the science that deals with the properties, composition, and structure of substances (defined as elements and compounds), the transformations they undergo, and the energy that is released or absorbed during these processes. Every substance, whether naturally occurring or artificially produced, consists of one or more of the hundred-odd species of atoms that have been identified as elements. Although these atoms, in turn, are composed of more elementary particles, they are the basic building blocks of chemical substances; there is no quantity of oxygen, mercury, or gold, for example, smaller than an atom of that substance. Chemistry, therefore, is concerned not with the subatomic domain but with the properties of atoms and the laws governing their combinations and how the knowledge of these properties can be used to achieve specific purposes.
The great challenge in chemistry is the development of a coherent explanation of the complex behaviour of materials, why they appear as they do, what gives them their enduring properties, and how interactions among different substances can bring about the formation of new substances and the destruction of old ones. From the earliest attempts to understand the material world in rational terms, chemists have struggled to develop theories of matter that satisfactorily explain both permanence and change. The ordered assembly of indestructible atoms into small and large molecules, or extended networks of intermingled atoms, is generally accepted as the basis of permanence, while the reorganization of atoms or molecules into different arrangements lies behind theories of change. Thus chemistry involves the study of the atomic composition and structural architecture of substances, as well as the varied interactions among substances that can lead to sudden, often violent reactions.
Chemistry also is concerned with the utilization of natural substances and the creation of artificial ones. Cooking, fermentation, glass making, and metallurgy are all chemical processes that date from the beginnings of civilization. Today, vinyl, Teflon, liquid crystals, semiconductors, and superconductors represent the fruits of chemical technology. The 20th century has seen dramatic advances in the comprehension of the marvelous and complex chemistry of living organisms, and a molecular interpretation of health and disease holds great promise. Modern chemistry, aided by increasingly sophisticated instruments, studies materials as small as single atoms and as large and complex as DNA (deoxyribonucleic acid), which contains millions of atoms. New substances can even be designed to bear desired characteristics and then synthesized. The rate at which chemical knowledge continues to accumulate is remarkable. Over time more than 8,000,000 different chemical substances, both natural and artificial, have been characterized and produced. The number was less than 500,000 as recently as 1965.
Intimately interconnected with the intellectual challenges of chemistry are those associated with industry. In the mid-19th century the German chemist Justus von Liebig commented that the wealth of a nation could be gauged by the amount of sulfuric acid it produced. This acid, essential to many manufacturing processes, remains today the leading chemical product of industrialized countries. As Liebig recognized, a country that produces large amounts of sulfuric acid is one with a strong chemical industry and a strong economy as a whole. The production, distribution, and utilization of a wide range of chemical products is common to all highly developed nations. In fact, one can say that the “iron age” of civilization is being replaced by a “polymer age,” for in some countries the total volume of polymers now produced exceeds that of iron.
The scope of chemistry
The days are long past when one person could hope to have a detailed knowledge of all areas of chemistry. Those pursuing their interests into specific areas of chemistry communicate with others who share the same interests. Over time a group of chemists with specialized research interests become the founding members of an area of specialization. The areas of specialization that emerged early in the history of chemistry, such as organic, inorganic, physical, analytical, and industrial chemistry, along with biochemistry, remain of greatest general interest. There has been, however, much growth in the areas of polymer, environmental, and medicinal chemistry during the 20th century. Moreover, new specialities continue to appear, as, for example, pesticide, forensic, and computer chemistry.
Most of the materials that occur on Earth, such as wood, coal, minerals, or air, are mixtures of many different and distinct chemical substances. Each pure chemical substance (e.g., oxygen, iron, or water) has a characteristic set of properties that gives it its chemical identity. Iron, for example, is a common silver-white metal that melts at 1,535° C, is very malleable, and readily combines with oxygen to form the common substances hematite and magnetite. The detection of iron in a mixture of metals, or in a compound such as magnetite, is a branch of analytical chemistry called qualitative analysis. Measurement of the actual amount of a certain substance in a compound or mixture is termed quantitative analysis. Quantitative analytic measurement has determined, for instance, that iron makes up 72.3 percent, by mass, of magnetite, the mineral commonly seen as black sand along beaches and stream banks. Over the years, chemists have discovered chemical reactions that indicate the presence of such elemental substances by the production of easily visible and identifiable products. Iron can be detected by chemical means if it is present in a sample to an amount of 1 part per million or greater. Some very simple qualitative tests reveal the presence of specific chemical elements in even smaller amounts. The yellow colour imparted to a flame by sodium is visible if the sample being ignited has as little as one-billionth of a gram of sodium. Such analytic tests have allowed chemists to identify the types and amounts of impurities in various substances and to determine the properties of very pure materials. Substances used in common laboratory experiments generally have impurity levels of less than 0.1 percent. For special applications, one can purchase chemicals that have impurities totaling less than 0.001 percent. The identification of pure substances and the analysis of chemical mixtures enable all other chemical disciplines to flourish.
