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In chemistry, an ionic compound is a chemical compound in which ions are held together in a structure by ionic bonds. The positively charged ions are called cations and the negatively charged ions are called anions. These can be simple ions as in the sodium (Na+) and chloride (Cl-) in sodium chloride. Many ionic compounds are composed of more complex polyatomic species such as the carbonate (CO32-) in calcium carbonate. Individual ions can have multiple nearest neighbours, so are not considered to be part of molecules, but instead part of a continuous network. Ionic compounds have high melting and boiling points, and they are hard and very brittle. As solids they are almost always electrically insulating, but when melted or dissolved they become highly conductive, because the ions are mobilized.
Ions in ionic compounds are held together by the electrostatic forces between oppositely charged bodies. Ionic compounds are rarely purely ionic, i.e. held together only by electrostatic forces. The bonds between the most electronegative/electropositive pairs such as caesium fluoride exhibit a degree of covalency. Similarly, covalent compounds often exhibit charge separations. See also HSAB theory.
Ionic compounds have very strong electrostatic bonds between particles. As a result, they generally have very high melting and boiling points and a low vapour pressure. They also have good electrical conductivity when molten or in an aqueous solution. Ionic inorganic compounds typically have high melting points so are solids at room temperature and usually form crystals. Unlike organic compounds they do not char nor ignite. On the other hand organic compounds have low melting points, most of them are insoluble in water, and characteristically they ignite quite easily.
The ions produced by electron transfer attract each other by electrostatic attraction and this creates an ionic bond.
Ions typically pack into extremely regular crystalline structures, in an arrangement that minimizes the Coulomb energy (maximizing attractions and minimizing repulsions). For spherical ions (including all simple ions), the arrangement of anions in these systems are often related to close-packed arrangements of spheres, with the cations occupying interstices. Depending on the stoichiometry of the ionic compound, and the coordination (principally determined by the size ratio) of cations and anions, a variety of structures are commonly observed.
|Stoichiometry||Cation:anion coordination||Interstitial sites occupied||Cubic close packing||Hexagonal close packing|
|MX||6:6||all octahedral||sodium chloride||nickel arsenide|
|4:4||alternate tetrahedral||zinc blende||wurtzite|
|6:3||half octahedral (alternate layers fully occupied)||cadmium chloride||cadmium iodide|
|MX3||6:2||one-third octahedral||chromium(III) chloride||bismuth iodide|
|AB2O4||one-eighth tetrahedral and one-half octahedral||spinel, inverse spinel||olivine|
In some cases the anions take on a simple cubic packing, and the resulting common structures observed are:
|Stoichiometry||Cation:anion coordination||Interstitial sites occupied||Example structure|
|MX||8:8||all filled||cesium chloride|
|MX2||8:4||half filled||calcium fluoride|
|M2X||4:8||half filled||lithium oxide|
Following the aphorism, "like dissolves like", ionic compounds dissolve most readily in polar solvents (such as water) or ionic liquids. Ionic compounds tend not to dissolve in nonpolar solvents (such as diethyl ether or petrol/gasoline).
When the oppositely charged ions in the solid ionic lattice are surrounded by the opposite pole of a polar molecule, the solid ions are pulled out of the lattice and into the liquid. If the solvation energy exceeds the lattice energy, the energy released in solvation is used to overcome the lattice energy so that the ions are freed from their positions in the crystal and dissolve in the liquid.
Although ionic compounds contain charged atoms or clusters, these materials do not typically conduct electricity. In order to conduct, the charged particles must be mobile rather than stationary in a crystal lattice. When the ionic compounds are dissolved in a liquid or are themselves melted into a liquid, they can conduct electricity because the ions become mobile. In some unusual materials, fast ion conductors, one or more of the ionic components has a significant mobility, allowing conductivity.
According to the nomenclature recommended by IUPAC, in the most simple case of a binary ionic compound with no possible ambiguity about the stoichiometry, the common name is written using two words. The name of the cation comes first, followed by the name of the anion. For example, MgCl2 is named magnesium chloride, and Na2SO4 is named sodium sulfate (SO42−, sulfate, is an example of a polyatomic ion). To obtain the empirical formula from these names, the stoichiometry can be deduced from the charges on the ions, and the requirement of overall charge neutrality.
If the oxidation state of the cation is ambiguous, Stock nomenclature requires the oxidation number to be written in Roman numerals in parentheses directly after the name of the cation (without a space separating them). For example, FeSO4 is named iron(II) sulfate (with the 2+ charge on the Fe2+ ions balancing the 2− charge on the sulfate ion), whereas Fe2(SO4)3 is named iron(III) sulfate (because the two iron ions in each formula unit have a charge of 3+, to balance the 2− on each of the three sulfate ions). If the Classical naming system is being used, some ionic compounds have special "old" names, such as ferrous and ferric, for iron(II) and iron(III) respectively, and cuprous and cupric, for copper(I) and copper(II) respectively, so under that system Fe2(SO4)3 is named ferric sulfate.
- McQuarrie & Rock 1991, p. 503.
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- Moore, Lesley E. Smart; Elaine A. (2005). Solid state chemistry : an introduction (3. ed.). Boca Raton, Fla. [u.a.]: Taylor & Francis, CRC. p. 44. ISBN 9780748775163.
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- IUPAC 2005, p. 70.
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