A compound is a substance composed of two or more elements chemically combined with each other. 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% oxygen and 11.1% hydrogen by weight. However, a mixture of hydrogen and oxygen gases can have any composition whatsoever.
Second, the elements that make up a compound lose their characteristic elemental properties when they become part of the compound, while 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 can often be recognized. Also, the iron can be extracted from the mixture by means of a magnet, or the sulfur can be dissolved out with carbon disulfide. In a compound called iron(II) sulfide, however, both iron and sulfur lose these properties.
Third, the formation of a compound is typically accompanied by light and heat, while no observable change is detectable 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, they give off a glow.
The term compound is often used in fields of science other than chemistry as either an adjective or verb. For example, medical workers may talk about a compound fracture in referring to a broken bone that has cut through the flesh. Biologists use a compound microscope—one that has more than one lens. Pharmacologists may speak of compounding a drug, that is, putting together the components of that
medication. Finally, a compounded drug is often one that is covered by a patent.
Prior to the 1800s, the term compound had relatively little precise meaning. When used, it was often unclear whether the reference was to what scientists now call a mixture or to what is now known as a compound. During the nineteenth century, the debate about the meaning of the word intensified, and it became one of the key questions in the young science of chemistry.
A critical aspect of this debate focused on the issue of constant composition. The issue was whether all compounds always had the same composition, or whether their composition could vary. The primary spokesman for the latter position was the French chemist Claude Louis Berthollet. Berthollet pointed to a considerable body of evidence that suggested a variable composition for compounds. For example, when some metals are heated, they form oxides that appear to have a regularly changing percentage composition. The longer they are heated, the higher the percentage of oxygen found in the oxide. Berthollet also mentioned alloys and amalgams as examples of substances with varying composition.
Berthollet’s principal opponent in this debate was his countryman Joseph Louis Proust, who argued that Dalton’s atomic theory required compounds to have a constant composition, a position advanced by Dalton himself. Proust set out to counter each of the arguments set forth by Berthollet. In the case of metal oxides, for example, Proust was able to show that metals often form more than one oxide. As copper metal is heated, for example, it first forms copper(I) or cuprous oxide and then, copper(II) or cupric oxide. At any one time, then, an experimenter would be able to detect some mixture of the two oxides varying from pure copper(I) oxide to pure copper(II) oxide. However, each of the two oxides itself, Proust argued, has a set and constant composition.
Working in Proust’s favor was an argument that nearly everyone was willing to acknowledge, namely that quantitative techniques had not yet been developed very highly in chemistry. Thus, it could be argued that what appeared to be variations in chemical composition were really nothing other than natural variability in results coming about as a result of imprecise techniques.
Proust remained puzzled by some of Berthollet’s evidence, including the problemof alloys and amalgams. At the time, he had no way of knowing that such materials are not compounds but were, in fact, mixtures. These remaining problems notwithstanding, Proust’s arguments eventually won the day and by the end of the century, the constant composition of compounds was universally accepted in chemistry.
It is difficult for a reader in the twenty-first century to appreciate the challenge facing a chemist in 1850 trying to understand the nature of a compound. Today it is clear that atoms of elements combine with each other to form, in many cases, molecules of a compound. Even the beginning chemistry student can express this concept with facility by using symbols and formulas, as in the formation of iron(II) sulfide from its elements:
Fe + S → FeS.
The chemist of 1850 was just barely comfortable with the idea of an atom and had not yet heard of the concept of a molecule. Moreover, the connection between ultimate particles (such as atoms and molecules) and materials encountered in the everyday work of a laboratory was not at all clear. As a result, early theories about the nature of compounds were based on empirical data (information collected from experiments), not from theoretical speculation about the behavior of atoms.
One of the earliest theories of compounds was that of the Swedishchemist Jons Jacob Berzelius, who argued that all compounds consist of two parts, one charged positively and one negatively. The theory was at least partially based on Berzelius’s own studies of electrolysis, in which compounds would often be broken apart into two pieces by the passage of an electrical current. Thus he pictured salts as being composed of a positively charged metal oxide and a negatively charged nonmetallic oxide. According this theory, sodium sulfate,Na2 SO4 could be represented as Na20 • SO3.