The importance of analytical chemistry has never been greater than it is today. The demand in modern societies for a variety of safe foods, affordable consumer goods, abundant energy, and labour-saving technologies places a great burden on the environment. All chemical manufacturing produces waste products in addition to the desired substances, and waste disposal has not always been carried out carefully. Disruption of the environment has occurred since the dawn of civilization, and pollution problems have increased with the growth of global population. The techniques of analytical chemistry are relied on heavily to maintain a benign environment. The undesirable substances in water, air, soil, and food must be identified, their point of origin fixed, and safe, economical methods for their removal or neutralization developed. Once the amount of a pollutant deemed to be hazardous has been assessed, it becomes important to detect harmful substances at concentrations well below the danger level. Analytical chemists seek to develop increasingly accurate and sensitive techniques and instruments.
Sophisticated analytic instruments, often coupled with computers, have improved the accuracy with which chemists can identify substances and have lowered detection limits. An analytic technique in general use is gas chromatography, which separates the different components of a gaseous mixture by passing the mixture through a long, narrow column of absorbent but porous material. The different gases interact differently with this absorbent material and pass through the column at different rates. As the separate gases flow out of the column, they can be passed into another analytic instrument called a mass spectrometer, which separates substances according to the mass of their constituent ions. A combined gas chromatograph-mass spectrometer can rapidly identify the individual components of a chemical mixture whose concentrations may be no greater than a few parts per billion. Similar or even greater sensitivities can be obtained under favourable conditions using techniques such as atomic absorption, polarography, and neutron activation. The rate of instrumental innovation is such that analytic instruments often become obsolete within 10 years of their introduction. Newer instruments are more accurate and faster and are employed widely in the areas of environmental and medicinal chemistry.
Modern chemistry, which dates more or less from the acceptance of the law of conservation of mass in the late 18th century, focused initially on those substances that were not associated with living organisms. Study of such substances, which normally have little or no carbon, constitutes the discipline of inorganic chemistry. Early work sought to identify the simple substances-namely, the elements-that are the constituents of all more complex substances. Some elements, such as gold and carbon, have been known since antiquity, and many others were discovered and studied throughout the 19th and early 20th centuries. Today, more than 100 are known. The study of such simple inorganic compounds as sodium chloride (common salt) has led to some of the fundamental concepts of modern chemistry, the law of definite proportions providing one notable example. This law states that for most pure chemical substances the constituent elements are always present in fixed proportions by mass (e.g., every 100 grams of salt contains 39.3 grams of sodium and 60.7 grams of chlorine). The crystalline form of salt, known as halite, consists of intermingled sodium and chlorine atoms, one sodium atom for each one of chlorine. Such a compound, formed solely by the combination of two elements, is known as a binary compound. Binary compounds are very common in inorganic chemistry, and they exhibit little structural variety. For this reason, the number of inorganic compounds is limited in spite of the large number of elements that may react with each other. If three or more elements are combined in a substance, the structural possibilities become greater.
After a period of quiescence in the early part of the 20th century, inorganic chemistry has again become an exciting area of research. Compounds of boron and hydrogen, known as boranes, have unique structural features that forced a change in thinking about the architecture of inorganic molecules. Some inorganic substances have structural features long believed to occur only in carbon compounds, and a few inorganic polymers have even been produced. Ceramics are materials composed of inorganic elements combined with oxygen. For centuries ceramic objects have been made by strongly heating a vessel formed from a paste of powdered minerals. Although ceramics are quite hard and stable at very high temperatures, they are usually brittle. Currently, new ceramics strong enough to be used as turbine blades in jet engines are being manufactured. There is hope that ceramics will one day replace steel in components of internal-combustion engines. In 1987 a ceramic containing yttrium, barium, copper, and oxygen, with the approximate formula YBa2Cu3O7, was found to be a superconductor at a temperature of about 100 K. A superconductor offers no resistance to the passage of an electrical current, and this new type of ceramic could very well find wide use in electrical and magnetic applications. A superconducting ceramic is so simple to make that it can be prepared in a high school laboratory. Its discovery illustrates the unpredictability of chemistry, for fundamental discoveries can still be made with simple equipment and inexpensive materials.
Many of the most interesting developments in inorganic chemistry bridge the gap with other disciplines. Organometallic chemistry investigates compounds that contain inorganic elements combined with carbon-rich units. Many organometallic compounds play an important role in industrial chemistry as catalysts, which are substances that are able to accelerate the rate of a reaction even when present in only very small amounts. Some success has been achieved in the use of such catalysts for converting natural gas to related but more useful chemical substances. Chemists also have created large inorganic molecules that contain a core of metal atoms, such as platinum, surrounded by a shell of different chemical units. Some of these compounds, referred to as metal clusters, have characteristics of metals, while others react in ways similar to biologic systems. Trace amounts of metals in biologic systems are essential for processes such as respiration, nerve function, and cell metabolism. Processes of this kind form the object of study of bioinorganic chemistry. Although organic molecules were once thought to be the distinguishing chemical feature of living creatures, it is now known that inorganic chemistry plays a vital role as well.
