Carbon - chemistry.

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Carbon - chemistry.
I

INTRODUCTION

Carbon, nonmetallic chemical element, known by the symbol C, that is the fundamental building block of material in living organisms and is important to many
industries. Carbon occurs in nature in nearly pure form in diamond and graphite. It is also the major component of coal, petroleum, asphalt, limestone, and most
materials made by plants and animals. The name carbon is derived from the Latin word carbo, meaning charcoal, a material that is composed primarily of carbon.
A carbon atom can chemically combine with atoms of other elements, as well as with other carbon atoms, to form molecules. Molecules that contain two or more
elements make up compounds. Carbon can form more compounds than can any other element except hydrogen.
Carbon is present in all substances known as organic compounds (see Chemistry, Organic). Originally, scientists used the term organic compounds for materials that
could only be obtained from living or dead organisms. Today chemists consider nearly any compound that contains carbon to be organic, whether they obtain it from
organisms or synthesize it in a laboratory or in factories. Compounds that do not contain carbon are called inorganic compounds.
Carbon atoms form part or all of the backbone for the major molecules of all living things on Earth, including sugars, proteins, fats, and deoxyribonucleic acids (DNA),
the molecules that carry the genetic code of living organisms. Many of the materials that we use in everyday life contain carbon-rich organic compounds. For instance,
we wear clothing made of organic compounds--either natural fibers, such as wool, silk, or cotton; or synthetic ones, such as nylon or polyester. We construct our
houses and furnishings from organic materials, such as wood and plastics. We burn carbon-rich fossil fuels, including gasoline, natural gas, and coal, for heat and
energy. In addition, we use organic compounds as pesticides and medicines, and the foods we eat are carbon compounds.

II

PROPERTIES

Of all the elements, carbon is the only one suitable for building the variety of molecules necessary to sustain life. Carbon atoms can attach to each other to form chains,
rings, or a crystal mesh. The chains may be thousands of carbon atoms long and either linear or branched, and the rings usually contain from three to six carbon
atoms. Most organic compounds contain many carbon-hydrogen bonds. Some of the other elements that bond to carbon include oxygen, nitrogen, fluorine, chlorine,
bromine, iodine, sulfur, and phosphorus.

A

Isotopes

Every carbon atom contains six positively charged particles called protons in its nucleus and six or more neutral particles called neutrons. The carbon atom's nucleus is
surrounded by six negatively charged electrons. The number of neutrons in a carbon atom's nucleus determines which isotope it is. Isotopes are atoms of the same
element that have different numbers of neutrons in the nucleus. Three different isotopes of carbon exist in nature. The important isotopes of carbon are carbon-12,
carbon-13, and carbon-14. Scientists identify them by their mass number, which is the sum of the number of protons and neutrons in an atom. Carbon-12 contains six
protons and six neutrons, carbon-13 contains six protons and seven neutrons, and carbon-14 contains six protons and eight neutrons.
In nature, carbon-12 accounts for about 98.89 percent of all carbon. Carbon-13 has a natural abundance of 1.11 percent, and the amount of carbon-14 is negligible.
The atomic mass of carbon is 12.011 atomic mass units (AMU), which is the average mass of the isotopes of carbon based on their abundance.
Scientists have found some important uses for the less abundant isotopes of carbon. The nucleus of carbon-13 is magnetic. This property enables scientists to detect
nuclei of carbon-13 atoms using a technique called nuclear magnetic resonance (NMR). By detecting the location of carbon-13 atoms in carbon-based molecules,
scientists can learn about the structure of these molecules. Carbon-14 is radioactive, that is, its nucleus is unstable and can spontaneously change into the nucleus of
another element (see Radioactivity). In a given sample, half of the carbon-14 nuclei will disintegrate in about 5,730 years. Living organisms constantly replenish carbon
in their systems, so that the amount of carbon-14 remains constant as long as an organism is alive. Knowing the original amount of carbon-14 in organisms, scientists
can measure the amount of carbon-14 that has disintegrated in a fossilized organism and determine the amount of time that has passed since it died. This technique for
determining the age of fossils is called carbon dating.

