A compound is a chemical substance in which atoms combine in such a way that the compound always has the same composition, unless it is chemically altered in some way. Elements make up compounds, and although there are only about 90 elements that occur in nature, there are literally many millions of compounds. A compound is not the same as a mixture, which has a variable composition, but until chemists understood the atomic and molecular substructure of compounds, the distinction was not always clear. When atoms of one element bond to atoms of another, they form substances quite different from either element. Sugar, for instance, is made up of carbon, the material in coal and graphite, along with two gases, hydrogen and oxygen. None of these is sweet, yet when they are brought together in just the right way, they make the compound that sweetens everything from breakfast cereals to colas. A compound cannot be understood purely in terms of its constituent elements; an awareness of the structure is also needed. Likewise, it is important to know just how to name a compound, using a uniform terminology, since there are far too many substances in the world to give each an individual name.
HOW IT WORKS
Elements and Compounds
A compound is a substance made up of atoms representing more than one element, and these atoms are typically joined in molecules. The composition of a compound is always the same: for instance, water always contains molecules composed of two hydrogen atoms bonded to a single oxygen atom. It can be frozen or boiled, but the molecules themselves are unchanged, because freezing and boiling are merely physical processes. To transform one compound into another, on the other hand, requires a chemical change or reaction.
Likewise, a chemical change is required to break down a compound into its constituent elements. Water can be broken down by passing a powerful electric current through it. This process, known as electrolysis, separates water into hydrogen and oxygen, both of which are highly flammable gases. The fact that they can be joined to form water, a substance used for putting out most kinds of fires, illustrates the types of changes elements undergo when they join to form compounds.
The atoms in a compound do not change into other atoms; or to put it another way, the elements do not change into other elements. Though two hydrogen atoms bond with an oxygen atom to form water, they are still hydrogens, and the oxygen is still an oxygen. The atoms’ elemental identity thus remains intact, and if these elements are separated, they can join with other elements to form entirely different compounds.
Letters and Elements; Words and Compounds
The nature of a compound is almost paradoxical: the constituent elements remain the same, yet the compound typically bears little resemblance to the elements that form it. How can this be? Perhaps an analogy to language, and the symbols used to express it, will serve to show that this
A mixture versus a compound.
apparent contradiction is not a contradiction at all—it is merely evidence of the complexities that occur when a simple particle contributes to a larger combination.
There are between 88 and 92 elements that appear in nature; figures vary, because a handful of the elements with atomic numbers less than 92 have not actually been found in nature, but were produced in laboratories. (All elements with an atomic number higher than that of uranium, which is 92, are artificial.) In any case, there are far more elements than there are letters in the English alphabet; yet even with 26 characters, it is possible to form an almost limitless vocabulary.
Let us focus on a single letter, g. Phonetically, it can make a hard sound, as in great, or a soft one, as in geranium. It may be silent, as in light, or it may combine with another letter to make either a typical “g” sound (edge) or a totally unexpected one (rough). It can slide into a vowel sound smoothly, as in singer, or with an almost imperceptible pause, as in finger. The complexities multiply when we consider the range of possibilities for the letter g, and all the resultant meanings involved: from God to dog, or from glory to degeneration.
What this analogy shows about chemistry
One could go on endlessly in this vein, and all with just one letter; however, a few points need to be made regarding the analogy between letters of the alphabet and elements. First of all, in all of the examples given above, or any number of others, the g still remained a g. In the same way, an atom of hydrogen (another g-word!) is always a hydrogen, whether it combines with oxygen to form water, nitrogen to make ammonia, or carbon to produce petroleum.
Secondly, note that this illustration would have been more difficult if the letter chosen had been q or z, since these are used more rarely. Likewise, there are elements such as technetium or praseodymium that seldom come up in discussions of compounds. On the other hand, one cannot go far in the study of chemistry without running across compounds involving hydrogen, carbon, oxygen, nitrogen, various metals, or halogens such as fluorine.