Organic compounds are based on the chemistry of carbon. Carbon is unique in the variety and extent of structures that can result from the three-dimensional connections of its atoms. The process of photosynthesis converts carbon dioxide and water to oxygen and compounds known as carbohydrates. Both cellulose, the substance that gives structural rigidity to plants, and starch, the energy storage product of plants, are polymeric carbohydrates. Simple carbohydrates produced by photosynthesis form the raw material for the myriad organic compounds found in the plant and animal kingdoms. When combined with variable amounts of hydrogen, oxygen, nitrogen, sulfur, phosphorus, and other elements, the structural possibilities of carbon compounds become limitless, and their number far exceeds the total of all nonorganic compounds. A major focus of organic chemistry is the isolation, purification, and structural study of these naturally occurring substances. Many natural products are simple molecules. Examples include formic acid (HCO2H) in ants, ethyl alcohol (C2H5OH) in fermenting fruit, and oxalic acid (C2H2O4) in rhubarb leaves. Other natural products, such as penicillin, vitamin B12, proteins, and nucleic acids, are exceedingly complex. The isolation of pure natural products from their host organism is made difficult by the low concentrations in which they may be present. Once they are isolated in pure form, however, modern instrumental techniques can reveal structural details for amounts weighing as little as one-millionth of a gram. The correlation of the physical and chemical properties of compounds with their structural features is the domain of physical organic chemistry. Once the properties endowed upon a substance by specific structural units termed functional groups are known, it becomes possible to design novel molecules that may exhibit desired properties. The preparation, under controlled laboratory conditions, of specific compounds is known as synthetic chemistry. Some products are easier to synthesize than to collect and purify from their natural sources. Tons of vitamin C, for example, are synthesized annually. Many synthetic substances have novel properties that make them especially useful. Plastics are a prime example, as are many drugs and agricultural chemicals. A continuing challenge for synthetic chemists is the structural complexity of most organic substances. To synthesize a desired substance, the atoms must be pieced together in the correct order and with the proper three-dimensional relationships. Just as a given pile of lumber and bricks can be assembled in many ways to build houses of several different designs, so too can a fixed number of atoms be connected together in various ways to give different molecules. Only one structural arrangement out of the many possibilities will be identical with a naturally occurring molecule. The antibiotic erythromycin, for example, contains 37 carbon, 67 hydrogen, and 13 oxygen atoms, along with one nitrogen atom. Even when joined together in the proper order, these 118 atoms can give rise to 262,144 different structures, only one of which has the characteristics of natural erythromycin. The great abundance of organic compounds, their fundamental role in the chemistry of life, and their structural diversity have made their study especially challenging and exciting. Organic chemistry is the largest area of specialization among the various fields of chemistry.
As understanding of inanimate chemistry grew during the 19th century, attempts to interpret the physiological processes of living organisms in terms of molecular structure and reactivity gave rise to the discipline of biochemistry. Biochemists employ the techniques and theories of chemistry to probe the molecular basis of life. An organism is investigated on the premise that its physiological processes are the consequence of many thousands of chemical reactions occurring in a highly integrated manner. Biochemists have established, among other things, the principles that underlie energy transfer in cells, the chemical structure of cell membranes, the coding and transmission of hereditary information, muscular and nerve function, and biosynthetic pathways. In fact, related biomolecules have been found to fulfill similar roles in organisms as different as bacteria and human beings. The study of biomolecules, however, presents many difficulties. Such molecules are often very large and exhibit great structural complexity; moreover, the chemical reactions they undergo are usually exceedingly fast. The separation of the two strands of DNA, for instance, occurs in one-millionth of a second. Such rapid rates of reaction are possible only through the intermediary action of biomolecules called enzymes. Enzymes are proteins that owe their remarkable rate-accelerating abilities to their three-dimensional chemical structure. Not surprisingly, biochemical discoveries have had a great impact on the understanding and treatment of disease. Many ailments due to inborn errors of metabolism have been traced to specific genetic defects. Other diseases result from disruptions in normal biochemical pathways.
Frequently, symptoms can be alleviated by drugs, and the discovery, mode of action, and degradation of therapeutic agents is another of the major areas of study in biochemistry. Bacterial infections can be treated with sulfonamides, penicillins, and tetracyclines, and research into viral infections has revealed the effectiveness of acyclovir against the herpes virus. There is much current interest in the details of carcinogenesis and cancer chemotherapy. It is known, for example, that cancer can result when cancer-causing molecules, or carcinogens as they are called, react with nucleic acids and proteins and interfere with their normal modes of action. Researchers have developed tests that can identify molecules likelyto be carcinogenic. The hope, of course, is that progress in the prevention and treatment of cancer will accelerate once the biochemical basis of the disease is more fully understood.
The molecular basis of biologic processes is an essential feature of the fast-growing disciplines of molecular biology and biotechnology. Chemistry has developed methods for rapidly and accurately determining the structure of proteins and DNA. In addition, efficient laboratory methods for the synthesis of genes are being devised. Ultimately, the correction of genetic diseases by replacement of defective genes with normal ones may become possible.
The simple substance ethylene is a gas composed of molecules with the formula CH2CH2. Under certain conditions, many ethylene molecules will join together to form a long chain called polyethylene, with the formula (CH2CH2)n, where n is a variable but large number. Polyethylene is a tough, durable solid material quite different from ethylene. It is an example of a polymer, which is a large molecule made up of many smaller molecules (monomers), usually joined together in a linear fashion. Many naturally occurring substances, including cellulose, starch, cotton, wool, rubber, leather, proteins, and DNA, are polymers. Polyethylene, nylon, and acrylics are examples of synthetic polymers. The study of such materials lies within the domain of polymer chemistry, a specialty that has flourished in the 20th century. The investigation of natural polymers overlaps considerably with biochemistry, but the synthesis of new polymers, the investigation of polymerization processes, and the characterization of the structure and properties of polymeric materials all pose unique problems for polymer chemists.