B

Bonding

As with all atoms, the electrons in a carbon atom reside in layers, or shells, around the nucleus. Carbon atoms have two electrons in their inner shell, and this shell can
only contain two electrons, so it is full. Carbon atoms have four outer, or valence, electrons in their next shell. This outer electron shell can hold eight electrons, and
atoms in general are much more stable when their outer shell is full. To obtain a full outer shell, carbon atoms form four covalent bonds with other atoms. A covalent
bond is a bond formed when two atoms share a pair of electrons. When two atoms share one pair of electrons, the covalent bond is called a sigma bond and it holds the
electrons tightly between the two atoms. One pair of shared electrons is also called a single bond. When two atoms share two pairs of electrons (creating a double
bond), the first shared pair forms a sigma bond, while the second pair forms a pi bond. The pi bond does not hold electrons as tightly as the sigma bond holds the first
pair. When two atoms share three pairs of electrons (creating a triple bond), two of the bonds are pi bonds. Electrons in pi bonds are much more reactive than are
electrons in sigma bonds. That is, pi electrons more easily split away from the bond and create bonds with other atoms, adding those atoms to the molecule.
Carbon atoms can bond together in chains, rings, and meshlike networks. If a carbon atom bonds with four identical atoms, those atoms will be equally distant from
each other--at the tips of an imaginary tetrahedron, or a pyramid with a triangular base. Any two of the bonds form an angle of 109.5° when carbon is in a tetrahedral
form.

C

Allotropes

Carbon has multiple allotropes. Allotropes are different physical forms of the same element, such as a hard, highly structured crystal and a soft, less-structured
substance. Allotropes differ in the way the atoms bond with each other and arrange themselves into a structure. Because of their different structures, allotropes have
different physical and chemical properties. The three common allotropes of carbon are diamond, graphite, and amorphous carbon (examples of amorphous carbon
include charcoal, soot, and the coal-derived fuel called coke). The density of diamond is about 3.5 grams per cubic centimeter (g/cm3), graphite ranges from 1.9 to 2.3
g/cm3, and amorphous carbon ranges from 1.8 to 2.1 g/cm3. Diamond is one of the hardest known materials, while graphite is one of the softest. These differences
arise from the differences in bonding between the carbon atoms.
In diamond, each carbon atom bonds tetrahedrally to four other carbon atoms to form a three-dimensional lattice. The shared electron pairs are held tightly in sigma
bonds between adjacent atoms. Pure diamond is an electrical insulator--it does not conduct electric current. It is colorless and, because of its hardness, is used in
industrial cutting tools. Cut diamonds sparkle brilliantly, which makes them treasured gemstones in jewelry.

Graphite is black and slippery and conducts electricity. In graphite, the atoms form planar, or flat, layers. Each layer is made up of rings containing six carbon atoms.
The rings are linked to each other in a structure that resembles the hexagonal mesh of chicken wire. Each atom has three sigma bonds (with 120° between any two of
the bonds) and belongs to three neighboring rings. The fourth electron of each atom becomes part of an extensive pi bond system. Graphite conducts electricity,
because the electrons in the pi bond system can move around throughout the graphite. Bonds between atoms within a layer of graphite are strong, but the forces
between the layers are weak. Because the layers can slip past each other, graphite is soft and can be used as a lubricant. Rubbing off layers of carbon in graphite is
easy; you do it every time you write with a "lead" pencil. The "lead" is not actually lead at all but graphite mixed with clay. Diamond makers can transform graphite into
diamond by applying extremely high pressure (more than 100,000 times the atmospheric pressure at sea level) and temperature (about 3000°C or 5000°F). High
temperatures break the strong bonds in graphite so that the atoms can rearrange themselves into a diamond lattice. About 90 percent of the diamonds used in tools in
the United States are made this way.
Amorphous carbon is actually made up of tiny crystal-like bits of graphite with varying amounts of other elements, which are considered impurities. For example, the
coal industry divides coal up into various grades depending on the amount of carbon in the coal and the amount of impurities. The highest grade, anthracite, contains
about 90 percent carbon. Lower grades include bituminous coal, which is 76 percent to 90 percent carbon, subbituminous coal, with 60 percent to 80 percent, and
lignite, with 55 percent to 73 percent.
In 1985 chemists created a new allotrope of carbon by heating graphite to extremely high temperatures. They named the allotrope buckminsterfullerene, after
American architect R. Buckminster Fuller. Fuller designed geodesic domes, rigid structures with a three-dimensional geometry that resemble this form of carbon. Unlike
diamond and graphite, which can have an unending crystal structure, the original fullerene forms molecules of 60 carbon atoms (with a molecular formula of C60). The
molecules are shaped like tiny soccer balls (called buckyballs), with an atom at each point where the lines on a soccer ball would normally meet. The 60 carbon atoms
bond in 20 six-membered rings and 12 five-membered rings. Each carbon atom is at a corner where two six-membered rings and one five-membered ring come
together. Scientists have since discovered other fullerenes, including very narrow, long tubes and the C70 fullerene, an elongated structure shaped more like a football
but rounded on the ends. After scientists discovered fullerenes in the lab, geologists discovered fullerenes in nature--in ancient rocks in New Zealand and in the
meteorite-created Ries Crater in Germany.
Scientists, excited by the properties of these recently discovered materials, are exploring ways to use them. When cooled, some fullerene-based compounds that include
other noncarbon atoms are superconductors, that is, they can conduct electricity with no resistance. Some pure carbon fullerene tubes are stronger than metals and
conduct electricity. Someday we may use them as electrical wires or as fibers to reinforce plastic, making materials that are even stronger than those reinforced with
current carbon fibers (see Composite Material). Other compounds based on C60 appear to inhibit the activity of the virus that causes acquired immunodeficiency
syndrome (AIDS).

