A chemical compound is a substance composed of two or more elements chemically combined with each other. Compounds are one of three general forms of matter. The other two are elements and mixtures. Historically, the distinction between compounds and mixtures was often unclear. Today, however, the two can be distinguished from each other on the basis of three primary criteria.
First, compounds have constant and definite compositions, while mixtures may exist in virtually any proportion. A sample of water always consists of 88.9 percent oxygen and 11.1 percent hydrogen. It makes no difference whether the water comes from Lake Michigan, the Grand River, or a cloud in the sky. Its composition is always the same.
By comparison, a mixture of hydrogen and oxygen gases can have any composition whatsoever. You can make a mixture of 90 percent hydrogen and 10 percent oxygen; 75 percent hydrogen and 25 percent oxygen; 50 percent hydrogen and 50 percent oxygen; or any other combination.
Family: A group of chemical compounds with similar structure and properties.
Functional group: A group of atoms that give a molecule certain distinctive chemical properties.
Mixture: A combination of two or more substances that are not chemically combined with each other and that can exist in any proportion.
Molecule: A particle made by the chemical combination of two or more atoms; the smallest particle of which a compound is made.
Octet rule: A hypothesis that atoms having eight electrons in their outermost energy level tend to be stable and chemically unreactive.
Oxide: An inorganic compound whose only negative part is the element oxygen.
Second, the elements that make up a compound lose their characteristic elemental properties when they become part of the compound. In contrast, the elements that make up a mixture retain those properties. In a mixture of iron and sulfur, for example, black iron granules and yellow sulfur crystals often remain recognizable. Also, the iron can be extracted from the mixture by means of a magnet, or the sulfur can be dissolved out with carbon disulfide. Once the compound called iron(II) sulfide has been formed, however, both iron and sulfur lose those properties. Iron cannot be extracted from the compound with a magnet, and sulfur cannot be dissolved out of the compound with carbon disulfide.
Third, some visible evidence—usually heat and light—accompanies the formation of a compound. But no observable change takes place in the making of a mixture. A mixture of iron and sulfur can be made simply by stirring the two elements together. But the compound iron(II) sulfide is produced only when the two elements are heated. Then, as they combine with each other, they give off a glow.
Prior to the 1800s, the term "compound" did not have a precise meaning: the same word was used to describe both a mixture and a compound. Scientists at that time could not measure the composition of materials very accurately. Only very rough balances were available, so no measurement of weight could be trusted to any great extent.
Thus, suppose that a chemist in 1800 reported the composition of water as 88.9 percent oxygen and 11.1 percent hydrogen, and a second chemist reported 88.6 percent oxygen and 11.4 percent hydrogen. The question, then, was whether different samples of water had different compositions or whether the balances used to measure the components were just inaccurate. In the former case, the term compound would have no meaning, since water's composition would not always be the same. In the latter case, water could be thought of as a compound, and the differences in composition reported could be attributed to problems with weighing, not with the composition of water.
This debate raged for many years among chemists. Gradually, balances became more and more accurate, and experimental results became more and more consistent. By about 1800, it had become obvious that something like a compound really did exist. And the most important characteristic of the compound was that its composition was always and everywhere exactly the same.
Compounds form when two or more elements combine with each other. When a sodium metal is added to chlorine gas, a burst of light is produced. The elements sodium and chlorine come together to form the compound known as sodium chloride, or ordinary table salt.
What do hemoglobin, chlorophyll, and vitamin B 12 all have in common? They are members of a class of compounds known as coordination compounds. Coordination compounds consist of two parts: a central metal atom surrounded by a group of atoms known as ligands. Some common ligands are water, ammonia, carbon monoxide, the chloride ion, the cyanide ion, and the thiocyanate ion. In coordination compounds, ligands cluster around the central metal atom in groups of four, six, or some other number. The central difference among hemoglobin, chlorophyll, and vitamin B 12 is the metal at the center of the compound.
A special class of coordination compounds is known as chelates. Their name comes from the Greek word chela , or "claw." The ligands in a chelate grab the metal ion like a claw and hold it tightly. One example of a chelating agent is the compound known as ethylenediaminetetraacetic acid (or EDTA). EDTA can be used to soften water because it "clamps on" to calcium ions that make water hard. It is also used to treat people who have been poisoned by lead, mercury, or other toxic metals. The EDTA grabs onto the metal ions in the blood and removes them from the body, thus preventing harm to a person.
The formation of compounds can be understood by examining changes that take place on an atomic level. (An atom is the smallest part of an element that can exist alone.) Those changes are covered by a scientific law known as the octet rule. The octet rule states that all atoms tend to be stable if they have eight electrons (an octet) in their outermost energy level. (That law is modified somewhat for the very lightest elements.) The tendency of elements to combine with each other to form compounds is an effort on the part of atoms to form complete octets. In the case of sodium chloride, a compound is formed when sodium atoms give away electrons to chlorine atoms. At the conclusion of this exchange, both sodium atoms and chlorine atoms have complete outer energy levels.
Atoms can satisfy the octet rule by methods other than the gain and loss of electrons. They can, for example, share electrons with each other. The joining together of two atoms as a result of the gain and loss or sharing of electrons is known as chemical bonding. Chemical bonds are the forces that hold elements together in a 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 percent of these compounds are 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 carboxylic acids is the carboxyl group (—COOH).
An important subset of organic compounds are those that occur in living organisms: the biochemical compounds. In general, biochemical compounds can be classified into four major families: 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 classified as such on the basis of their solubility (ability to dissolve). They tend not to be soluble in water, but soluble in organic (or carbon-containing) liquids.
Inorganic compounds are typically classified into one of five major groups: acids, bases, salts, oxides, and others. Acids can be defined as compounds that produce hydrogen ions when placed into water. Bases, in contrast, are compounds that produce hydroxide ions when placed into water. Oxides are compounds whose only negative part is oxygen. Salts are compounds that consist of two parts, one positive (the cation) and one negative (the anion). The cation can be of any element or group of elements except hydrogen, while the anion may be of any atom or group of atoms except the hydroxide group.
This system of classification is useful in grouping compounds that have many similar properties. For example, all acids have a sour taste, leave a pink stain on 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.
[ See also Element, chemical ]