electrolyte and the electrical circuit that provides the energy. Electrodes of metal, graphite and semiconductor material are widely used, choice of suitable electrode depends on chemical reactivity between the electrode and electrolyte and manufacturing cost. The key process of electrolysis is the interchange of atoms and ions by the removal or addition of electrons from the external circuit, the desired products of electrolysis are often in a different physical state from the electrolyte and can be removed by some physical processes. For example, in the electrolysis of brine to produce hydrogen and chlorine and these gaseous products bubble from the electrolyte and are collected. Each electrode attracts ions that are of the opposite charge, positively charged ions move towards the electron-providing cathode. Negatively charged ions move towards the electron-extracting anode, in this process electrons are either absorbed or released. Neutral atoms gain or lose electrons and become charged ions that pass into the electrolyte. The formation of uncharged atoms from ions is called discharging, when an ion gains or loses enough electrons to become uncharged atoms, the newly formed atoms separate from the electrolyte. Positive metal ions like Cu++ deposit onto the cathode in a layer, the terms for this are electroplating, electrowinning, and electrorefining
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Lone pair
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In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom and is sometimes called a non-bonding pair. Lone pairs are found in the outermost electron shell of atoms and they can be identified by using a Lewis structure. Electron pairs are therefore considered lone pairs if two electrons are paired but are not used in chemical bonding, thus, the number of lone pair electrons plus the number of bonding electrons equals the total number of valence electrons around an atom. Lone pair is a used in VSEPR theory which explains the shapes of molecules. They are also referred to in the chemistry of Lewis acids, however, not all non-bonding pairs of electrons are considered by chemists to be lone pairs. Examples are the transition metals where the non-bonding pairs do not influence molecular geometry and are said to be stereochemically inactive, in VSEPR theory the electron pairs on the oxygen atom in water form the vertices of a tetrahedron with the lone pairs on two of the four vertices. The H–O–H bond angle is with 104. 5°, less than the 109° predicted for an angle. The pairs often exhibit a negative polar character with their charge density and are located closer to the atomic nucleus on average compared to the bonding pair of electrons. The presence of a lone pair decreases the angle between the bonding pair of electrons, due to their high electric charge which causes great repulsion between the electrons. They are also used in the formation of a dative bond, for example, the creation of the hydronium ion occurs when acids are dissolved in water and is due to the oxygen atom donating a lone pair to the hydrogen ion. This can be more clearly when looked at it in two more common molecules. For example, in carbon dioxide, the atoms are on opposite sides of the carbon. Due to the force of the oxygen atoms lone pairs. This is an illustration of the VSEPR theory, lone pairs can make a contribution to a molecules dipole moment. NH3 has a moment of 1.47 D. There is also an associated with the lone pair and this reinforces the contribution made by the polar covalent N-H bonds to ammonias dipole moment. In contrast to NH3, NF3 has a lower dipole moment of 0.24 D. A stereochemically active lone pair is expected for divalent lead
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Nitrogen
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Nitrogen is a chemical element with symbol N and atomic number 7. It was first discovered and isolated by Scottish physician Daniel Rutherford in 1772, although Carl Wilhelm Scheele and Henry Cavendish had independently done so at about the same time, Rutherford is generally accorded the credit because his work was published first. Nitrogen is the lightest member of group 15 of the periodic table, the name comes from the Greek πνίγειν to choke, directly referencing nitrogens asphyxiating properties. It is an element in the universe, estimated at about seventh in total abundance in the Milky Way. At standard temperature and pressure, two atoms of the element bind to form dinitrogen, a colourless and odorless diatomic gas with the formula N2, dinitrogen forms about 78% of Earths atmosphere, making it the most abundant uncombined element. Nitrogen occurs in all organisms, primarily in amino acids, in the nucleic acids, the human body contains about 3% nitrogen by mass, the fourth most abundant element in the body after oxygen, carbon, and hydrogen. The nitrogen cycle describes movement of the element from the air, into the biosphere and organic compounds, many industrially important compounds, such as ammonia, nitric acid, organic nitrates, and cyanides, contain nitrogen. The extremely strong bond in elemental nitrogen, the second strongest bond in any diatomic molecule. Synthetically produced ammonia and nitrates are key industrial fertilisers, and fertiliser nitrates are key pollutants in the eutrophication of water systems. Apart from its use in fertilisers and energy-stores, nitrogen is a constituent of organic compounds as diverse as Kevlar used in high-strength fabric, Nitrogen is a constituent of every major pharmacological drug class, including antibiotics. Many notable nitrogen-containing drugs, such as the caffeine and morphine or the synthetic amphetamines. Nitrogen compounds have a long history, ammonium chloride having been known to Herodotus. They were well known by the Middle Ages, alchemists knew nitric acid as aqua fortis, as well as other nitrogen compounds such as ammonium salts and nitrate salts. The mixture of nitric and hydrochloric acids was known as aqua regia, celebrated for its ability to dissolve gold, the discovery of nitrogen is attributed to the Scottish physician Daniel Rutherford in 1772, who called it noxious air. Though he did not recognise it as a different chemical substance, he clearly distinguished it from Joseph Blacks fixed air. The fact that there was a component of air that does not support combustion was clear to Rutherford, Nitrogen was also studied at about the same time by Carl Wilhelm Scheele, Henry Cavendish, and Joseph Priestley, who referred to it as burnt air or phlogisticated air. Nitrogen gas was inert enough that Antoine Lavoisier referred to it as air or azote, from the Greek word άζωτικός. In an atmosphere of nitrogen, animals died and flames were extinguished
