Inorganic Compounds

Type of physical science: Chemistry

Field of study: Chemistry of molecules: types of molecules

Inorganic chemistry is loosely defined as the study of materials that do not contain both the elements carbon and hydrogen. Inorganic compounds run the gamut from common household items to large biologically important materials, from minerals to industrially important catalysts, making inorganic chemistry an enormous and very diversified field of study.

Overview

Elements are the atomic building blocks of nature. Each element consists ultimately of atoms, and each is given a simple one- or two-letter symbol, such as C for carbon, H for hydrogen, Cl for chlorine, and Ca for calcium. There are slightly more than one hundred known elements.

Compounds are materials (or "substances") that consist of atoms of more than one type of element. Each compound is assigned a formula, which is a listing of the elements present, with subscripts used to indicate the relative number of atoms of each element. For example, calcium chloride has as its formula CaCl₂, meaning that there is twice as much chlorine as calcium.

Historically, those compounds that contain, at the least, both the elements carbon and hydrogen have been called organic. The name derives from the importance of such compounds to life. Hence, all other compounds are called inorganic. It would seem that, since there are more than one hundred elements, and since a compound is inorganic as long as it does not contain both carbon and hydrogen, then there ought to be, statistically, many more inorganic compounds than organic ones. This is not true, as a result of the abundant possible kinds of compounds containing carbon and hydrogen. It is not an exaggeration, however, to say that the variety of types of inorganic compounds far surpasses the variety found in their organic counterparts.

There are six major classes of inorganic compounds: strong (and some weak) acids in water; strong (and some weak) bases in water; salts; complex ions and coordination compounds; organometallic compounds; and inorganic compounds not falling into one of the above categories. Each of these will briefly be examined here.

For a variety of reasons, water is of primal importance in the study of chemistry. Water is a molecular compound having the formula H₂O. Molecular compounds typically consist of small, electrically neutral units called molecules. Each molecule is held together by covalent bonds. A covalent bond may be thought of as a bit of atomic glue holding the disparate atoms together to form a coherent whole: the molecule. Molecules, like atoms, can gain a net charge, in which case they are called ions. When a molecule or atom gains a net negative charge, it is called an anion; conversely, when an atom or molecule gains a net positive charge, it is called a cation.

While water consists principally of neutral water molecules, a small portion of the water is transformed chemically into hydronium cations (H₃O⁺) and hydroxide anions (OH^-). Note that the net charge is included as a superscript of the formula. In water, the amounts of hydronium and hydroxide are equal.

When various materials are added to water, the relative proportions of hydronium and hydroxide ions become altered. Acids are materials that, when added to water, cause hydronium to become more prevalent than hydroxide. A strong acid is one that is converted with virtually 100 percent chemical efficiency into hydronium in water. There are only a handful of strong acids, and they are all inorganic compounds. Examples include hydrochloric acid (HCl), nitric acid (HNO₃), sulfuric acid (H₂ SO₄), and a handful of others. Weak acids are those that are chemically transformed into hydronium, but with less than 100 percent efficiency. Most weak acids are organic, but a few are inorganic, such as hydrosulfuric acid (H₂S) and hydrofluoric acid (HF).

Whereas acids increase the relative amount of hydronium in water, bases increase the relative amount of hydroxide ions. Strong bases and weak bases are defined much as are strong and weak acids. All strong bases are inorganic compounds, such as sodium hydroxide, NaOH; potassium hydroxide, KOH; calcium hydroxide, Ca(OH)2; and a few others. As before, most weak bases are organic compounds, but a few (such as ammonia, NH3) are inorganic.

Whereas water and all the inorganic acids mentioned above are molecular compounds, the strong bases, it is important to note, are ionic compounds. Ionic compounds consist of both anions and cations in a proportion such that the overall compound is electrically neutral. Sodium hydroxide, for example, consists of sodium (Na⁺) cations and OH-anions in a one-to-one proportion (NaOH), so that the entire substance is neutral. Whereas molecular compounds are held together by covalent bonds, ionic compounds are held together by ionic bonds. These are simply the electrical attractions between the oppositely charged ions.