Polymer chemists have designed and synthesized polymers that vary in hardness, flexibility, softening temperature, solubility in water, and biodegradability. They have produced polymeric materials that are as strong as steel yet lighter and more resistant to corrosion. Oil, natural gas, and water pipelines are now routinely constructed of plastic pipe. In recent years, automakers have increased their use of plastic components to build lighter vehicles that consume less fuel. Other industries such as those involved in the manufacture of textiles, rubber, paper, and packaging materials are built upon polymer chemistry.
Besides producing new kinds of polymeric materials, researchers are concerned with developing special catalysts that are required by the large-scale industrial synthesis of commercial polymers. Without such catalysts, the polymerization process would be very slow in certain cases.
Many chemical disciplines, such as those already discussed, focus on certain classes of materials that share common structural and chemical features. Other specialties may be centred not on a class of substances but rather on their interactions and transformations. The oldest of these fields is physical chemistry, which seeks to measure, correlate, and explain the quantitative aspects of chemical processes. The Anglo-Irish chemist Robert Boyle, for example, discovered in the 17th century that at room temperature the volume of a fixed quantity of gas decreases proportionally as the pressure on it increases. Thus, for a gas at constant temperature, the product of its volume V and pressure P equals a constant number-i.e., PV = constant. Such a simple arithmetic relationship is valid for nearly all gases at room temperature and at pressures equal to or less than one atmosphere. Subsequent work has shown that the relationship loses its validity at higher pressures, but more complicated expressions that more accurately match experimental results can be derived. The discovery and investigation of such chemical regularities, often called laws of nature, lie within the realm of physical chemistry. For much of the 18th century the source of mathematical regularity in chemical systems was assumed to be the continuum of forces and fields that surround the atoms making up chemical elements and compounds. Developments in the 20th century, however, have shown that chemical behaviour is best interpreted by a quantum mechanical model of atomic and molecular structure. The branch of physical chemistry that is largely devoted to this subject is theoretical chemistry. Theoretical chemists make extensive use of computers to help them solve complicated mathematical equations. Other branches of physical chemistry include chemical thermodynamics, which deals with the relationship between heat and other forms of chemical energy, and chemical kinetics, which seeks to measure and understand the rates of chemical reactions. Electrochemistry investigates the interrelationship of electric current and chemical change. The passage of an electric current through a chemical solution causes changes in the constituent substances that are often reversible-i.e., under different conditions the altered substances themselves will yield an electric current. Common batteries contain chemical substances that, when placed in contact with each other by closing an electrical circuit, will deliver current at a constant voltage until the substances are consumed. At present there is much interest in devices that can use the energy in sunlight to drive chemical reactions whose products are capable of storing the energy. The discovery of such devices would make possible the widespread utilization of solar energy.
There are many other disciplines within physical chemistry that are concerned more with the general properties of substances and the interactions among substances than with the substances themselves. Photochemistry is a specialty that investigates the interaction of light with matter. Chemical reactions initiated by the absorption of light can be very different from those that occur by other means. Vitamin D, for example, is formed in the human body when the steroid ergosterol absorbs solar radiation; ergosterol does not change to vitamin D in the dark.
A rapidly developing subdiscipline of physical chemistry is surface chemistry. It examines the properties of chemical surfaces, relying heavily on instruments that can provide a chemical profile of such surfaces. Whenever a solid is exposed to a liquid or a gas, a reaction occurs initially on the surface of the solid, and its properties can change dramatically as a result. Aluminum is a case in point: it is resistant to corrosion precisely because the surface of the pure metal reacts with oxygen to form a layer of aluminum oxide, which serves to protect the interior of the metal from further oxidation. Numerous reaction catalysts perform their function by providing a reactive surface on which substances can react.
The manufacture, sale, and distribution of chemical products is one of the cornerstones of a developed country. Chemists play an important role in the manufacture, inspection, and safe handling of chemical products, as well as in product development and general management. The manufacture of basic chemicals such as oxygen, chlorine, ammonia, and sulfuric acid provides the raw materials for industries producing textiles, agricultural products, metals, paints, and pulp and paper. Specialty chemicals are produced in smaller amounts for industries involved with such products as pharmaceuticals, foodstuffs, packaging, detergents, flavours, and fragrances. To a large extent, the chemical industry takes the products and reactions common to “bench-top” chemical processes and scales them up to industrial quantities.
The monitoring and control of bulk chemical processes, especially with regard to heat transfer, pose problems usually tackled by chemists and chemical engineers. The disposal of by-products also is a major problem for bulk chemical producers. These and other challenges of industrial chemistry set it apart from the more purely intellectual disciplines of chemistry discussed above. Yet, within the chemical industry, there is a considerable amount of fundamental research undertaken within traditional specialties. Most large chemical companies have research-and-development capability. Pharmaceutical firms, for example, operate large research laboratories in which chemists test molecules for pharmacological activity. The new products and processes that are discovered in such laboratories are often patented and become a source of profit for the company funding the research. A great deal of the research conducted in the chemical industry can be termed applied research because its goals are closely tied to the products and processes of the company concerned. New technologies often require much chemical expertise. The fabrication of, say, electronic microcircuits involves close to 100 separate chemical steps from start to finish. Thus, the chemical industry evolves with the technological advances of the modern world and at the same time often contributes to the rate of progress.
