Carbon
Carbon
Carbon is the non-metallic chemical element of atomic number 6 in group 14 of the periodic table. Its symbol is C, atomic weight 12.01, and specific gravity as graphite 2.25 and as diamond 3.51. Its stable isotopes are12C (98.90%) and 13C (1.10%). The weight of the12C atom is the international standard on which atomic weights are based. It is defined as weighing exactly 12.00000 atomic mass units.
Humans have known about various forms of carbon for thousands of years, back somewhere in prehistoric times. When cave people made a fire, they observed the formation of soot (finely divided carbon particles), although they had, of course, no detailed understanding of the chemistry involved. The production of charcoal by heating wood in the absence of air and of coke by heating coal in the absence of air were both well known to ancient peoples. Lampblack was also mixed with olive oil or balsam gum to make ink by early peoples, and ancient Egyptians are known to have used lampblack as eyeliner.
The person who first discovered carbon is unknown. However, the first recognition that carbon might be a chemical element is often attributed to the French physicist Rene´ Antoine Ferchault Re´aumur (1683–1757), who attributed the difference among wrought iron, cast iron, and steel to the presence of some “black combustible material” that he knew was present in charcoal. Carbon was classified officially as an element near the end of the eighteenth century. The name chosen for the element was based on an earlier Latin name for charcoal, carbo.
In various forms, carbon is found not only on Earth, but in the atmospheres of other planets, in the Sun and stars, in comets, and in some meteorites.
On Earth, carbon can be considered to be the most important of all the chemical elements because it is the essential element in practically all of the chemical compounds in living things. Carbon compounds make the processes of life work. Beyond Earth, carbon-atom nuclei are an essential part of the nuclear fusion reactions that produce the energy of the Sun and other stars. Without carbon, the Sun would be cold and dark.
How carbon is found
In the form of chemical compounds, carbon is distributed throughout the world as carbon dioxide gas, CO2, in the atmosphere and dissolved in all the rivers, lakes, and oceans. In the form of carbonates, mostly calcium carbonate (CaCO3), it occurs as huge rocky masses of limestone, marble, and chalk. In the form of hydrocarbons, it occurs as great deposits of natural gas, petroleum, and coal. Coal is important not only as a fuel, but because it is the source of the carbon that is dissolved in molten iron to make steel.
All plants and animals on Earth contain a substantial proportion of carbon. After hydrogen and oxygen, carbon is the most abundant element in the human body, making up 10.7% of all the body’s atoms.
Carbon is found as the free (uncombined) element in three different allotropic forms—different geometrical arrangements of the atoms in the solid. The two crystalline forms (forms containing very definite atomic arrangements) are graphite and diamond. Graphite is one of the softest known materials, while diamond is one of the hardest.
There is also a shapeless, or amorphous, form of carbon in which the atoms have no particular geometric arrangement. Carbon black, a form of amorphous carbon obtained from smoky flames, is used to make rubber tires and black inks. Charcoal—wood or other plant material that has been heated in the absence of enough air to actually burn—is mostly amorphous carbon, but it retains some of the microscopic structure of the plant cells in the wood from which it was made. Activated charcoal is charcoal that has been steam-purified of all the gummy wood-decomposition products, leaving porous grains of pure carbon that have an enormous microscopic surface area. It is estimated that one cubic inch of activated charcoal contains 200, 000 square feet (18, 580 meters2) of microscopic surface. This huge surface has a stickiness, called adsorption, for molecules of gases and solids; activated charcoal is therefore used to remove impurities from water and air, such as in home water purifiers and in gas masks.
