Chemical Compound

Most of the ten million or so chemical compounds that are known today can be classified into a relatively small number of subgroups or families. More than 90% of these compounds are, in the first place, designated as organic compounds because they contain the element carbon. In turn, organic compounds can be further subdivided into a few dozen major families such as the alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, carboxylic acids, and amines. Each of these families can be recognized by the presence of a characteristic functional group that strongly determines the physical and chemical properties of the compounds that make up that family. For example, the functional group of the alcohols is the hydroxyl group (-OH) and that of the carboxylicacids, the carboxyl group (-COOH).

An important subset of organic compounds are those that occur in living organisms, the biochemical compounds. Biochemical compounds can largely be classified into four major families: the carbohydrates, proteins, nucleic acids, and lipids. Members of the first three families are grouped together because of common structural features and similar physical and chemical properties. Members of the lipid family are so classified on the basis of their solubility. They tend not to be soluble in water, but soluble in organic liquids.

Inorganic compounds are typically classified into one of five major groups: acids, bases, salts, oxides, and others. Acids are defined as compounds which ionize or dissociate in water solution to yield hydrogen ions. Bases are compounds that ionize or dissociate in water solution to yield hydroxide ions. Oxides are compounds whose only negative part is oxygen. Salts are compounds whose cations are any ion but hydrogen and whose anions are any ion but the hydroxide ion. Salts are often described as the compounds formed (other than water) when an acid and a base react with each other.

This system of classification is useful in grouping compounds that have many similar properties. For example, all acids have a sour taste, impart a pink color to litmus paper, and react with bases to form salts. One drawback of the system, however, is that it may not give a sense of the enormous diversity of compounds that exist within a particular family. For example, the element chlorine forms at least five common acids, known as hydrochloric, hypochlorous, chlorous, chloric, and perchloric acids. For all their similarities, these five acids also have important distinctive properties.

The "others" category of compound classification includes all those compounds that don't fit into one of the other four categories. Perhaps the most important group of compounds contained in this "others" category is the coordination compounds. Coordination compounds are different from acids, bases, salts, and oxides primarily because of their method of bonding. Members of the last four groups are formed when atoms give or take electrons to form ionic bonds, share pairs of electrons to form covalent bonds, or exchange electrons in some fashion intermediary between these cases to form polar covalent bonds. Coordination compounds, on the other hand, are formed when one or more ions or molecules contributes both electrons in a bonding pair to a metallic atom or ion. The contributing species in such a compound is (or are) known as ligands and the compound as a whole is often called a metal complex.