In chemistry, the compound ionic is a chemical compound consisting of ions united by an electrostatic force called ionic bond. This compound is neutral in its entirety, but consists of positively charged ions called cations and negatively charged ions called anions. These can be simple ions such as sodium (Na ) and chloride (Cl - ) in sodium chloride, or polyatomic species such as ammonium ( NH
4 ) and carbonate ( CO 2 -
3 ) ions in ammonium carbonate. Individual ions in ionic compounds usually have some nearest neighbors, so are not considered as part of the molecule, but as part of a continuous three-dimensional network, usually in a crystal structure.
Ionic compounds containing hydrogen ions (H ) are classified as acids, and contain basic hydroxide ions (OH - ) or oxides (O 2 - ) are classified as base. Ionic-ionic compounds are also known as salts and can be formed by acid-base reactions. The ionic compound may also be produced from its constituent ion by evaporating its solvent, precipitate, freezing, solid-state reaction, or reactive metal transfer reaction with a non-metallic reactive, such as a halogen gas.
Ionic compounds usually have high melting and boiling points, and are hard and brittle. As solids they almost always isolate electricity, but when it melts or dissolves they become highly conductive, because the ions are mobilized.
Video Ionic compound
History of discovery
The word ion is the Greek ??? , ion , "going", participle present of ????? , ienai , "to go". The term was introduced by the English physicist and chemist Michael Faraday in 1834 for an unknown species that goes from one electrode to another via an aqueous medium.
In 1913 the structure of sodium chloride crystals was determined by William Henry Bragg and William Lawrence Bragg. It reveals that there are six closest neighbors adjacent to each atom, indicating that the constituents are not arranged in finite molecules or aggregates, but as networks with a crystal frame remotely. Many other inorganic compounds are also found to have similar structural features. These compounds were soon described as being composed of ions rather than neutral atoms, but evidence of this hypothesis was not discovered until the mid-1920s, when an X-ray reflection experiment (which detected electron density) was performed.
The main contributors to the development of theoretical treatment of the ionic crystal structure are Max Born, Fritz Haber, Alfred Landà ©  ©, Erwin Madelung, Paul Peter Ewald, and Kazimierz Fajans. The predicted birth of crystal energy is based on the assumption of ionic constituents, which indicates good correspondence to thermochemical measurements, further supporting assumptions.
Maps Ionic compound
Formation
The ionic compound may be produced from its constituent ion by evaporation, precipitation, or freezing. Reactive metals such as alkali metals can react directly with highly electronegative halogen gases to form ionic products. They can also be synthesized as a product of high temperature reactions between solids.
If the ionic compound dissolves in a solvent, it can be obtained as a solid compound by evaporating the solvent from this electrolyte solution. When the solvent is evaporated, the ions do not enter the vapor, but remain in the remaining solution, and when they become sufficiently concentrated, nucleation occurs, and they crystallize into ionic compounds. This process occurs widely in nature, and is a means of evaporative mineral formation. Another method of recovering the compound from the solution involves saturating the solution at high temperatures and then reducing the solubility by reducing the temperature until the solution is saturated and the nucleation of the solid compound.
The insoluble ionic compounds can be precipitated by mixing two solutions, one with a cation and one with anions in it. Since all solutions are electrically neutral, the two mixed solutions must also contain the contraversy of the opposite charge. To ensure that this does not contaminate the deposited ionic compounds, it is important to ensure that they do not precipitate. If both solutions have hydrogen ions and hydroxide ions as opposed, they will react with each other in so-called acid-base reactions or neutralization reactions to form water. Alternately, counterions can be selected to ensure that even when combined into one solution they will remain soluble as audience ions.
If the solvent is water either in the evaporation or precipitation method, in many cases the ionic crystals formed also include water crystallisation, so this product is known as hydrate, and can have very different chemical properties.
The liquid salt will solidify on cooling to below their freezing point. This is sometimes used for the synthesis of solid-state ionic compounds of solid reactants, which first melt together. In other cases, the solid reactants need not be melted, but may react through solid state reaction routes. In this method, the reactants are repeatedly finely ground into a paste, and then heated to a temperature where the ions in the neighboring reactant can diffuse together during the time the reactant mixture remains in the oven. Other synthetic routes use solid precursors with correct stoichiometric ratios of non-volatile ions, which are heated to expel other species.
In some reactions between highly reactive metals (usually from Group 1 or Group 2) and highly electronegative halogen gases, or water, the atoms can be ionized by electron transfer, a process that is thermodynamically understood using the Born-Haber cycle.
