Ionic compound

The crystal structure of sodium chloride, NaCl, a typical ionic compound. The purple spheres are sodium cations, Na⁺, and the green spheres are chloride anions, Cl⁻.

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.

Characteristics

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.

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 an aqueous solution.^[1] Ionic inorganic compounds are solids at room temperature and usually form crystals, they have high melting points, and are soluble in water. 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.^[2]

The ions produced by electron transfer attract each other by electrostatic attraction and this creates an ionic bond.

Solubility

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/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. When this force is more than the electrostatic attraction of the lattice, the ions become dissolved in the liquid.

Electrical conductivity

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 state, they can conduct electricity with the mobile ions that allowed the electricity passed through it. ^[3]

Name

According to the IUPAC, an ionic compound's common name is written using two words. The name of the cation comes first (with the oxidation number written in parentheses when using Stock nomenclature), followed by the name of the anion.^[4] 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 ferrous and ferric, for iron(II) and iron(III) respectively, and cuprous and cupric, for copper(I) and copper(II) respectively.

See also

• IUPAC nomenclature
• Bonding in solids
• Solubility

References