The methodology of chemistry
Chemistry is to a large extent a cumulative science. Over time the number and extent of observations and phenomena studied increase. Not all hypotheses and discoveries endure unchallenged, however. Some of them are discarded as new observations or more satisfying explanations appear. Nonetheless, chemistry has a broad spectrum of explanatory models for chemical phenomena that have endured and been extended over time. These now have the status of theories, interconnected sets of explanatory devices that correlate well with observed phenomena. As new discoveries are made, they are incorporated into existing theory whenever possible. However, as the discovery of high-temperature superconductors in 1986 illustrates, accepted theory is never sufficient to predict the course of future discovery. Serendipity, or chance discovery, will continue to play as much a role in the future as will theoretical sophistication.
Studies of molecular structure
The chemical properties of a substance are a function of its structure, and the techniques of X-ray crystallography now enable chemists to determine the precise atomic arrangement of complex molecules. A molecule is an ordered assembly of atoms. Each atom in a molecule is connected to one or more neighbouring atoms by a chemical bond. The length of bonds and the angles between adjacent bonds are all important in describing molecular structure, and a comprehensive theory of chemical bonding is one of the major achievements of modern chemistry. Fundamental to bonding theory is the atomic-molecular concept.
Atoms and elements
As far as general chemistry is concerned, atoms are composed of the three fundamental particles: the proton, the neutron, and the electron. Although the proton and the neutron are themselves composed of smaller units, their substructure has little impact on chemical transformation. As was explained in an earlier section, the proton carries a charge of +1, and the number of protons in an atomic nucleus distinguishes one type of chemical atom from another. The simplest atom of all, hydrogen, has a nucleus composed of a single proton. The neutron has very nearly the same mass as the proton, but it has no charge. Neutrons are contained with protons in the nucleus of all atoms other than hydrogen. The atom with one proton and one neutron in its nucleus is called deuterium. Because it has only one proton, deuterium exhibits the same chemical properties as hydrogen but has a different mass. Hydrogen and deuterium are examples of related atoms called isotopes. The third atomic particle, the electron, has a charge of -1, but its mass is 1,836 times smaller than that of a proton. The electron occupies a region of space outside the nucleus termed an orbital. Some orbitals are spherical with the nucleus at the centre. Because electrons have so little mass and move about at speeds close to half that of light, they exhibit the same wave-particle duality as photons of light. This means that some of the properties of an electron are best described by considering the electron to be a particle, while other properties are consistent with the behaviour of a standing wave. The energy of a standing wave, such as a vibrating string, is distributed over the region of space defined by the two fixed ends and the up-and-down extremes of vibration. Such a wave does not exist in a fixed region of space as does a particle. Early models of atomic structure envisioned the electron as a particle orbiting the nucleus, but electron orbitals are now interpreted as the regions of space occupied by standing waves called wave functions. These wave functions represent the regions of space around the nucleus in which the probability of finding an electron is high. They play an important role in bonding theory, as will be discussed later.
Each proton in an atomic nucleus requires an electron for electrical neutrality. Thus, as the number of protons in a nucleus increases, so too does the number of electrons. The electrons, alone or in pairs, occupy orbitals increasingly distant from the nucleus. Electrons farther from the nucleus are attracted less strongly by the protons in the nucleus, and they can be removed more easily from the atom. The energy required to move an electron from one orbital to another, or from one orbital to free space, gives a measure of the energy level of the orbitals. These energies have been found to have distinct, fixed values; they are said to be quantized. The energy differences between orbitals give rise to the characteristic patterns of light absorption or emission that are unique to each chemical atom.
A new chemical atom-that is, an element-results each time another proton is added to an atomic nucleus. Consecutive addition of protons generates the whole range of elements known to exist in the universe. Compounds are formed when two or more different elements combine through atomic bonding. Such bond formation is a consequence of electron pairing and constitutes the foundation of all structural chemistry.
Ionic and covalent bonding
When two different atoms approach each other, the electrons in their outer orbitals can respond in two distinct ways. An electron in the outermost atomic orbital of atom A may move completely to an outer but stabler orbital of atom B. The charged atoms that result, A+ and B-, are called ions, and the electrostatic force of attraction between them gives rise to what is termed an ionic bond. Most elements can form ionic bonds, and the substances that result commonly exist as three-dimensional arrays of positive and negative ions. Ionic compounds are frequently crystalline solids that have high melting points (e.g., table salt).
The second way in which the two outer electrons of atoms A and B can respond to the approach of A and B is to pair up to form a covalent bond. In the simple view known as the valence-bond model, in which electrons are treated strictly as particles, the two paired electrons are assumed to lie between the two nuclei and are shared equally by atoms A and B, resulting in a covalent bond. Atoms joined together by one or more covalent bonds constitute molecules. Hydrogen gas is composed of hydrogen molecules, which consist in turn of two hydrogen atoms linked by a covalent bond. The notation H2 for hydrogen gas is referred to as a molecular formula. Molecular formulas indicate the number and type of atoms that make up a molecule. The molecule H2 is responsible for the properties generally associated with hydrogen gas. Most substances on Earth have covalently bonded molecules as their fundamental chemical unit, and their molecular properties are completely different from those of the constituent elements. The physical and chemical properties of carbon dioxide, for example, are quite distinct from those of pure carbon and pure oxygen.