Graphite
Graphite is a soft, shiny, dark gray or black, greasy-feeling mineral that is found in large masses throughout the world, including the United States, Brazil, England, western Europe, Siberia, and Sri Lanka. It is a good conductor of electricity and resists temperatures up to about 6, 332°F (3, 500°C), which makes it useful as brushes (conductors that slide along rotating parts) in electric motors and generators, and as electrodes in high-temperature electrolysis cells. Because of its slipperiness, it is used as a lubricant. For example, powdered graphite is used to lubricate locks, where oil might “gum up the works.” The so-called lead in pencils is actually a mixture of graphite, clay, and wax. It is called lead because the metallic element lead (Pb) leaves gray marks on paper and was used for writing in ancient times. When graphite-based pencils came into use, they were called lead pencils.
The reason for graphite’s slipperiness is its unusual crystalline structure. It consists of a stack of one-atom-thick sheets of carbon atoms, bonded tightly together into a hexagonal pattern in each sheet, but with only very weak attractions—much weaker than actual chemical bonds—holding the sheets together.
The sheets of carbon atoms can therefore slide easily over one another; graphite is slippery in the same way as layers of wet leaves on a sidewalk.
Diamond
Diamond, the other crystalline form of pure carbon, is the world’s hardest natural material, and is used in industry as an abrasive and in drill tips for drilling through rock in oil fields and human teeth in dentists’ offices. On a hardness scale of one to ten, which mineralogists refer to as the Moh scale of hardness, diamond is awarded a perfect ten. However, that is not why diamonds are so expensive. They are the most expensive of all gems, and they are kept that way, by supply and demand. The supply is largely controlled by the De Beers Consolidated Mines, Inc. in South Africa, where most of the world’s diamonds are mined, and the demand is kept high by the social importance that is widely attributed to diamonds.
A diamond can be considered to be a single huge molecule consisting of nothing but carbon atoms that are strongly bonded to each other by covalent bonds, just as in other molecules. A one-carat diamond molecule contains 1022 carbon atoms.
The beauty of gem-quality diamonds—industrial diamonds are small, dark, and cloudy—comes from their crystal clarity, their high refractivity (ability to bend light rays), and their high dispersion (their ability to spread light of different colors apart, which makes the diamond’s rainbow fire). Skillful chipping of the gems into facets (flat faces) at carefully calculated angles makes the most of their sparkle. Even though diamonds are hard, meaning that they cannot be scratched by other materials, they are brittle—they can be cracked.
The chemistry of carbon
Carbon is unique among the elements because its atoms can form an endless variety of molecules with an endless variety of sizes, shapes, and chemical properties. No other element can do that to the degree that carbon can. In the evolution of life on Earth, nature has always been able to find just the right carbon compound out of the millions available, to serve just about any required function in the complicated chemistry of living things.
Carbon-containing compounds are called organic compounds, and the study of their properties and reactions is called organic chemistry. The name organic was originally given to those substances that are found in living organisms: plants and animals. As scientists now know, almost all of the chemical substances in living things are carbon compounds (water and minerals are the obvious exceptions). The name organic was eventually applied to the chemistry of all carbon compounds, regardless of where they come from.
Until the early nineteenth century, it was believed that organic substances contained a supernatural life force that made them special, and that they were not susceptible to chemical experimentation. But in 1828, German chemist named Friedrich Wöhler (1800–1882) apparently broke down the mysterious barrier between living and non-living things. By simply heating a non-organic, non-living chemical called ammonium cyanate (NH4 OCN), he converted it into a chemical called urea (H2 N-CO-NH2), which was known to be a waste product in the urine of mammals and was therefore an organic substance. As Wöhler put it, he was amazed to be able to create an organic substance “without benefit of a kidney, a bladder or a dog.” What had happened in Wöhler’s experiment was that the eight atoms in the ammonium cyanate molecule—two nitrogen atoms, four hydrogen atoms, one oxygen atom, and one carbon atom—simply rearranged themselves into a molecule having a different geometry. In chemical language, the two molecules are isomers of one another.
After Wöhler, chemists boldly synthesized (made artificially) many of the chemical compounds that formerly had been observed only in living things. Today, biochemistry—living chemistry—is one of the most active and productive fields of scientific research. It has taught scientists more about the processes of life than could ever have been imagined.