Bond
Ions in ionic compounds are primarily held together by the electrostatic forces between the charge distributions of these bodies, and in particular the ionic bonds resulting from the long-range Coulomb appeal between the net negative charge of the anions and the net positive charge of the cations. There is also an interesting little addition to the van der Waals interaction that only accounts for about 1-2% of the cohesive energy for small ions. When a pair of ions is close enough for their outermost electron shell (most of the simple ions have a closed skin) to overlap, a short-run repulsion force occurs, because of the Pauli exclusion principle. The balance between these forces leads to potential energy well with minimum energy when the nucleus is separated by a certain equilibrium distance.
If the electronic structure of two interacting objects is affected by the presence of each other, covalent (non-ionic) interactions also contribute to the overall energy of the compound formed. The ionic compounds are rarely pure ionic, ie merely united by the electrostatic forces. The bond between even the most electronegative/electropositive pair as in cesium fluoride shows a small degree of covalency. In contrast, covalent bonds between different atoms often exhibit some separation of charge and may be considered to have partial ionic characters. The circumstances in which a compound will have an ionic or covalent character can usually be understood using the Fajans rule, which only uses the charge and size of each ion. According to this rule, the compound with the most ionic character will have a large positive ion with a low charge, tied to a small negative ion with a high charge. More general HSAB theory can be applied, wherein the compound with the most ionic character is composed of hard acid and hard base: small, high-charged ions with high differences in electronegativity between anions and cations. This difference in electronegativity means that the separation of charge, and the resulting dipole moment, is maintained even when the ions are in contact (the excess electrons in the anion are not transferred or polarized to neutralize the cations).
Structure
Ions are usually packed in very regular crystal structures, in settings that minimize lattice energy (maximize attraction and minimize repulsion). Lattice energy is the sum of the interactions of all sites with all the other sites. For non-polarized spherical ions only charge and distance to determine the energy of the electrostatic interaction. For certain ideal crystal structures, all geometric distances are related to the smallest internuclear distance. Thus for every possible crystal structure, the total electrostatic energy can be connected to the electrostatic energy of unit charges at the nearest neighboring distance to a multiplication constant called the Madelung constant which can be calculated efficiently by the number of Ewald. When a reasonable form is assumed for additional repulsive energy, the total lattice energy can be modeled using the Born-LandÃÆ'Â ©, Born-Mayer equation, or in the absence of structural information, the Kapustinskii equation.
Using a simpler approach of ions as impenetrable hard balls, the anion arrangement in these systems is often associated with the arrangement of covert balls, with cations occupying the tetrahedral or octahedral gaps. Depending on the stoichiometry of ionic compounds, and coordination (mainly determined by the radius ratio) of the cations and anions, various structures are generally observed, and rationalized theoretically by Pauling rules.
In some cases the anion takes a simple cubic packaging, and the resulting general structure observed is:
Some ionic liquids, especially with anion or cation mixtures, can be cooled rapidly enough so that there is not enough time for the formation of crystals, thus forming ionic glass (without a long sequence).
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In ionic crystals, there will usually be some defect points, but to maintain electroneutrality, these defects come in pairs. Frenkel defects consist of cations paired with interstitial cations and can be produced anywhere in most crystals, occurring most often in compounds with low coordination amounts and much smaller cations than anions. Schottky defects consist of one void of each type, and are produced on the surface of the crystal, occurring most often in compounds with high coordination numbers and when the anions and cations are of the same size. If the cation has some possible oxidation state, then it is possible for the cation vacancy to compensate for electron deficiencies at the cation site with higher oxidation numbers, resulting in non-stoichiometric compounds. Another non-stoichiometric possibility is the formation of F-center, the free electrons occupying the anion position. When a compound has three or more ionic components, even more types of defects are possible. All defects of this point can be generated through thermal vibrations and have an equilibrium concentration. Because they are very expensive, but entropically advantageous, they occur in larger concentrations at higher temperatures. Once generated, these defective pairs can spread largely independently of each other, by jumping between lattice sites. The mobility of this defect is the source of most transport phenomena within ionic crystals, including diffusion and conductivity of solid state ionic. When vacancies collide with interstitials (Frenkel), they can rejoin and annihilate each other. Similarly vacancies are removed when they reach the crystal surface (Schottky). Defects in crystal structures generally extend lattice parameters, reducing overall crystal density. Defects also produce ions in very different local environments, which cause them to experience different crystal-symmetry fields, especially in the case of different cations swapping the lattice locations. This produces a different separation of the d-electron orbital, so the optical absorption (and hence the color) can change with the concentration of the defect.
Properties
Acidity/alkalinity
Ionic compounds containing hydrogen ions (H ) are classified as acids, and containing electropositive cations and basic anionic hydroxide ions (OH - ) or oxides (O 2 - ) are classified as bases. Other ionic compounds are known as salts and can be formed by acid-base reactions. If the compound is the result of a reaction between a strong acid and a weak base, the result is an acid salt. If it is the result of a reaction between a strong base and a weak acid, the result is a base salt. If it is the result of a reaction between strong acid and a strong base, the result is a neutral salt. Weak acids reacting with weak bases can produce ionic compounds with both conjugate base ions and conjugate acid ions, such as ammonium acetate.
