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 a halogen or polyatomic ion. Ions in ionic compounds are held together by the electrostatic force between oppositely charged bodies. Ionic compounds have a high melting and boiling point, and they have a high hardness and are very brittle.

Ions can be single atoms, as in common table salt sodium chloride, or more complex groups such as 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 a real world analogy, experiment with a pair of bar magnets.)

Chemical compounds are rarely strictly ionic or strictly covalent. Except for the most electronegative/electropositive pairs such as cesium fluoride, ionic compounds usually exhibit a degree of covalency. Similarly, covalent compounds often exhibit charge separations. See also HSAB theory.

Physical properties of ionic and molecular compounds:

Characteristics

Ionic compounds have strong electrostatic bonds between particles. As a result, they generally have high melting and boiling points. They have good electrical conductivity when molten or in aqueous solution. While ionic inorganic compounds are solids at room temperature and will usually form crystals, organic ionic liquids are of increasing interest.

Solubility

Following the aphorism, "like dissolves like", ionic compounds dissolve in polar solvents, especially those which 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 an ionic compound is named, the cation is named first and then the anion. When an elemental anion is named, the suffix, -ide, is added to the name of the element. There are two common types of cations: Type I and Type II. Type I cations have only one charge and their name is simply listed when the compound is named. Type II cations have more than one charge and when the ionic compound is named, a Roman numeral is used to denote the charge of the cation. In addition, there are common polyatomic anions which do not have suffixes in their name such as hypochlorite (ClO^–).

Nomenclature

According to the IUPAC, an ionic compound's common name is written using two words. The name of the cation comes first (when using the Stock system) with the oxidation number written in parenthesis, 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)).