Why carbon is special
There are now more than ten million organic compounds known by chemists. Many more undoubtedly exist in nature, and organic chemists are continually creating (synthesizing) new ones. Carbon is able to form the most compounds of any element because each carbon atom can form four chemical bonds to other atoms and because the carbon atom is just the right, small size to fit in comfortably as parts of very large molecules.
Having the atomic number 6, every carbon atom has a total of six electrons. Two are in a completed inner orbit, while the other four are valence electrons—outer electrons that are available for forming bonds with other atoms.
The carbon atom’s four valence electrons can be shared by other atoms that have electrons to share, thus forming covalent (shared-electron) bonds. They can even be shared by other carbon atoms, which in turn can share electrons with other carbon atoms and so on, forming long strings of carbon atoms, bonded to each other like links in a chain. Silicon (Si), another element in group 14 of the periodic table, also has four valence electrons and can make large molecules called silicones, but its atoms are too large to fit together into as great a variety of molecules as carbon atoms can.
Carbon’s ability to form long carbon-to-carbon chains is the first of five reasons that there can be so many different carbon compounds; a molecule that differs by even one atom is, of course, a molecule of a different compound. The second reason for carbon’s astounding compound-forming ability is that carbon atoms can bind to each other not only in straight chains, but in complex branchings, like the branches of a tree. They can even join head-to-tail to make rings of carbon atoms. There is practically no limit to the number or complexity of the branches or the number of rings that can be attached to them, and hence no limit to the number of different molecules that can be formed.
The third reason is that carbon atoms can share not only a single electron with another atom to form a single bond, but it can also share two or three electrons, forming a double or triple bond. This makes for a huge number of possible bond combinations at different places, making a huge number of different possible molecules. And a molecule that differs by even one atom or one bond position is a molecule of a different compound.
The fourth reason is that the same collection of atoms and bonds, but in a different geometrical arrangement within the molecule, makes a molecule with a different shape and hence different properties. These different molecules are called isomers.
The fifth reason is that all of the electrons that are not being used to bond carbon atoms together into chains and rings can be used to form bonds with atoms of several other elements. The most common other element is hydrogen, which makes the family of compounds known as hydrocarbons. However, nitrogen, oxygen, phosphorus, sulfur, halogens, and several other kinds of atoms can also be attached as part of an organic molecule. There is a huge number of ways in which they can be attached to the carbon-atom branches, and each variation makes a molecule of a different compound. It is just as if moving a Christmas tree ornament from one branch to another created a completely different tree.
Classes of carbon compounds
It is obviously impossible to summarize the properties of carbon’s millions of compounds in one place. Introductory textbooks of organic chemistry generally run well over a thousand pages, and biochemistry textbooks run thousands more. But organic compounds can be classified into families that have similar properties because they have certain groupings of atoms in common.
See also Alcohol; Aldehydes; Alkaloid; Amides; Amino acid; Barbiturates; Chemical bond; Carbohydrate; Carbon cycle; Carboxylic acids; Ester; Ether; Fat; Fatty acids; Glycol; Halide, organic; Hydrocarbon; Isomer; Lipid; Polymer; Proteins.
Resources
BOOKS
Bloch, Daniel R., ed. Organic Chemistry Demystified. New York: McGraw-Hill, 2006.
Carey, Francis A. Organic Chemistry. Dubuque, IA: McGraw-Hill, 2006.
Hoffman, Robert V. Organic Chemistry: An Intermediate Text. Hoboken, NJ: Wiley-Interscience, 2004. Messina, Giacoma, and Saveria Santangelo, eds. Carbon:
The Future Material for Advanced Technology Applications. New York: Springer, 2006.
Yasuda, Ei-ichi, ed. Carbon Alloys: Novel Concepts to Develop Carbon Science and Technology. Boston, MA: Elsevier, 2003.
Robert L. Wolke