A binary compound is a substance whose molecules are comprised of atoms of two elements. The number of individual atoms in each molecule may vary, but they must belong to only two elements, although isotopes of elements are permitted. Binary compounds are, by definition, some of the simplest compounds in all of nature but are also some of the most important in both organic and inorganic chemistry. They are broadly categorized as either ionic or covalent, depending on the type of bonds between the atoms. Several subgroups and divisions of binary compounds are recognized by chemists for classification and teaching purposes.
A substance consisting of one metal and one nonmetal element is an ionic binary compound. These compounds form when positively charged ions bond with negatively charged ions. The ionic bonds these atoms form are very strong and require a great deal of energy to break. Most often, the positively charged ions are atoms of a metal, and the negatively charged ions come from a nonmetal. Many compounds of this sort are salts such as common table salt which consists of one sodium atom and one chlorine atom.
The second main type is formed by the combination of atoms bound with covalent bonds. Covalent chemical bonds are not as strong as ionic bonds, meaning that relatively little energy is required to break them. Hydrocarbons consisting of only carbon and hydrogen atoms are one type of covalent binary compound. Water is perhaps the most familiar. This covalent compound consists of two hydrogen atoms and one oxygen atom.
Chemists have developed a system for naming chemicals and other compounds, and knowing a little bit about the conventions of this system can help in identifying some binary compounds from their names alone. Names for ionic compounds, which tend to be a metal bonded to a nonmetal, are usually formulated with the name of the metal followed by a shortened form of the name of the nonmetal element, with the suffix "-ide." For example, salt is sodium chloride, the "-ide" helping to identify it as a binary compound. Covalent compounds follow similar conventions, and binary compounds of either type may have the second term modified by a prefix as well to denote multiple atoms of one element, such as carbon dioxide, which has two oxygen atoms. Other rules are used for certain other special cases, and occasionally, a compound may not follow the conventions at all.