A third point to consider is the fact that there is nothing about g itself that provides any information as to the meaning of the word it helps to form. Here the analogy to elements is imperfect, because the nature of the element’s subatomic structure—particularly the configuration of electrons on the outer shell of the atom—provides a great deal of information as to how it will combine with other atoms.
Still, it is evident, as we have seen, that the properties of a compound cannot easily be predicted by studying the properties of its constituent elements—just as one cannot begin to define a word simply because it contains a g.On the other hand, knowledge of the sounds that g makes may help us to pronounce a word containing that letter. In the same way, the fact that a compound includes carbon provides a clue that the compound might be organic.
Even when we have all the letters to form a word, we still need to know how they are ordered. Loge and ogle contain the same letters, but one is a noun referring to a theater box, whereas the other is a verb meaning the act of looking at someone in an improper fashion. In chemistry, these are called isomers: two substances having identical chemical formulas, but differing chemical structures.
It so happens that g is also part of the suffix -ing, used in forming a gerund. Aside from being yet another g-word, a gerund is a substantive, or noun form of a verb: for instance, going. Once again, there is a similarity between words and compounds. Just as certain classes of words are formed by the addition of regular suffixes, so the naming of whole classes of compounds can be achieved by means of a uniform system of nomenclature.
Distinguishing Between Compounds and Mixtures
To continue the analogy used above just one step further, a word is not just a collection of letters; it has to have a meaning. Likewise, the fact that various substances are mixed together does not necessarily make them a compound. Actually, the difference between a word and a mere collection of letters is somewhat greater than the difference between a compound and a mixture—a substance in which elements are not chemically bonded, and in which the composition is variable. A nonsensical string of characters serves no linguistic purpose; on the other hand, mixtures are an integral part of life.
Tea, whether iced or hot, is a mixture. So is coffee, or even wine. In each case, substances are added together and subjected to a process, but the result is not a compound. We know this because the composition varies. Depending on the coffee beans used, for instance, coffee can have a wide variety of flavors. If, in the brewing process, too much coffee is used in proportion to the water, the resulting mixture will be strong or bitter; on the hand, an insufficient coffee-to-water ratio will produce coffee that is too weak.
Note that a number of terms have been used here that, from a scientific standpoint at least, are vague. How weak is “too weak”? That all depends on the tastes of the person making the coffee. But as long as coffee beans and hot water are used, no matter what the proportion, the mixture is still coffee. On the other hand, when two oxygen atoms, rather than one, are chemically combined with two hydrogen atoms, the result is not “strong water” Nor is it “oxygenated water”: it is hydrogen peroxide, a substance no one should drink.
Three principal characteristics serve to differentiate a compound from a mixture. First, as we have seen, a compound has a definite and constant composition, whereas a mixture can exist with virtually any proportion between its constituent parts. Second, elements lose their characteristic elemental properties once they form a compound, but the parts of a mixture do not. (For example, when mixed with water, sugar is still sweet.) Third, the formation of a compound involves a chemical reaction, whereas a mixture can be created simply by the physical act of stirring items together.
The Atomic and Molecular Keys
The means by which compounds are formed are discussed numerous times, and in various ways, throughout this topic. A few of those particulars will be mentioned briefly below, in relation to the naming of compounds, but for the most part, there will be no attempt to explain the details of the processes involved in chemical bonding. The reader is therefore encouraged to consult the essays on Chemical Bonding and Electrons.
It is important, nonetheless, to recognize that chemists’ knowledge is based on their understanding of the atom and the ways that electrons, negatively charged particles in the atom, bring about bonds between elements. Awareness of these specifics emerged only at the beginning of the twentieth century, with the discovery of subatomic particles. Another important threshold had been crossed a century before, with the development of atomic theory by English chemist John Dalton (1766-1844), and of molecular theory by Italian physicist Amedeo Avogadro (1776-1856).
Around the same time, French chemists Antoine Lavoisier (1743-1794) and Joseph-Louis Proust (1754-1826), respectively, clarified the definitions of “element” and “compound.” Until then, the idea of a compound had little precise meaning for chemists, who often used the term to describe a mixture. Thus, French chemist Claude Louis Berthollet (1748-1822) asserted that compounds have variable composition, and for evidence he pointed to the fact that when some metals are heated, they form oxides, in which the percentage of oxygen increases with temperature.