It is perhaps not surprising that acids and bases react chemically with one another. The result of such a reaction is always water, plus an ionic compound called a salt. Most salts are inorganic in nature. As an example, if nitric acid, HNO₃, is mixed with calcium hydroxide, Ca(OH)₂, in water, these original materials are consumed. In their places, one finds some additional water, plus the compound calcium nitrate, Ca(NO₃)₂. Calcium nitrate is an ionic compound, consisting of the ions Ca⁺² and NO₃^-. Calcium nitrate is called a salt because it is formed as a result of reaction of an acid with a base. Ordinary table salt (the most famous of all the salts) is the inorganic ionic compound NaCl, and is formed by action of hydrochloric acid with sodium hydroxide in water. In modern times, the term "salt" has come to mean virtually any simple ionic compound, excluding the strong bases.

The above analysis of acids, bases, and salts has been strongly linked with water.

Acids, bases, and salts all can be defined in more general terms, with no reference made to water, but this more or less obscures the central issues being considered here. Indeed, much of modern inorganic chemistry has centered on broadening and generalizing the concepts of acids, bases, and salts.

The next broad class of inorganic substances consists of complex ions and coordination compounds. The great majority (about 80 percent) of the elements are metals, and, as such, these elements are good conductors of heat and electricity, and good reflectors of light. The metals can be split into three groups: the representative metals are those including sodium, potassium, calcium, and a few others; the main-body transition elements comprise a broad class of metals containing many familiar examples such as copper (Cu), iron (Fe), silver (Ag), gold (Au), mercury (Hg), and many others; and lastly, the inner-transition metals consist of uranium, cerium, thorium, and several others. Largely, the representative metals form ionic compounds (strong bases and salts), which we have discussed previously. The inner-transition elements have relatively little known chemistry associated with them. The main-body transition metals form a wide variety of inorganic compounds. While these elements form simple ionic compounds (salts), they also form a huge selection of more complicated inorganic substances. Many of these compounds are called complex ions; others are coordination compounds.

A complex ion starts off (usually) as a main-body transition metal atom or cation, such as Hg⁺². Then some organic or inorganic molecules or ions attach themselves to this cation.

These incoming groups are called ligands. The attachments between metal and ligands are typically covalent bonds, so the entire moiety is held together as a molecule. As an example, when the inorganic ion cyanide (CN^-), comes into contact with an Hg⁺² cation, the complex ion [Hg(CN)₄]^-2 can form. The mercury cation is centrally located, and the cyanide ions attach themselves to this ion. The entire unit has a net charge of minus two, which derives from the fact that the mercury has a plus-two charge and each of the four cyanides has a charge of minus one.

It is customary to enclose each complex ion in brackets, and include the net charge as a superscript. The number of possible complex ions is staggering. There are numerous possible transition metal cations, and an even greater variety of possible ligands. Two other common examples are [Ag(NH₃)]⁺² and [Co(NH₃)₄Cl]²⁺. Note that in the latter of these, there are two different kinds of ligands (ammonia, NH₃, and chlorine anion, Cl^-) attached to the same cobalt (Co⁺³) cation.

Sometimes the charge on the transition metal cation is exactly canceled by the charge(s) on the ligands. An example is [Hg(CN)²], where the charge on the mercury cation is plus two and the charge on each cyanide is minus one. Thus the charges cancel, and the entire unit has no net electrical charge. Such complexes are called coordination compounds. The only difference between a complex ion and a coordination compound is that the former bears a net charge while the latter does not.

Organometallic compounds are intimately related to complex ions and coordination compounds. Again, the central ion (or atom) is frequently from the main-body transition metals.

The ligands are organic molecules or anions, and there is typically covalent bonding between the metal and carbon atoms in the ligand. In a sense, organometallic chemistry is a transitional area between organic and inorganic chemistry. One of the most famous organometallic compounds is ferrocene, Fe(C₅H₅)₂. There is a central iron ion covalently bonded to two organic cyclopentadienyl anions, C₅H₅^-. As a second example, the molecule carbon monoxide (CO) is not ordinarily considered to be organic, as it contains carbon but not hydrogen. Nevertheless, there are abundant, interesting inorganic complexes between transition metal atoms (and ions) and carbon monoxide. These are often classified as organometallic compounds. Examples include Fe(CO)₆ and Ni(CO)₄, the central atoms being iron and nickel (Ni), respectively.

Lastly, there are numerous inorganic compounds that do not fall into any of the foregoing categories. There is a wide spectrum of inorganic compounds of the elements boron, nitrogen, phosphorous, silicon, and sulfur, to name but a few. As another interesting example, the noble gases include the elements helium, neon, argon, and xenon. Up until the 1960's, no stable compounds of the noble gases were known. Within the past several decades, some compounds have been discovered. These are all inorganic in nature, and frequently involve the elements xenon and fluorine; XeF₆ is a famous example.