The interpretation of a covalent bond as a localized electron pair is an oversimplification of the bonding situation. A more comprehensive description of bonding that considers the wave properties of electrons is the molecular-orbital theory. According to this theory, electrons in a molecule, rather than being localized between atoms, are distributed over all the atoms in the molecule in a spatial distribution described by a molecular orbital. Such orbitals result when the atomic orbitals of bonded atoms combine with each other. The total number of molecular orbitals present in a molecule is equal to the sum of all atomic orbitals in the constituent atoms prior to bonding. Thus, for the simple combination of atoms A and B to form the molecule AB, two atomic orbitals combine to generate two molecular orbitals. One of these, the so-called bonding molecular orbital, represents a region of space enveloping both the A and B atoms, while the other, the anti-bonding molecular orbital, has two lobes, neither of which occupies the space between the two atoms. The bonding molecular orbital is at a lower energy level than are the two atomic orbitals, while the anti-bonding orbital is at a higher energy level. The two paired electrons that constitute the covalent bond between A and B occupy the bonding molecular orbital. For this reason, there is a high probability of finding the electrons between A and B, but they can be found elsewhere in the orbital as well. Because only two electrons are involved in bond formation and both can be accommodated in the lower energy orbital, the anti-bonding orbital remains unpopulated. This theory of bonding predicts that bonding between A and B will occur because the energy of the paired electrons after bonding is less than that of the two electrons in their atomic orbitals prior to bonding. The formation of a covalent bond is thus energetically favoured. The system goes from a state of higher energy to one of lower energy.
Another feature of this bonding picture is that it is able to predict the energy required to move an electron from the bonding molecular orbital to the anti-bonding one. The energy required for such an electronic excitation can be provided by visible light, for example, and the wavelength of the light absorbed determines the colour displayed by the absorbing molecule (e.g., violets are blue because the pigments in the flower absorb the red rays of natural light and reflect more of the blue). As the number of atoms in a molecule increases, so too does the number of molecular orbitals. Calculation of molecular orbitals for large molecules is mathematically difficult, but computers have made it possible to determine the wave equations for several large molecules. Molecular properties predicted by such calculations correlate well with experimental results.
Many elements can form two or more covalent bonds, but only a few are able to form extended chains of covalent bonds. The outstanding example is carbon, which can form as many as four covalent bonds and can bond to itself indefinitely. Carbon has six electrons in total, two of which are paired in an atomic orbital closest to the nucleus. The remaining four are farther from the nucleus and are available for covalent bonding. When there is sufficient hydrogen present, carbon will react to form methane, CH4. When all four electron pairs occupy the four molecular orbitals of lowest energy, the molecule assumes the shape of a tetrahedron, with carbon at the centre and the four hydrogen atoms at the apexes. The C-H bond length is 110 picometres (1 picometre = 10-12 metre), and the angle between adjacent C-H bonds is close to 110°. Such tetrahedral symmetry is common to many carbon compounds and results in interesting structural possibilities. If two carbon atoms are joined together, with three hydrogen atoms bonded to each carbon atom, the molecule ethane is obtained. When four carbon atoms are joined together, two different structures are possible: a linear structure designated n-butane and a branched structure called iso-butane. These two structures have the same molecular formula, C4H10, but a different order of attachment of their constituent atoms. The two molecules are termed structural isomers. Each of them has unique chemical and physical properties, and they are different compounds. The number of possible isomers increases rapidly as the number of carbon atoms increases. There are five isomers for C6H14, 75 for C10H22, and 6.2 × 1013 for C40H82. When carbon forms bonds to atoms other than hydrogen, such as oxygen, nitrogen, and sulfur, the structural possibilities become even greater. It is this great potential for structural diversity that makes carbon compounds essential to living organisms.
Even when the bonding sequence of carbon compounds is fixed, further structural variation is still possible. When two carbon atoms are joined together by two bonding pairs of electrons, a double bond is formed. A double bond forces the two carbon atoms and attached groups into a rigid, planar structure. As a result, a molecule such as CHCl=CHCl can exist in two nonidentical forms called geometric isomers. Structural rigidity also occurs in ring structures, and attached groups can be on the same side of a ring or on different sides. Yet another opportunity for isomerism arises when a carbon atom is bonded to four different groups. These can be attached in two different ways, one of which is the mirror image of the other. This type of isomerism is called optical isomerism, because the two isomers affect plane-polarized light differently. Two optical isomers are possible for every carbon atom that is bonded to four different groups. For a molecule bearing 10 such carbon atoms, the total number of possible isomers will be 210 = 1,024. Large biomolecules often have 10 or more carbon atoms for which such optical isomers are possible. Only one of all the possible isomers will be identical to the natural molecule. For this reason, the laboratory synthesis of large organic molecules is exceedingly difficult. Only in the last few decades of the 20th century have chemists succeeded in developing reagents and processes that yield specific optical isomers. They expect that new synthetic methods will make possible the synthesis of ever more complex natural products.