Proust, on the other hand, maintained that compounds must have a constant composition, an argument supported by Dalton’s atomic theory. Proust worked to counter Berthollet’s positions on a number of particulars, but was still unable to explain why metals form variable alloys, or combinations of metals; no chemist at the time understood that an alloy is a mixture, not a compound.
Nonetheless, Proust was right in his theory of constant composition, and Berthollet was incorrect on this score. With the discovery of subatomic structures, it became possible to develop highly sophisticated theories of chemical bonding, which in turn facilitated understanding of compounds.
Types of Compounds
organic compounds. Though there are millions of compounds, these can be grouped into just a few categories. Organic compounds, of which there are many millions, are compounds containing carbon. The only major groupings of carbon-containing compounds that are not considered organic are carbonates, such as limestone, and oxides, such as carbon dioxide. Organic compounds can be further subdivided into a number of functional groups, such as alcohols. Within the realm of organic compounds, whether natural or artificial, are petroleum and its many products, as well as plastics and other synthetic materials.
The term “organic,” as applied in chemistry, does not necessarily refer to living things, since the definition is based on the presence of carbon. Nonetheless, all living organisms are organic, and the biochemical compounds in living things form an important subset of organic compounds. Biochemical compounds are, in turn, divided into four families: carbohydrates, proteins, nucleic acids, and lipids.
Inorganic compounds can be classified according to five major groupings: acids, bases, salts, oxides, and others. An acid is a compound which, when dissolved in water, produces positive ions (atoms with an electric charge) of hydrogen. Bases are substances that produce negatively charged hydroxide (OH-) ions when dissolved in water. An oxide is a compound in which the only negatively charged ion is an oxygen, and a salt is formed by the reaction of an acid with a base. Generally speaking, a salt is any combination of a metal and a nonmetal, and it can contain ions of any element but hydrogen.
The remaining inorganic compounds, classified as “others,” are those that do not fit into any of the groupings described above. An important subset of this broad category are the coordination compounds, formed when one or more ions or molecules contributes both electrons necessary for a bonding pair, in order to bond with a metallic ion or atom.
In the early days of chemistry as a science, common names were applied to compounds. Water is an example of a common name; so is sugar, as well as salt. These names work well enough in everyday life, and in fact, chemists still refer to water simply as “water.” (The only other common name still used in chemistry is ammonia.)
But as the number of compounds discovered and developed by chemists began to proliferate, the need for a systematic means of naming them became apparent. With millions of compounds, it would be nearly impossible to come up with names for every one. Furthermore, common names tell chemists nothing about the chemical properties of a particular substance.
Today, chemists use a system of nomenclature that is rather detailed but fairly easy to understand, once the rules are understood. We will examine this system briefly, primarily as it relates to binary compounds—compounds containing just two elements. Binary compounds are divided into three groups. The first two are ionic compounds, involving metals that form positively charged ions, or cations (pronounced KAT-ie-unz). The third consists of compounds that contain only nonmetals. These groups are:
• Type I: Ionic compounds involving a metal that always forms a cation of a certain electric charge.
• Type II: Ionic compounds involving a metal (typically a transition metal) that forms cations with differing charges.
• Type III: Compounds containing only non-metals.
cations and anions. Cations are represented symbolically thus: H+,or Mg2+. The first, a hydrogen cation, has a positive charge of 1, but note that the 1 is not shown—just as the first power of a number is never designated in mathematics. In the second example, a magnesium cation, the superscript number, combined with the plus sign (which can either follow the number, as is shown here, or proceed it) indicates that the atom has a positive charge of two. Thus, even if one were not told that this is a cation, it would be easy enough to discern from the notation.
