Ionic compounds are held together by electrical attraction. One ion has a positive charge and the other a negative charge and they stick together the way the opposite poles of magnets do. Ionic compounds often make hard visible crystals, as a consequence of the regular way the ions attract one another, and have high melting points because they are so strongly held in their crystal lattices.
Ions are atoms that hold a positive or negative charge. They have a charge because one or more electrons are missing from their outer shell or one or more extra electrons have been added. It is these out of place electrons that form the ionic, or electrovalent, bond.
An atom that is missing an electron has a positive charge, because electrons are negative. It is called a cation. An atom with one or more extra electrons has a negative charge (again because electrons are negative) and it is called an anion.
Cations and anions
Cations, the positively charged atoms in ionic compounds, are usually metals. They do not necessarily appear as the hard shiny stuff people think of as a metal, however. In fact, the sodium in sodium chloride, salt, is a metal.
The anion, the negatively charged ion in an ionic compound, can be a single element, like the chlorine in salt. However, it can also be a polyatomic ion, like hydroxide, carbonate, bicarbonate, sulfate and phosphate. These are combinations of elements that act together as an anion in an ionic bond.
A polyatomic ion may be held together by covalent bonds, in which atoms share electrons, instead of giving up or taking them as in ionic bonds. Even after the covalent bonds form, however, a polyatomic ion is still negatively charged as a group. That is why it can bond with the positively charged cation.
Ionic compounds are hard, like salt crystals, but like salt, some ionic compounds dissolve readily, whether in water or another liquid. When they dissolve, the solution ionic compounds form can be a strong conductor of electricity. Solid ionic compounds are poor conductors, because the electrons are held tightly in a crystal lattice and cannot move. Moving electrons are what make an electric current.
However, when an ionic compound is dissolved, the ions are let loose to wander, and can readily conduct a current. Salt water conducts a current quite well, but pure water does not. However, pure water is quite rare; most water has ionic forms of minerals dissolved in it. Therefore, stay away from water in thunderstorms.
Ionic compounds are useful as solutions that conduct electricity. They are the form in which many metals are mined, as well. They are also widely used to remove dyes. Chemists and physicists study them to gain insight into the way the world is put together, and perhaps admire them for the strict beauty of their crystalline forms.