III

OCCURRENCE

Carbon is widely distributed in nature and the universe. We have already discussed how carbon occurs as a pure element and in countless organic compounds on Earth.
But carbon also abounds in the Sun, stars, comets, and in the atmospheres of most planets. The atmosphere of Mars is mostly carbon dioxide (carbon bonded with two
oxygen atoms, or CO2). Earth's atmosphere contains only 0.03 percent CO2. Like virtually all atoms, carbon atoms are made in the interior of stars during a supernova,
an explosion of a star that emits vast amounts of energy. These explosions build atoms in thermonuclear reactions, high temperature events that fuse two nuclei
together. Hydrogen atoms fuse together into a helium atom, then helium atoms fuse into carbon. Carbon atoms can then fuse with helium into oxygen.
The total mass of carbon on Earth is about 7.5 × 1019 kg (about 1.7 × 1020 lb). When written out, 7.5 × 1019 is 75 followed by 18 zeros. Only about 0.001 percent of
this total is found in living plants and animals. As noted earlier, carbon is found in elemental form as amorphous carbon (mostly coal), graphite, and diamond. Large
deposits of coal are found in Europe, Asia, Australia, and North America. Large deposits of graphite are found in China, India, North Korea, Mexico, Brazil, the Czech
Republic, and the Ukraine. Natural diamonds are found in deposits that are believed to be the remains of ancient volcanic pipes, long tubes of rocky material formed by
volcanoes. Diamond-containing pipes occur in South Africa, Russia, and the state of Arkansas in the United States, and in the ocean floor off the Cape of Good Hope in
South Africa. Some meteorites contain microscopic diamonds.
Carbon is also found in inorganic compounds bound up in rocks and, most importantly to living organisms, as carbon dioxide in the air and water. Rocks can contain
carbon-based inorganic compounds such as carbonates of calcium and magnesium, which make up limestone. Carbon dioxide occurs as a gas in the atmosphere of
Earth and also as a dissolved gas in all natural water. Although the percentage of carbon dioxide in Earth's atmosphere is small, it helps keep the planet warm enough
to sustain life. Carbon dioxide traps some of the solar radiation, in the same way that a greenhouse or a car with closed windows traps heat (see Greenhouse Effect).
Based on the distance from the Sun and the amount of solar radiation, Earth would have an average temperature of -18°C (0°F) without this blanket of carbon dioxide.
The oceans would be frozen.
Carbon dioxide also provides the carbon needed by living organisms to build organic molecules. During the carbon cycle--the continuous exchange of carbon among
plants, animals, and their environment--plants capture carbon dioxide from the air. With the aid of sunlight, the plants use the carbon to build complex organic
molecules, such as starches and sugars. This process is called photosynthesis. When animals eat the plants or the plants otherwise decompose, the complex organic
molecules are broken down again. To complete the cycle, animals exhale carbon dioxide back into the atmosphere. In addition, some carbon gets deposited in rock, but
as the rocks weather, they release the carbon. Carbon dioxide also escapes through the vents of volcanoes. In natural processes, the total amount of carbon dioxide
returned to the atmosphere equals the amount extracted.
Plants, animals, and other life forms make carbon-based organic molecules that range from small to enormous in size. Small molecules include acetic acid (C2H4O 2),
which gives vinegar its sour taste; the simple sugar glucose (C6H12O 6); and common table sugar, sucrose (C12H22O 11). The three basic energy-providing nutrients of
living organisms, carbohydrates, fats, and proteins, are all based on carbon. Wood from plants is made of a very large carbohydrate, called cellulose, which consists of
many, many glucose molecules bonded together.
The human body is about 18 percent carbon by mass, and the biologically significant molecules (other than water) have carbon as part or all of the backbone of their
structure. Cell membranes are made up of lipids, which are large organic molecules of carbon, hydrogen, oxygen, nitrogen, and phosphorus. Other large organic
molecules of the body are the proteins found in blood, muscle, skin, hair, and every living cell. Ribonucleic acids (RNA) and deoxyribonucleic acids (DNA) are gigantic
carbon-based molecules that contain the genetic information, or the blueprints, for a living organism. Biochemical processes, the chemical reactions that create and
sustain life, rely on the chemical reactions of carbon-based substances. These life processes involve the complex and coordinated making or breaking of carbon bonds.
Fossil fuels, such as coal, petroleum, and natural gas, are mainly hydrocarbons (molecules containing only carbon and hydrogen). They most likely formed from the
remains of organisms that lived approximately 500 million years ago. Coal formed from the remains of plants that were buried and subjected to high pressure and heat
over long periods of time. Petroleum, formed from microscopic sea plants and bacteria, is a thick, dark liquid composed of a variety of hydrocarbons. Natural gas, also
formed from tiny sea creatures, is usually found with petroleum deposits. It consists mostly of methane (CH4), but it also contains significant amounts of ethane (C2H6),
propane (C3H8), and butane (C4H10). Liquid petroleum is mainly composed of hydrocarbon molecules that contain from 5 to more than 25 carbons. Products made from
petroleum include gasoline, kerosene, jet fuel, diesel fuel, heating oil, lubricating oil, and asphalt.

