Chemical Substances

There are several ways to classify chemical substances. One scheme uses two major categories: molecular and ionic. Most molecular substances, and the molecules of which they are composed are formed by a combination of two or more nonmetals. When metal atoms combine with nonmetal atoms, they usually form ionic compounds. The smallest characteristic entity of an ionic compound is a formula unit made up of ions.

Ions

An atom or molecule that picks up an extra electron becomes a negatively charged ion, called an anion.  An atom or molecule that loses an electron becomes a positively charged ion, called a cation.  Although electrical charge has a unit, the Coulomb, in chemistry when we refer to the charge on an ion we really mean relative charge, which does not have units.  For example a chlorine atom , which has 17 protons and 17 electrons, tends to pick up one additional electron.  The chloride anion would have 17 protons and 18 electrons, thus its actual electrical charge would be -1.60218 x 10^-19 Coulombs (the charge of one electron).   In chemistry we would say that the chloride ion has a charge of -1 and we would write its symbol as Cl ^-.

Ionic Formulas

The formulas of ionic compounds are always the same as their empirical formulas since ionic compounds do not consist of discrete molecules. For example a solid sample of potassium chloride (KCl) consists of equal numbers of K⁺ and Cl^- ions arranged in a three-dimensional network. In this substance there is a 1:1 ratio of cations to anions so that the substance is electrically neutral.