Investigations of chemical transformations
The structure of ionic substances and covalently bonded molecules largely determines their function. As noted above, the properties of a substance depend on the number and type of atoms it contains and on the bonding patterns present. Its bulk properties also depend, however, on the interactions among individual atoms, ions, or molecules. The force of attraction between the fundamental units of a substance dictate whether, at a given temperature and pressure, that substance will exist in the solid, liquid, or gas phase. At room temperature and pressure, for example, the strong forces of attraction between the positive ions of sodium (Na+) and the negative ions of chlorine (Cl-) draw them into a compact solid structure. The weaker forces of attraction among neighbouring water molecules allow the looser packing characteristic of a liquid. Finally, the very weak attractive forces acting among adjacent oxygen molecules are exceeded by the dispersive forces of heat; oxygen, consequently, is a gas. Interparticle forces thus affect the chemical and physical behaviour of substances, but they also determine to a large extent how a particle will respond to the approach of a different particle. If the two particles react with each other to form new particles, a chemical reaction has occurred. Notwithstanding the unlimited structural diversity allowed by molecular bonding, the world would be devoid of life if substances were incapable of change. The study of chemical transformation, which complements the study of molecular structure, is built on the concepts of energy and entropy.
Energy and the first law of thermodynamics
The concept of energy is a fundamental and familiar one in all the sciences. In simple terms, the energy of a body represents its ability to do work, and work itself is a force acting over a distance.
Chemical systems can have both kinetic energy (energy of motion) and potential energy (stored energy). The kinetic energy possessed by any collection of molecules in a solid, liquid, or gas is known as its thermal energy. Since liquids expand when they have more thermal energy, a liquid column of mercury, for example, will rise higher in an evacuated tube as it becomes warmer. In this way a thermometer can be used to measure the thermal energy, or temperature, of a system. The temperature at which all molecular motion comes to a halt is known as absolute zero.
Energy also may be stored in atoms or molecules as potential energy. When protons and neutrons combine to form the nucleus of a certain element, the reduction in potential energy is matched by the production of a huge quantity of kinetic energy. Consider, for instance, the formation of the deuterium nucleus from one proton and one neutron. The fundamental mass unit of the chemist is the mole, which represents the mass, in grams, of 6.02 × 1023 individual particles, whether they be atoms or molecules. One mole of protons has a mass of 1.007825 grams and one mole of neutrons has a mass of 1.008665 grams. By simple addition the mass of one mole of deuterium atoms (ignoring the negligible mass of one mole of electrons) should be 2.016490 grams. The measured mass is 0.00239 gram less than this. The missing mass is known as the binding energy of the nucleus and represents the mass equivalent of the energy released by nucleus formation. By using Einstein’s formula for the conversion of mass to energy (E = mc2), one can calculate the energy equivalent of 0.00239 gram as 2.15 × 108 kilojoules. This is approximately 240,000 times greater than the energy released by the combustion of one mole of methane. Such studies of the energetics of atom formation and interconversion are part of a specialty known as nuclear chemistry.
The energy released by the combustion of methane is about 900 kilojoules per mole. Although much less than the energy released by nuclear reactions, the energy given off by a chemical process such as combustion is great enough to be perceived as heat and light. Energy is released in so-called exothermic reactions because the chemical bonds in the product molecules, carbon dioxide and water, are stronger and stabler than those in the reactant molecules, methane and oxygen. The chemical potential energy of the system has decreased, and most of the released energy appears as heat, while some appears as radiant energy, or light. The heat produced by such a combustion reaction will raise the temperature of the surrounding air and, at constant pressure, increase its volume. This expansion of air results in work being done. In the cylinder of an internal-combustion engine, for example, the combustion of gasoline results in hot gases that expand against a moving piston. The motion of the piston turns a crankshaft, which then propels the vehicle. In this case, chemical potential energy has been converted to thermal energy, some of which produces useful work. This process illustrates a statement of the conservation of energy known as the first law of thermodynamics. This law states that, for an exothermic reaction, the energy released by the chemical system is equal to the heat gained by the surroundings plus the work performed. By measuring the heat and work quantities that accompany chemical reactions, it is possible to ascertain the energy differences between the reactants and the products of various reactions. In this manner, the potential energy stored in a variety of molecules can be determined, and the energy changes that accompany chemical reactions can be calculated.
Entropy and the second law of thermodynamics
Some chemical processes occur even though there is no net energy change. Consider a vessel containing a gas, connected to an evacuated vessel via a channel wherein a barrier obstructs passage of the gas. If the barrier is removed, the gas will expand into the evacuated vessel. This expansion is consistent with the observation that a gas always expands to fill the volume available. When the temperature of both vessels is the same, the energy of the gas before and after the expansion is the same. The reverse reaction does not occur, however. The spontaneous reaction is the one that yields a state of greater disorder. In the expanded volume, the individual gas molecules have greater freedom of movement and thus are more disordered. The measure of the disorder of a system is a quantity termed entropy. At a temperature of absolute zero, all movement of atoms and molecules ceases, and the disorder-and entropy-of such perfectly compacted substances is zero. (Zero entropy at zero temperature is in accord with the third law of thermodynamics.) All substances above absolute zero will have a positive entropy value that increases with temperature. When a hot body cools down, the thermal energy it loses passes to the surrounding air, which is at a lower temperature. As the entropy of the cooling body decreases, the entropy of the surrounding air increases. In fact, the increase in entropy of the air is greater than the decrease in entropy of the cooling body. This is consistent with the second law, which states that the total entropy of a system and its surroundings always increases in a spontaneous reaction. Thus the first and second laws of thermodynamics indicate that, for all processes of chemical change throughout the universe, energy is conserved but entropy increases.