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Proton
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A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and mass slightly less than that of a neutron. Protons and neutrons, each with masses of one atomic mass unit, are collectively referred to as nucleons. One or more protons are present in the nucleus of every atom, the number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number. Since each element has a number of protons, each element has its own unique atomic number. The word proton is Greek for first, and this name was given to the nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the nucleus could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a particle, and hence a building block of nitrogen. In the modern Standard Model of particle physics, protons are hadrons, and like neutrons, although protons were originally considered fundamental or elementary particles, they are now known to be composed of three valence quarks, two up quarks and one down quark. The rest masses of quarks contribute only about 1% of a protons mass, the remainder of a protons mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. At sufficiently low temperatures, free protons will bind to electrons, however, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, the result is a protonated atom, which is a chemical compound of hydrogen. In vacuum, when electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom. Such free hydrogen atoms tend to react chemically with other types of atoms at sufficiently low energies. When free hydrogen atoms react with other, they form neutral hydrogen molecules. Protons are spin-½ fermions and are composed of three quarks, making them baryons. Protons have an exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm. Protons and neutrons are both nucleons, which may be together by the nuclear force to form atomic nuclei. The nucleus of the most common isotope of the atom is a lone proton
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Covalent bond
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A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, for many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration. Covalent bonding includes many kinds of interactions, including σ-bonding, π-bonding, metal-to-metal bonding, agostic interactions, bent bonds, the term covalent bond dates from 1939. In the molecule H2, the atoms share the two electrons via covalent bonding. Covalency is greatest between atoms of similar electronegativities, thus, covalent bonding does not necessarily require that the two atoms be of the same elements, only that they be of comparable electronegativity. Covalent bonding that entails sharing of electrons more than two atoms is said to be delocalized. The term covalence in regard to bonding was first used in 1919 by Irving Langmuir in a Journal of the American Chemical Society article entitled The Arrangement of Electrons in Atoms and Molecules. Langmuir wrote that we shall denote by the term covalence the number of pairs of electrons that an atom shares with its neighbors. The idea of covalent bonding can be traced several years before 1919 to Gilbert N. Lewis and he introduced the Lewis notation or electron dot notation or Lewis dot structure, in which valence electrons 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 bonds and triple bonds. An alternative form of representation, not shown here, has bond-forming electron pairs represented as solid lines, Lewis proposed that an atom forms enough covalent bonds to form a full outer electron shell. In the diagram of methane shown here, the atom has a valence of four and is, therefore, surrounded by eight electrons, four from the carbon itself. Each hydrogen has a valence of one and is surrounded by two electrons – its own one electron plus one from the carbon, walter Heitler and Fritz London are credited with the first successful quantum mechanical explanation of a chemical bond in 1927. Their work was based on the valence bond model, which assumes that a bond is formed when there is good overlap between the atomic orbitals of participating atoms. Atomic orbitals have specific directional properties leading to different types of covalent bonds, sigma bonds are the strongest covalent bonds and are due to head-on overlapping of orbitals on two different atoms. A single bond is usually a σ bond, pi bonds are weaker and are due to lateral overlap between p orbitals. A double bond between two given atoms consists of one σ and one π bond, and a bond is one σ. Covalent bonds are also affected by the electronegativity of the atoms which determines the chemical polarity of the bond
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Tetrahedral molecular geometry