Applications

The applications of inorganic chemistry are as widespread and varied as one might expect of such a far-ranging discipline.

The strong acids and bases all find huge industrial niches. Because it is a gentle oxidant, sulfuric acid is widely used in the preparation of organic chemicals, and is one of the most-manufactured chemicals in the world. Other strong acids are useful in dissolving metals; hydrofluoric acid is used to etch glass. The strong bases have assorted applications, from drain cleaners to saponification, the process of making soap. The weak base ammonia is a commonly used cleaner.

Salts play an enormous role in daily life. Salts of lithium are used to treat various mental disorders, while salts of sodium and potassium play crucial physiological roles. As an example, a nerve impulse originates with the exchange of sodium and potassium ions across a membrane surrounding the nerve cord. Also, there are large amounts of sodium salts in the kidneys, and these are important in the retention and regulation of water in the body. Potassium and ammonium salts are widely used as fertilizers. Salts of calcium, strontium, and barium are abundantly present in minerals such as limestone, dolomite, and carnallite. Similarly, bauxite is an aluminum-containing mineral from which most commercial aluminum metal is prepared. It is hardly an exaggeration to say that the study of geology is frequently the study of inorganic compounds.

Complex ions and coordination compounds have a tremendous number of applications, particularly in biology and medicine. Hemoglobin, for example, consists of a large organic protein molecule wrapped around a porphyrin group. A porphyrin group is basically a complex ion, with the central transition metal ion being iron. The iron ion forms six covalent bonds to its ligands. Four of these are to nitrogen atoms of the porphyrin, and a fifth is to one of the amino acids of the protein chain. The sixth bond is to either a molecule of oxygen (O₂) or to a water molecule. In the lungs, oxygen becomes bonded to the iron in hemoglobin. Then when the blood carries the hemoglobin to the tissues, the oxygen is released and replaced by a water molecule, thus providing the tissue with the oxygen needed for life. One problem with this scheme is that oxygen is not a particularly good ligand, and so is easily replaced by other, better ligands. One such ligand is carbon monoxide (CO). The carbon monoxide (present in automobile exhaust, for example) preferentially binds to the hemoglobin in the lungs, excluding oxygen and potentially causing death. There are several other iron-containing proteins in the body, such as cytochromes and myoglobins. Enzymes are special proteins that act to catalyze (or speed up) the biochemical reactions occurring in the body. Several iron-containing enzymes are also known. As a final example, chlorophyll is the compound in plants that aids photosynthesis (the conversion of carbon dioxide into oxygen). Chlorophyll, like hemoglobin, is basically a complex ion, the metal ion being magnesium.

Metal ions are both a curse and a blessing to biological systems. Mercury ion, for example, is believed to poison the body by binding strongly to the sulfur atom of some amino acids in enzymes, thus reducing the effectiveness of the enzymes. On the other hand, metal ions can be used as antidotes to several poisons. As long as the poison is a good ligand, administering doses of appropriate metal ions causes the toxic agents to form complex ions with the metal.

These complexes can then be eliminated from the body. As a final example, the coordination compound of platinum, [Pt(NH₃)₂Cl₂], is frequently used as an anticancer agent. The action of this substance is not entirely understood, but it is believed that the platinum binds to the DNA in the cancerous tissues, rendering it dysfunctional.

The primary use of organometallic compounds industrially is as catalysts. A staggering amount of organic compounds is produced annually. To make such processes economical, catalysts are needed to speed up the reactions. For example, several million tons of aldehydes and alcohols (both organic materials) are produced each year. The starting material is an alkene, another organic substance. The catalyst necessary to carry out these organic reactions, however, is an organometallic compound of cobalt, Co₂(CO)₈. Similarly, a rhodium and carbon monoxide organometallic compound is the catalyst in the conversion of methyl alcohol into acetic acid (the active ingredient in vinegar).

Context

While both ancients and alchemists had some success in preparing both elements and inorganic compounds, chemistry did not come into existence as a scientific discipline until the end of the eighteenth century. By the latter half of the nineteenth century, it was recognized that there were fundamental differences between organic and inorganic substances. Organic chemistry, being more narrowly focused and amenable to systemization and prediction, began to flourish, and continues to do so. Inorganic chemistry, however, was not so fortunate.