Other theories followed, many of them developed in an effort to explain the rapidly growing number of organic compounds being discovered and studied. According to the radical theory, for example, compounds were viewed as consisting of two parts, one of which was one of a few standard radicals, or groups of atoms. Organic compounds were explained as being derived from the methyl, ethyl, benzyl, cyanogen, or some other radical.
The type theory, proposed by Charles Gerhardt in the 1840s, said that compounds could be understood as derivatives of one or more basic types, such as water or ammonia. According to this theory, bases such as sodium hydroxide (NaOH) were thought to be derivatives of water (HOH) in which one hydrogen atom is replaced by a metal.
The most fundamental change that has taken place in chemistry since the nineteenth century is that atomic theory now permits an understanding of chemical compounds from the particle level rather than from purely empirical data. That is, as our knowledge of atomic structure has grown and developed, and our understanding of the reasons that atoms (elements) combine with each other has improved. For example, the question of how and why iron and sulfur combine with each other to form a compound is now approached in terms of how and why an iron atom combines with a sulfur atom to form a molecule of iron(II) sulfide.
A key solving that puzzle was suggested by the German chemist Albrecht Kossel in 1916. In considering the unreactivity of inert gases, Kossel concluded that the presence of eight electrons in the outermost energy level of an atom (as is the case with all inert gases) gave them a certain stability. Perhaps, Kossel said, the tendency of atoms to exchange electrons in such a way as to achieve a full octet (eight) of electrons could explain chemical reactions in which elements combine to form compounds.
Although Kossel had hit on a key concept, he did not fully develop this theory. That work was left to the American chemist Gilbert Newton Lewis. At about the same time that Kossel was proposing his octet theory, Lewis was developing a comprehensive explanation showing how atoms can gain a complete octet either by the gain and loss or by the sharing of pairs of electrons with other atoms. Although Lewis’s theory has undergone many transformations, improvements, and extensions (especially in the work of Linus Pauling), his explanation of compound formation still constitutes the heart of such theory today.
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 organic compounds because they contain the element carbon. These in turn, can be subdivided into a few dozen major families such as the alkanes, alkenes, alkynes, alcohols, aldehydes, ketones, carboxylic acids, and amines. Each family can be recognized by the presence of a characteristic functional group that strongly determines the physical and chemical properties of its members. 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. These 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 that 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 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—hydrochloric, hypochlorous, chlorous, chloric, and perchloric. For all their similarities, these five acids also have important distinctive properties.
The “other” category of compound classification includes all those compounds that don’t fit into one of the other four categories. Perhaps the most important compounds in this category are the coordination compounds, primarily because of their method of bonding. Acids, bases, oxides, and salts 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 contribute both electrons in a bonding pair to a metallic atom or ion. The contributing species in such a compound are known as ligands and the compound as a whole is often called a metal complex.
Alloy— A mixture of two or more metals with properties distinct from the metals of which it is made.
Amalgam— An alloy that contains the metal mercury.
Coordination compounds— Compounds formed when metallic ions or atoms are joined to other atoms, ions, or molecules by means of coordinate covalent bonds.
Empirical— Evidence that is obtained from some type of experimentation.
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— An hypothesis that atoms that have 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.
Radical— A group of atoms that behaves as if it were a single atom.
Masterson, William L., Emil J. Slowinski, and Conrad L. Stanitski. Chemical Principles. Philadelphia: Saunders, 1983, Chapter 3.
Moore, John, and Nicholas D. Spencer. Encyclopedia of Chemical Physics and PhysicalChemistry. Washington, DC: Institute of Physics, 2001.
Williams, Arthur L., Harland D. Embree, and Harold J. DeBey. Introduction to Chemistry. 3rd ed. Reading, MA: Addison-Wesley Publishing Company, 1986, Chapter 11.
David E. Newton