Application of the laws of thermodynamics to chemical systems allows chemists to predict the behaviour of chemical reactions. When energy and entropy considerations favour the formation of product molecules, reagent molecules will act to form products until an equilibrium is established between products and reagents. The ratio of products to reagents is specified by a quantity known as an equilibrium constant, which is a function of the energy and entropy differences between the two. What thermodynamics cannot predict, however, is the rate at which chemical reactions occur. For fast reactions an equilibrium mixture of products and reagents can be established in one millisecond or less; for slow reactions the time required could be hundreds of years.
Rates of reaction
When the specific rates of chemical reactions are measured experimentally, they are found to be dependent on the concentrations of reacting species, temperature, and a quantity called activation energy. Chemists explain this phenomenon by recourse to the collision theory of reaction rates. This theory builds on the premise that a reaction between two or more chemicals requires, at the molecular level, a collision between two rapidly moving molecules. If the two molecules collide in the right way and with enough kinetic energy, one of the molecules may acquire enough energy to initiate the bond-breaking process. As this occurs, new bonds may begin to form, and ultimately reagent molecules are converted into product molecules. The point of highest energy during bond breaking and bond formation is called the transition state of the molecular process. The difference between the energy of the transition state and that of the reacting molecules is the activation energy that must be exceeded for a reaction to occur. Reaction rates increase with temperature because the colliding molecules have greater energies, and more of them will have energies that exceed the activation energy of reaction. The modern study of the molecular basis of chemical change has been greatly aided by lasers and computers. It is now possible to study short-lived collision products and to better determine the molecular mechanisms that fix the rate of chemical reactions. This knowledge is useful in designing new catalysts that can accelerate the rate of reaction by lowering the activation energy. Catalysts are important for many biochemical and industrial processes because they speed up reactions that ordinarily occur too slowly to be useful. Moreover, they often do so with increased control over the structural features of the product molecules. A rhodium phosphine catalyst, for example, has enabled chemists to obtain 96 percent of the correct optical isomer in a key step in the synthesis of L-dopa, a drug used for treating Parkinson’s disease.
Chemistry and society
For the first two-thirds of the 20th century, chemistry was seen by many as the science of the future. The potential of chemical products for enriching society appeared to be unlimited. Increasingly, however, and especially in the public mind, the negative aspects of chemistry have come to the fore. Disposal of chemical by-products at waste-disposal sites of limited capacity has resulted in environmental and health problems of enormous concern. The legitimate use of drugs for the medically supervised treatment of diseases has been tainted by the growing misuse of mood-altering drugs. The very word chemicals has come to be used all too frequently in a pejorative sense. There is, as a result, a danger that the pursuit and application of chemical knowledge may be seen as bearing risks that outweigh the benefits.
It is easy to underestimate the central role of chemistry in modern society, but chemical products are essential if the world’s population is to be clothed, housed, and fed. The world’s reserves of fossil fuels (e.g., oil, natural gas, and coal) will eventually be exhausted, some as soon as the 21st century, and new chemical processes and materials will provide a crucial alternative energy source. The conversion of solar energy to more concentrated, useful forms, for example, will rely heavily on discoveries in chemistry. Long-term, environmentally acceptable solutions to pollution problems are not attainable without chemical knowledge. There is much truth in the aphorism that “chemical problems require chemical solutions.” Chemical inquiry will lead to a better understanding of the behaviour of both natural and synthetic materials and to the discovery of new substances that will help future generations better supply their needs and deal with their problems.
Progress in chemistry can no longer be measured only in terms of economics and utility. The discovery and manufacture of new chemical goods must continue to be economically feasible but must be environmentally acceptable as well. The impact of new substances on the environment can now be assessed before large-scale production begins, and environmental compatibility has become a valued property of new materials. For example, compounds consisting of carbon fully bonded to chlorine and fluorine, called chlorofluorocarbons (or Freons), were believed to be ideal for their intended use when they were first discovered. They are nontoxic, nonflammable gases and volatile liquids that are very stable. These properties led to their widespread use as solvents, refrigerants, and propellants in aerosol containers. Time has shown, however, that these compounds decompose in the upper regions of the atmosphere and that the decomposition products act to destroy stratospheric ozone. Limits have now been placed on the use of chlorofluorocarbons, but it is impossible to recover the amounts already dispersed into the atmosphere.
The chlorofluorocarbon problem illustrates how difficult it is to anticipate the overall impact that new materials can have on the environment. Chemists are working to develop methods of assessment, and prevailing chemical theory provides the working tools. Once a substance has been identified as hazardous to the existing ecological balance, it is the responsibility of chemists to locate that substance and neutralize it, limiting the damage it can do or removing it from the environment entirely. The last years of the 20th century will see many new, exciting discoveries in the processes and products of chemistry. Inevitably, the harmful effects of some substances will outweigh their benefits, and their use will have to be limited. Yet, the positive impact of chemistry on society as a whole seems beyond doubt.