IV

USES OF CARBON

Scientists, industry, and consumers use different forms of carbon and carbon-containing compounds in many ways. Scientists use the carbon atom as the basic unit of
mass and as a clue to the age of an object. Industries use carbon to make steel from iron, purify metals, and add strength to rubber. In the form of diamond, carbon
can cut most other substances and shine more brilliantly in jewelry than most other gems. Carbon compounds can be burned as fuel to heat food or homes, as well as
form many different molecules for all sorts of human needs.
In 1961 the international unions of physicists and chemists agreed to use the mass of the isotope carbon-12 as the basis for atomic weights. Carbon-12 is defined to
have an atomic mass of exactly 12 atomic mass units (AMU). The atomic mass of an element is the average mass of an atom of that element as compared to the mass
of a carbon-12 atom.
Carbon-14 dating, a technique originated by American chemist Willard F. Libby in 1947, uses carbon to estimate the age of things that were once alive or artifacts made
from them, such as wood sculptures or cloth. The carbon dioxide in the atmosphere includes one atom of radioactive carbon-14 for every 1012 (1,000 billion) atoms of
the nonradioactive carbon-12. While living, an organism contains this same ratio because it is continuously exchanging carbon with the atmosphere through
photosynthesis or through eating and respiration. When an organism dies, exchange with the environment stops, and no additional carbon-14 is taken in. The
radioactive isotope carbon-14 decays into nitrogen-14, and the carbon-14 concentration decreases with time. By measuring the carbon-14 to carbon-12 ratio in an
archaeological sample, a scientist can estimate how much time has passed since the organism died. (See also Dating Methods: Carbon-14 Method.)
Carbon has many industrial uses. At high temperatures, carbon combines with iron to make steel. The chemical composition of steel determines its physical properties.
Carbon steel with about 1.5 percent carbon is used to make sheet steel and tools. Steel used for automobile and aircraft engine parts contains about 1 percent carbon.
High strength steel used for transportation equipment and structural beams contains about 0.25 percent carbon. Stainless steel for engine parts or kitchen utensils
contains from 0.03 to 1.2 percent carbon. Carbon, in the form of coke, can also react with tin oxide and lead oxide to yield the pure metals tin and lead. Carbon black,
made of fine particles of amorphous carbon, is produced by incomplete combustion of natural gas. It is mainly used as a filler and reinforcing agent for rubber.
Natural and synthetic diamonds can cut nearly every other known material. Gem cutters, surgeons, and manufacturers use diamond knives and drills. General Electric
Company produced the first synthetic diamond in 1955. Today tiny synthetic diamonds are commonly used as abrasives. Producers of metal tools use lasers to heat
carbon dioxide over a metal surface, making the carbon atoms coat the surface with a diamond film. This diamond coating can make cutting tools last much longer than
untreated tools.
People burn fossil fuels to generate energy. Burning, or combustion, is the reaction of a substance with oxygen to produce new substances and energy (in the form of
heat). When coal burns, carbon reacts with oxygen to yield carbon dioxide and heat. The higher the carbon content, the greater the energy released in combustion.
Therefore, anthracite (containing the most carbon) is the most valuable coal, and lignite (containing the least amount of carbon) is the least valuable. In petroleum, oil,
and natural gas, burning releases energy when bonds between the atoms break and when carbon and hydrogen atoms recombine with oxygen to form carbon dioxide
and water.
Carbon compounds are the basis of the synthetic organic chemicals, which account for many of the products of the chemical industry. Pharmaceuticals, pesticides,
paints, and coatings are among the products made from synthetic organic chemicals. The synthetic fiber, synthetic rubber, and plastics industries depend upon the
unique ability of carbon to form stable, long chains, or polymers, made from small organic molecules bonded together. Carbon-based polymers form synthetic fibers,
such as nylon, rayon, and polyester. All the plastics, from polyethylene terephthalate (PET) in soft drink bottles to polyvinyl chloride (PVC) in window frames to styrene
in car parts, depend on the properties of carbon.

Contributed By:
Patricia Lutz
Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.