Anions (AN-ie-unz), or negatively charged ions, are represented in a similar way: H- for hydride, an anion of hydrogen; or O2- for oxide. Note, however, that the naming of anions and cations is different. Cations are always called, for example, a hydrogen cation, or a magnesium cation. On the other hand, names of simple anions (involving a single atom) are formed by taking the root of the element name and adding an -ide: for example, fluoride (F-).
Type I binary compounds
In a binary ionic compound, a metal combines with a nonmetal. The metal loses one or more electrons to become a cation, while the nonmetal gains one or more electrons to become an anion. Thus, table salt is formed by the joining of a cation of the metal sodium (Na+) and an anion of the nonmetal chlorine (Cl-). Instead of the common name “salt,” which can apply to a range
of substances, its chemical name is sodium chloride.
In naming Type I binary compounds, of which sodium chloride is an example, the cation is always represented first by the name of the element. The anion follows, with the root name of the element attached to the -ide suffix, as described above. Another example is calcium sulfide, formed by a cation of the metal calcium (Ca2+) and an anion of the nonmetal sulfur (S2-).
Type II binary compounds
The chemical nomenclature for type II binary compounds is somewhat more complicated, because they involve metals that can have multiple positive charges. This is particularly true of the transition metals, a family of 40 elements in the middle of the periodic table distinguished from other elements by a number of characteristics. The name “transition” thus implies a break in the even pattern of the periodic table.
Iron (Fe), for instance, is a transition metal, and it can form cations with charges of 2+ or 3+, while copper (Cu) can form cations with charges of 1+ or 2+. When encountering positively charged cations, it is not enough to say, for instance, “iron oxide,” or “copper sulfide,” because it is not clear which iron cation is involved. To solve the problem, chemists use a system of Roman numerals.
According to this system, the cations referred to above are expressed in the name of a Type II binary compound thus: iron (II), iron (III), copper (I), and copper (II). This is followed with the name of the anion, as before, using the -ide suffix. Note that the Roman numeral is usually the same as the number of positive charges in the cation.
It should be noted, also, that there is an older system for naming Type II binary compounds with terms that incorporate the element name— often the Latin original, reflected in the chemical symbol—with a suffix. For instance, this system uses the word “ferrous” for iron (II) and “ferric” for iron (III). However, this method of nomenclature is increasingly being replaced by the one we have described here.
Type III binary compounds.
In a Type III binary compound involving only nonmetals, the first element in the formula is referred to simply by its element name, as though it were a cation, while the second element is given an -ide suffix, as though it were an anion. If there is more than one atom present, prefixes are used to indicate the number of atoms. These prefixes are listed below. It should be noted that mono- is never used for naming the first element in a type III binary compound.
• mono-: 1
• tri-: 3
• tetra-: 4
• penta-: 5
• hexa-: 6
• hepta-: 7
• octa-: 8
Thus CO2 is called carbon dioxide, indicating one carbon atom and two oxygens. Again, mono- is not used for the first element, but it is used for the second: hence, the name of the compound with the formula CO is carbon monoxide. It is also possible to know the formula for a compound simply from the name: if confronted with a name such as “dinitrogen pentoxide,” for instance, it is fairly easy to apply the rules governing these prefixes to discern that the substance is N2O5. Note that vowels at the end of a prefix are dropped when the name ofthe element that follows it also begins with a vowel: we say monoxide, not “monooxide”; and pentoxide, not “pentaoxide.” This makes pronunciation much easier.
Polyatomic ions and acids
More complicated rules apply for polyatomic ions, which are charged groupings of atoms such as NH4NO3, or ammonium nitrate. The only way to learn how to name polyatomic ions is by memorizing the names of the constituent parts. In the above example, for instance, the first part of the formula, which has a positive charge, is always called ammonium, while the second part, which has a negative charge, is always called nitrate. A good chemistry text topic should provide a table listing the names of common polyatomic ions.
A number of polyatomic ions are called oxyanions, meaning that they include varying numbers of oxygen atoms combined with atoms of other elements. There are rules for designating the names of these polyatomic ions, some of which are listed in the essay on Ions and Ionization.
