Carbon
Carbon is the non-metallic chemical element of atomic number 6 in Group 14 of the periodic table, symbol C, atomic weight 12.01, specific gravity as graphite 2.25, as diamond 3.51. Its stable isotopes are 12C (98.90%) and 13C (1.10%). The weight of the 12C atom is the international standard on which atomic weights are based. It is defined as weighing exactly 12.00000 atomic mass units.
Carbon has been known since prehistoric times. It gets its name from carbo, the Latin word for charcoal, which is almost pure carbon. 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 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 are what 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 of many other stars. Without carbon, the Sun would be cold and dark.
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 inks black. 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 ft2 (18,580 m2) 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 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 electrolysiscells. Because of its slipperiness, it is used as a lubricant. For example, powdered graphite is used to lubricate locks, where oil might be too viscous. The "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, 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 Mohs scale of hardness, diamond is awarded a perfect ten. But that's not why diamonds are so expensive. They are the most expensive of all gems, and 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 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 1022carbon atoms.
The beauty of gem-quality diamonds 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 can't be scratched by other materials, they are brittle—they can be cracked.
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 anywhere near 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. Almost all of the chemical substances in living things are carbon compounds (water and minerals are the obvious exceptions), and the name organic was eventually applied to the chemistry of all carbon compounds, regardless of where they come from.
Having the atomic number six, every carbon atom has a total of six protons. Therefore, all carbon atoms with a neutral charge also have a total of six electrons. Two are in a completed inner orbit, while the other four are valence electronsouter electrons that are available for forming bonds with other atoms. An ion is an atom with either a negative or positive charge has either less or more electrons than the number of protons (respectively), and is referred to as either an anion (negatively charged) or a cation (positively charged).
It is impossible to summarize the properties of carbon's millions of compounds. Organic compounds can be classified into families that have similar properties, because they have certain groupings of atoms in common.
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