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In a tetrahedral molecular geometry, a central atom is located at the center with four substituents that are located at the corners of a tetrahedron. The bond angles are cos−1 =109.4712206. ° ≈109. 5° when all four substituents are the same, the perfectly symmetrical tetrahedron belongs to point group Td, but most tetrahedral molecules have lower symmetry. Aside from virtually all saturated organic compounds, most compounds of Si, Ge, often tetrahedral molecules feature multiple bonding to the outer ligands, as in xenon tetroxide, the perchlorate ion, the sulfate ion, the phosphate ion. Thiazyl trifluoride is tetrahedral, featuring a triple bond. Other molecules have an arrangement of electron pairs around a central atom. However the usual classification considers only the atoms and not the lone pair. The H–N–H angles are 107°, contracted from 109.5 and this difference is attributed to the influence of the lone pair which exerts a greater repulsive influence than a bonded atom. Again the geometry is widespread, particularly so for complexes where the metal has d0 or d10 configuration, illustrative examples include tetrakispalladium, nickel carbonyl, and titanium tetrachloride. Many complexes with incompletely filled d-shells are often tetrahedral, e. g. the tetrahalides of iron, cobalt, however in liquid water or in ice, the lone pairs form hydrogen bonds with neighboring water molecules. The most common arrangement of hydrogen atoms around an oxygen is tetrahedral with two hydrogen atoms bonded to oxygen and two attached by hydrogen bonds. Since the hydrogen bonds vary in many of these water molecules are not symmetrical. Many compounds and complexes adopt bitetrahedral structures, in this motif, the two tetrahedra share a common edge. The inorganic polymer silicon disulfide features a chain of edge-shared tetrahedra. Inversion of tetrahedral occurs widely in organic and main group chemistry, the so-called Walden inversion illustrates the stereochemical consequences of inversion at carbon. Nitrogen inversion in ammonia also entails transient formation of planar NH3, geometrical constraints in a molecule can cause a severe distortion of idealized tetrahedral geometry. In compounds featuring inverted tetrahedral geometry at a carbon atom, all four groups attached to carbon are on one side of a plane. The carbon atom lies at or near the apex of a pyramid with the other four groups at the corners. The simplest examples of organic molecules displaying inverted tetrahedral geometry are the smallest propellanes, such as propellane, or more generally the paddlanes, such molecules are typically strained, resulting in increased reactivity
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Isoelectronicity
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The species concerned are termed isoelectronic. This definition is sometimes termed valence isoelectronicity, in contrast with various alternatives, at one extreme these require identity of the total electron count and with it the entire electron configuration. More usually, alternatives are broader, and may extend to allowing different numbers of atoms in the species being compared, the importance of the concept lies in identifying significantly related species, as pairs or series. Isoelectronic species can be expected to show consistency and predictability in their properties. Electron-density calculations have been performed on many common substances, resulting in reaction predictions, identifying a new, rare or odd compound as isoelectronic with one already characterised offers clues to possible properties and reactions. The N atom and the O+ radical ion are isoelectronic because each has five electrons in the electronic shell. Similarly, the cations K+, Ca2+, and Sc3+ and the anions Cl−, S2−, in such monatomic cases, there is a clear trend in the sizes of such species, with atomic radius decreasing as charge increases. CO, CN−, N2 and NO+ are isoelectronic because each has two nuclei and 10 valence electrons, with each considered to have 5 of them. CO2, FCN, N2O, NO2+, N3−, and NCO− are all isoelectronic, isoelectronicity leads to the concept of hydrogen-like atoms, ions with one electron which are thus isoelectronic with hydrogen. The uncharged H 2C=C=O molecule and the zwitterionic H 2C=N+=N− molecule are isoelectronic, CH 3COCH3 and CH 3N 2CH3 are not isoelectronic. The amino acids tellurocysteine, selenocysteine, cysteine and serine are also considered isoelectronic
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Borohydride
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Borohydride refers to the anion BH4− and its salts. Borohydride is also the used for compounds containing BH4−nXn−, for example cyanoborohydride and triethylborohydride. Such compounds find use as reducing agents in organic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, tetrahydroborates are also of academic and industrial interest in inorganic chemistry. Alkali metal borohydrides were described in 1940 by Hermann Irving Schlesinger and they synthesized lithium borohydride from diborane,2 MH + B2H6 →2 M Current methods involve reduction of trimethyl borate with sodium hydride. The borohydride anion has a structure with boron in the center. In metal complexes, the ion is bound to the metal by means of bridging hydrogen atoms. The preferred coordination mode is strongly affected by the nature of the metal, sodium borohydride is the borohydride that is produced on the largest scale industrially, estimated at 5000 tons/y in 2002. The main use is for the reduction of sulfur dioxide to give sodium dithionite, because of their high hydrogen content, borohydride complexes and salts have been of interest in the context of hydrogen storage. Reminiscent of related work on ammonia borane, challenges are associated with slow kinetics, borohydrides can serve as ligands for many metals, and there is a large coordination chemistry of the borohydride ion. In most such compounds, the BH4− ligand is bidentate, Some binary borohydrides are highly volatile. Metal borohydride complexes can often be prepared by a simple salt elimination reaction, when the borohydride complex is volatile, this decomposition pathway is the basis of chemical vapor deposition, a way of depositing thin films of metal borides. The diboride films can be deposited on a variety of substances including glass, copper, aluminum, and steel