Still other rules govern the naming of acids; here again, the operative factor is the presence of oxygen. Thus, if the anion does not contain oxygen, the name of the acid is created by adding the prefix hydro- and the suffix -ic, as in hydrochloric acid. If the anion does contain oxygen, the root name of the principal element is joined to a suffix: depending on the relative numbers of oxygen atoms, this may beic or ous.
As observed much earlier, isomers are like two words with the same letters, but arranged in different ways. Specifically, isomers are chemical compounds having the same formula, but in which the atoms are arranged differently, thereby forming different compounds. There are two principal types of isomer: structural isomers, which differ according to the attachment of atoms on the molecule, and stereoisomers, which differ according to the locations of the atoms in space.
An example of structural isomerism is the difference between propyl alcohol and isopropyl (rubbing) alcohol: these two have differing properties, because their alcohol functional groups are not attached to the same carbon atom in the carbon chain to which the functional group is attached.
In a stereoisomer, on the other hand, atoms are attached in the same order, but have different spatial relationships. If functional groups are aligned on the same side of a double bond between two carbon atoms, this is called a cis iso-mer, from a Latin word meaning “on this side.” If they are on opposite sides, it is called a trans (“across”) isomer. Hence the term “trans fats,” which are saturated fats that improve certain properties of foods—including taste—but which may contribute to heart disease.
Acid: An inorganic compound that, when dissolved in water, produces positive ions—that is, cations—of hydrogen, designated symbolically as H+ ions. (This is just one definition; see Acids and Bases; Acid-Base Reactions for more.)
Anion: The negatively charged ion that results when an atom gains one or more electrons. The word is pronounced “AN-ie-un.”
Base: An inorganic compound that produces negative hydroxide ions when it is dissolved in water. These anions are designated by the symbol OH-. (This is just one definition; see Acids and Bases; Acid-Base Reactions for more.)
Binary compound: A compound that contains just two elements. For the purposes of establishing compound names, binary compounds are divided into Type I, Type II, and Type III.
Cation: The positively charged ion that results when an atom loses one or more electrons. The word “cation” is pronounced “KAT-ie-un.”
Chemical bonding: The joining, through electromagnetic force, of atoms representing different elements.
Compound: A substance made up of atoms of more than one element, which are chemically bonded and usually joined in molecules. The composition of a compound is always the same, unless it is changed chemically.
Coordination compounds:Inorganic compounds formed when one or more ions or molecules contribute both electrons necessary for a bonding pair in order to bond with a metallic ion or atom.
Electron: A negatively charged particle in an atom.
Inorganic compound: For the most part, inorganic compounds are any compounds that do not contain carbon. However, carbonates and carbon oxides are also inorganic compounds. Compare with organic compounds.
Ion: An atom or group of atoms that has lost or gained one or more electrons, and thus has a net electric charge.
Ionic bonding: A form of chemical bonding that results from attractions between ions with opposite electric charges.
Ionic compound: A compound in which ions are present. Ionic compounds contain at least one metal joined to another element by an ionic bond.
Isomers: Substances which have the same chemical formula, but which have different chemical properties due to differences in the arrangement of atoms.
Mixture: A substance in which elements are not chemically bonded, and in which the composition is variable. A mixture is distinguished from a compound.
Organic compound: Generally speaking, a compound containing carbon. The only exceptions are the carbonates (for example, calcium carbonate or limestone) and oxides, such as carbon dioxide.
Oxide: An inorganic compound in which the only negatively charged ion is an oxygen.
Salt: An inorganic compound formed by the reaction of an acid with a base. Generally speaking, a salt is a combination of a metal and a nonmetal, and it can contain ions of any element but hydrogen.
Type i binary compounds:Ionic compounds involving a metal that always forms a cation of a certain electric charge.
Type ii binary compounds: Ionic compounds involving a metal (typically a transition metal) that forms cations with differing charges.
Type iii binary compounds:Compounds containing only nonmetals. valence electrons: Electrons that occupy the highest energy levels in an atom. These are the only electrons involved in chemical bonding.