Systemization was difficult, and predictability hardly existent.

Perhaps the one bright spot was the work of Alfred Werner in the late nineteenth century. Werner was the first to explain the bonding arrangement in coordination compounds and complex ions, materials that had puzzled chemists for some years.

The time between Werner and the end of World War II saw rapid and spectacular developments in many areas of science, but inorganic chemistry languished during this period.

Only after World War II, and largely as a result of the new interest in atomic energy, did inorganic chemistry gain renewed stature as a scientific discipline.

In the ensuing years, an enormous amount of ground has been covered. Much work has been done (and will continue to be done) on coordination compounds, complex ions, and organometallic substances. The discovery of stable compounds of the noble gases caused a mini-revolution in the way chemists think about molecules and chemical reactivity. It is clear that bioinorganic chemistry (metalloenzymes, metalloproteins, and so on ) is a burgeoning discipline, as are the fields of superconductors, the chemistry of the inner-transition elements, and the development of the quantum mechanical techniques and models necessary to interpret and predict much of inorganic chemistry.

Principal terms

ACID: a material that acts to increase the relative amount of hydronium ion in water

BASE: a material that acts to increase the relative amount of hydroxide ion in water

COMPLEX IONS: compounds formed between a metal ion (or atom) and a set of ligands; if the complex has no net electrical charge, it is called a coordination compound; a closely related group of compounds are the organometallics, in which the ligands are organic molecules or ions

INORGANIC COMPOUNDS: historically, chemical compounds not containing both the elements hydrogen and carbon; in contrast, organic compounds are those that do contain both carbon and hydrogen

IONIC COMPOUNDS: substances that consist of some positively charged ions and some negatively charged ions, present in a proportion that guarantees overall electrical neutrality of the material

IONS: atoms or molecules that have accumulated a net electrical charge; positively charged ions are called cations, while their negatively charged counterparts are called anions

LIGANDS: the entities connected to a (usually) metal atom or ion in a complex ion, coordination compound, or organometallic compound

METALS: those elements that are good conductors of heat and electricity, and good reflectors of light; approximately 80 percent of elements are metals

MOLECULAR COMPOUNDS: in contrast to ionic compounds, these typically consist ultimately of generally small, electrically neutral units called molecules; molecules are composed of atoms, all held together by covalent bonds

SALTS: historically, the compounds formed by reaction of an acid with a base in water; generically, any simple ionic compound, excluding the strong bases

Bibliography

Cotton, F. Albert, and Geoffrey Wilkinson. BASIC INORGANIC CHEMISTRY. New York: John Wiley, 1976. Written by two Nobel prize winners, this is a less detailed selection of topics from the authors' famous compendium, ADVANCED INORGANIC CHEMISTRY. The topics should be accessible to most undergraduates.

De Duve, Christian. A GUIDED TOUR OF THE LIVING CELL. New York: Scientific American Books, 1984. A splendid book (there are actually two volumes), with wonderful photographs and illustrations. Many of the applications of inorganic chemistry are discussed, including metalloproteins and metalloenzymes, and sodium and potassium transport. The emphasis is on the biological side of chemistry.

Farber, Eduard. THE EVOLUTION OF CHEMISTRY. New York: Ronald Press, 1952. A fairly compact and nontechnical introduction to the history of modern chemistry, including inorganic chemistry.

Huheey, James E. INORGANIC CHEMISTRY. 3d ed. New York: Harper & Row, 1983. A tremendous textbook for undergraduate students. Contains abundant diagrams and pictures that serve to supplement the text. The chapter on the inorganic chemistry of biological systems is excellent.

Ifde, Aaron J. THE DEVELOPMENT OF MODERN CHEMISTRY. New York: Harper & Row, 1964. An extremely thorough (yet relatively nontechnical) history of chemistry, including two extensive chapters on inorganic chemistry during both the nineteenth and twentieth centuries.

Moeller, Therald. INORGANIC CHEMISTRY. New York: Wiley, 1982. What makes this book interesting is its organization, which is both different from most other inorganic textbooks and more illuminating. Broad topics are covered, but in good detail. It is less a recounting of facts (although there are plenty) than a study of concepts. Easily understandable to undergraduate students.

Acids and Bases

Quantum Mechanics of Chemical Bonding