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In chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. Ions in ionic compounds are held together by the electrostatic forces between oppositely charged bodies. Ionic compounds have high melting and boiling points, and they are hard and very brittle.
Ions can be single atoms, as the sodium and chlorine in common table salt sodium chloride, or more complex groups such as the carbonate in calcium carbonate. But to be considered an ion, they must carry a positive or negative charge. Thus, in an ionic bond, one 'bonder' must have a positive charge and the other a negative one. By sticking to each other, they resolve, or partially resolve, their separate charge imbalances. Positive to positive and negative to negative ionic bonds do not occur. (For an easily visible analogy, experiment with a pair of bar magnets.)
Chemical compounds are never strictly ionic. Even 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.
|Physical Properties||Ionic Compounds|
|States (at room temperature)||Solid|
|Electrical conductivity||Solid: No Liquid: Yes Aqueous: Yes (only if soluble)|
|Boiling point and Melting Point||High|
|Solubility in water||Often high|
Ionic compounds have strong electrostatic bonds between particles. As a result, they generally have very high melting and boiling points. They also have good electrical conductivity when molten or in aqueous solution. While ionic inorganic compounds are solids at room temperature and usually form crystals, organic ionic liquids are of increasing interest.
The ions produced by electron transfer attract each other by electrostatic attraction and this creates an ionic bond.
Following the aphorism, "like dissolves like", ionic compounds dissolve in polar solvents, especially those that ionize, such as water and ionic liquids. They are usually appreciably soluble in other polar solvents such as alcohols, acetone and dimethyl sulfoxide as well. Ionic compounds tend not to dissolve in nonpolar solvents such as diethyl ether or petrol.
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. When this force is more than the electrostatic attraction of the lattice, the ions become dissolved in the liquid.
Solid ionic compounds cannot conduct electricity because there are no mobile ions or electrons present in the lattice. When the ionic compounds are dissolved in a liquid or molten, they can conduct electricity with the mobile ions that allowed the electricity passed through it.
According to the IUPAC, an ionic compound's common name is written using two words. The name of the cation comes first (when using stock nomenclature with the oxidation number written in parentheses, followed by the name of the anion. For example, Fe2(SO4)3 is named as iron(III) sulfate. If the Classical naming system is being used, some ionic compounds have special "old" names, such as ferric (iron(III)), ferrous (iron(II)), cupric (copper(II)), and cuprous (copper(I)).