Some ions are classified as amphoters, capable of reacting with acids or bases. This also applies to some compounds with ionic characters, usually oxides or hydroxides of less electropositive metals (so they also have significant covalent character), such as zinc oxide, aluminum hydroxide, aluminum oxide and lead (II) oxide.
Melting and boiling points
Electrostatic forces between particles are strongest when the load is high, and the distance between the ion cores is very small. In this case, the compound generally has a very high melting and boiling point and low vapor pressure. Melting point trends can be better explained when the structure and size ratio of ions is taken into account. Above their melting point, the ionic solid melts and becomes a molten salt (although some ionic compounds such as aluminum chloride and iron (III) chloride exhibit molecular-like structures in the liquid phase). Inorganic compounds with simple ions usually have small ions, and thus have high melting point, as well as solids at room temperature. Some substances with larger ions, however, have melting points below or near room temperature (often defined as up to 100 ° C), and are called ionic liquids. Ions in ionic liquids often have an uneven distribution of charge, or large substituents such as hydrocarbon chains, which also play a role in determining the strength of interactions and melting tendencies.
Even when local structures and bonds of ionic solids are sufficiently disrupted to melt them, there is still a strong long-range electrostatic force from the attraction of holding the liquid together and preventing the boiling ions to form the gas phase. This means that even ionic liquids at room temperature have a low vapor pressure, and require a much higher temperature to boil. The boiling point shows the same trend as the melting point in terms of ion size and other interaction strength. When vaporization, the ions are still not released from each other. For example, in the vapor phase sodium chloride exists as a diatomic "molecule".
Fragility
Most of the ionic compounds are very fragile. Once they reach their limits, they can not change shape easily, because the strict alignment of positive and negative ions must be maintained. Instead the material undergoes fractures through the hemisphere. As the temperature increases (usually close to the melting point) there is brittle transition, and plastic flow becomes possible by the dislocation movement.
Compressibility
The compressibility of an ionic compound is largely determined by its structure, and in particular the coordination number. For example, halides with cesium chloride structures (coordination number 8) are less compressible than sodium chloride structures (coordination number 6), and less than those with coordination numbers 4.
Solubility
When the ionic compound dissolves, individual ions dissociate and dissolve by the solvent and dispersed throughout the resulting solution. Since the ions are released into the solution when dissolved, and can fill, the dissolved ionic compound is the most commonly used strong electrolyte class, and the solution has high electrical conductivity.
The highest solubility in polar solvents (such as water) or ionic liquids, but tends to be low in nonpolar solvents (such as gasoline/petrol). This is mainly because the resulting ion-dipole interactions are significantly stronger than ion-induced dipole interactions, resulting in higher heat of the solution. When the opposite charged ions in a solid ionic lattice are surrounded by opposite poles of the polar molecule, solid ions are pulled out of the lattice and into the liquid. If the solvation energy exceeds the lattice energy, a negative net enthalpy change generates a thermodynamic impulse to remove the ion from its position in the crystal and dissolves in the liquid. In addition, the entropy changes of the solution are usually positive for solid solutes such as ionic compounds, which means that their solubility increases as the temperature increases. There are some unusual ionic compounds such as cerium (III) sulfate, where the entropy changes are negative, because of the extra sequence induced in water in the solution, and the solubility decreases with temperature.
Electrical Conductivity
Although ionic compounds contain atoms or charged groups, they usually do not conduct electricity to a significant degree when the substance is solid. To perform, charged particles must move rather than stationary in the crystal lattice. This is achieved to some extent at high temperatures when defect concentration increases ionic mobility and solid ionic ionic conductivity is observed. When ionic compounds are dissolved in liquids or melted into liquids, they can conduct electricity because the ions are actually moving. The advantage of this conductivity in dissolution or melting is sometimes used as a hallmark of ionic compounds.
In some unusual ionic compounds: fast ionic conductors, and ionic glasses, one or more of the ionic components have significant mobility, allowing conductivity even when the material as a whole remains solid. It often depends heavily on temperature, and may be the result of a high phase change or high defect concentration. These materials are used in all solid-state supercapacitors, batteries, and fuel cells, and in various types of chemical sensors.
Color
The colors of ionic compounds are often different from those of aqueous solutions containing constituent ions, or hydrated forms of the same compound.
Anions in compounds with bonds with most ionic characters tend to be colorless (with absorption bands in the ultraviolet part of the spectrum). In compounds with less ionic characters, their color becomes deeper through yellow, orange, red and black (since the absorption band shifts to a longer wavelength to the visible spectrum).
Simple cation uptake bands shift toward shorter wavelengths as they engage in more covalent interactions. This occurs during the hydration of the metal ions, so that the colorless anhydrous ion compounds with an anion absorbing in the infrared can become colored in solution.
