Carbon - chemistry.

« 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 ofthe 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 forcesbetween 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 iseasy; 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 intodiamond by applying extremely high pressure (more than 100,000 times the atmospheric pressure at sea level) and temperature (about 3000°C or 5000°F).

Hightemperatures 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 inthe 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, thecoal 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, containsabout 90 percent carbon.

Lower grades include bituminous coal, which is 76 percent to 90 percent carbon, subbituminous coal, with 60 percent to 80 percent, andlignite, 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, afterAmerican architect R.

Buckminster Fuller.

Fuller designed geodesic domes, rigid structures with a three-dimensional geometry that resemble this form of carbon.

Unlikediamond and graphite, which can have an unending crystal structure, the original fullerene forms molecules of 60 carbon atoms (with a molecular formula of C 60).

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 atomsbond 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 cometogether.

Scientists have since discovered other fullerenes, including very narrow, long tubes and the C 70 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 themeteorite-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 includeother noncarbon atoms are superconductors, that is, they can conduct electricity with no resistance.

Some pure carbon fullerene tubes are stronger than metals andconduct electricity.

Someday we may use them as electrical wires or as fibers to reinforce plastic, making materials that are even stronger than those reinforced withcurrent carbon fibers ( see Composite Material).

Other compounds based on C 60 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 twooxygen atoms, or CO 2).

Earth’s atmosphere contains only 0.03 percent CO 2.

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 nucleitogether.

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 × 10 19 kg (about 1.7 × 10 20 lb).

When written out, 7.5 × 10 19 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.

Largedeposits 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 CzechRepublic, 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 byvolcanoes.

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 inSouth 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 containcarbon-based inorganic compounds such as carbonates of calcium and magnesium, which make up limestone.

Carbon dioxide occurs as a gas in the atmosphere ofEarth 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 enoughto 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 amongplants, animals, and their environment—plants capture carbon dioxide from the air.

With the aid of sunlight, the plants use the carbon to build complex organicmolecules, such as starches and sugars.

This process is called photosynthesis.

When animals eat the plants or the plants otherwise decompose, the complex organicmolecules are broken down again.

To complete the cycle, animals exhale carbon dioxide back into the atmosphere.

In addition, some carbon gets deposited in rock, butas the rocks weather, they release the carbon.

Carbon dioxide also escapes through the vents of volcanoes.

In natural processes, the total amount of carbon dioxidereturned 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 (C 2H4O2), which gives vinegar its sour taste; the simple sugar glucose (C 6H12O6); and common table sugar, sucrose (C 12H22O11).

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 ofmany, 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 theirstructure.

Cell membranes are made up of lipids, which are large organic molecules of carbon, hydrogen, oxygen, nitrogen, and phosphorus.

Other large organicmolecules of the body are the proteins found in blood, muscle, skin, hair, and every living cell.

Ribonucleic acids (RNA) and deoxyribonucleic acids (DNA) are giganticcarbon-based molecules that contain the genetic information, or the blueprints, for a living organism.

Biochemical processes, the chemical reactions that create andsustain 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 theremains 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 heatover 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, alsoformed from tiny sea creatures, is usually found with petroleum deposits.

It consists mostly of methane (CH 4), but it also contains significant amounts of ethane (C 2H6), propane (C 3H8), and butane (C 4H10).

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.. »