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.
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. 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.
An ionic compound is named first by its cation and then by its anion. For cations that take on multiple charges, the charge is written using Roman numerals in parentheses immediately following the element name. For example, Cu(NO3)2 is copper(II) nitrate, because the charge of two nitrate ions (NO3−1) is 2 \cdot −1 = −2, and since the net charge of the ionic compound must be zero, the Cu ion has a 2+ charge. This compound is therefore copper(II) nitrate. The Roman numerals in fact show the oxidation number, but in simple ionic compounds this will always equal the ionic charge on the metal.
Salts, of which ordinary "salt" (sodium chloride) is the most common example, are all solids under ordinary conditions. A small number of these (such as NaCl) do retain their component ions and are properly called "ionic solids." In many cases, however, the ions lose their electrically charged character and form largely non-ionic solids, such as CuCl2. The term "ion-derived solids" encompasses both of these classes of compounds.
Most cations and anions can combine to form solid compounds that are usually known as salts. The one overriding requirement is that the resulting compound must be electrically neutral: therefore the ions Ca2+ and Br– combine only in a 1:2 ratio to form calcium bromide, CaBr2. Because no other, simpler formula is possible, there is no need to name it "calcium dibromide."
Since some metallic elements form cations that have different positive charges, the names of ionic compounds derived from these elements must contain some indication of the cation charge. The older method uses the suffixes -ous and -ic to denote the lower and higher charges, respectively. In the cases of iron and copper, the Latin names of the elements are used (ferrous/ferric, cuprous/cupric). This system is still widely used, although it has been officially supplanted by the more precise, if slightly cumbersome, Stock system. In the Stock system, the cationic charge (actually, the oxidation number) is indicated by means of Roman numerals following the cation symbol. In both systems, the name of the anion ends in -ide.