Usage
Ionic compounds have long had various uses and applications. Many minerals are ionic. Humans have been processing regular salt (sodium chloride) for more than 8000 years, using it for the first time as food seasonings and preservatives, and now also in manufacturing, agriculture, water conditioning, ice-free, and many other uses. Many ionic compounds are so widely used in society that they use common names unrelated to their chemical identities. Examples include borax, calomel, milk magnesia, muriatic acid, vitriol oil, saltpeter, and dead lime.
Soluble ionic compounds such as salts can be easily dissolved to provide an electrolyte solution. This is a simple way to control ionic concentration and strength. Concentration of the solute affects many of the colligative properties, including increasing osmotic pressure, and leading to depression of freezing point and boiling point increase. Because the solutes are filled their ions also increase the electrical conductivity of the solution. Increased ionic strength reduces the thickness of the double electrical layer around the colloidal particles, and hence the stability of the emulsion and suspension.
The chemical identity of the added ions is also important in many uses. For example, fluoride-containing compounds are dissolved to supply fluoride ions for water fluoridation.
Solid ionic compounds have long been used as paint pigments, and are resistant to organic solvents, but are sensitive to acidity or alkalinity. Since 1801 pyrotechnicians have described and widely used ionic compounds containing metals as a source of color in fireworks. Under intense heat, electrons in metal ions or small molecules can be excited. These electrons then return to a lower energy state, and release the light with the color spectrum characteristics of the existing species.
In chemistry, ionic compounds are often used as precursors for high-temperature solid-state synthesis.
Many metals are geologically most abundant as ionic compounds in ores. To obtain the elemental material, this ore is processed by fusion or electrolysis, in which a redox reaction occurs (often with a carbon reducing agent) such that the metal ion obtains the electron into a neutral atom.
Nomenclature
According to the nomenclature recommended by IUPAC, ionic compounds are named according to their composition, not their structure. In the simplest case of binary ionic compounds without the possibility of ambiguity about the charge and thus stoichiometry, the common name is written using two words. Cation names (names of unmodified elements for monatomic cations) appear first, followed by the anion name. For example, MgCl 2 is named magnesium chloride, and Na 2 SO 4 is named sodium sulfate ( SO 2 -
4 , sulphate, is an example of a polyatomic ion). To obtain the empirical formula of these names, stoichiometry can be inferred from the charge on the ion, and the overall charge neutrality requirements.
If there are several different cations and/or anions, multiplicative prefixes ( di - , tri - , tetra - , Ã,...) is often required to show relative composition, and then anion cations are listed in alphabetical order. For example, KMgCl 3 is named magnesium potassium trichloride to distinguish it from K 2 MgCl 4 , magnesium dipotassium tetrachloride (note that in both empirical formulas and written names, cations appear in alphabetical order, but the order varies between them because the symbol for potassium is K). When one of the ions already has a multiplicative prefix in its name, alternate multiplicative prefix ( bis - , tris - , tetrakis - ,...) is used. For example, Ba (BrF 4 ) 2 is named barium bis (tetrafluoridobromate).
Compounds containing one or more elements that can exist in various oxidation charges will have stoichiometry that depends on the existing oxidation state, to ensure overall neutrality. This can be indicated on the name by determining the oxidation state of an element present, or the charge on the ion. Due to the risk of ambiguity in allocating oxidation numbers, IUPAC prefers direct indication of ionic charge. It is written as an Arabic integer followed by a sign (... Ã ,, 2-, 1-, 1, 2, Ã,...) in parentheses immediately after the cation name (without spaces separating it). For example, FeSO 4 is given the name of iron (2) sulfate (with charge 2 on Fe > 2 ion balancing the 2- charge on sulfate ions), while Fe 2 3 is named iron (3) sulfate (because two iron ions in each unit of formula each have a cost of 3, to balance 2- in each - of three sulfate ions). Stock nomenclature, still commonly used, writes oxidation numbers in Roman numerals (... Ã ,, -II, -I, 0, I, II, Ã,...). So the examples given above will be named iron (II) sulfate and iron (III) sulphate respectively. For simple ions, ionic charges and oxidation numbers are identical, but for polyatomic ions they are often different. For example, uranil ions (2), UO 2
2 , has uranium in oxidation state 6, so will be called dioxouranium ion (VI) in the Stock nomenclature. Older naming systems for metal cations are also widely used, adding the -ous and -ic suffix to the Latin root of the name, to give a special name for the low oxidation rate and high. For example, this scheme uses "iron" and "iron", for iron (II) and iron (III) respectively, so the above given example is classically named ferrous sulfate and ferric sulfate.
See also
- Bond in solid form
- Ioliomics
- Ionic bond
Note
References
Bibliography
Source of the article : Wikipedia
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