The covalent bonding of two or more atoms of the same element is known as catenation. Although other elements display catenation, none show it to the same degree as carbon.
ALLOTROPIES OF CARBON:
Untill recently, carbon was thought to occur in only two principal allotropic forms: diamond and graphite. In diamonds, each carbon atom is tetrahedrally bonded to four carbon atoms.
To move one plane of atoms in the diamond, crystal relative to another requires breaking of many strong carbon-carbon bonds. Because of this diamond is one of the hardest substances known. As a pure substance, diamond is colorless, although natural diamond may be colored by impurities.
Graphite is a black substance having a layered structure. Each layer consists of carbon atoms bonded to three other carbon atoms to give hexagonal pattern of carbon atoms arranged in a plane.
One layer of carbon atoms, in graphite, is held to another layer only by van der Waals forces. Because of relative weakness of these forces, the layers in graphite easily slide over one another, resulting in a substance that is soft and slippery.
Graphite, unlike diamond, is a good electrical conductor, because of delocalized bonding within layers.
In 1985, a third allotropic form of carbon, known as buckmnisterfullerene (C60, was discovered by Harold W. Kroto, England. The molecule has a stable "soccer ball" structure, containing not only hexagons, as in graphite, but also pentagons with 60 vertices.
Artificial diamonds are produced from graphite that is subjected to high pressure (10,000 atm) and temperature (20000C), which find applications in jewellery and cutting tools.
OXIDES OF CARBON:
Carbon has two principal oxides: carbon monoxide, CO, and carbon dioxide, CO2. Carbon and organic compounds burn in an excess of oxygen to give carbon dioxide. However, an equilibrium exists among carbon, carbon monoxide, and carbon dioxide that favours carbon monoxide above 7000C.
CO2(g) + C(s) 2CO (g)
For this reason, carbon monoxide is almost always one of the products of combustion of carbon and organic compounds, unless an excess oxygen is present, in which case the carbon monoxide burns to give carbon dioxide.
These two oxides are different in their physical and chemical properties. Carbon monoxide is a colorless, odorless gas that burns in air with blue flame. It is a toxic gas, which poisons by attaching strongly to iron atoms in the hemoglobin of red blood cells, preventing them from carrying from carrying oxygen.
As a result, the cells of the body are starved of oxygen. Carbon monoxide is manufactured from natural gas (CH4) and petroleum hydrocarbond, either by reaction with steam or by partial oxidation.
The resulting product is a mixture of carbon monoxide and hydrogen, which is known as synthesis gas. Methanol, for examole, is produced from synthesis gas. Carbon monoxide is also used as a fuel and as a reducing agent.
Carbon dioxide is a colorless, oderless gas with a faint acid taste. Under normal circumstances, the gas is nontoxic, although at high concentrations it interferes with respiration.
Carbon dioxide does not support combustion, which is why it is used as fire extinguisher. Carbon dioxide is produced whenever carbon or organic materials are burned.
Carbon dioxide is obtained commercially as a by-product in the production of ammonia and in the calcining (strong heating) of limestone to give calcium oxide. Liquid carbon dioxide and solid carbon dioxide (dry ice) are used in large quantities as refrigerants.
Carbonated beverages are made by dissolving carbon dioxide gas under pressure in an aqueous solution of sugar and flavourings.
Carbonated water is acidic as the result of the formation of carbonic acid, although carbonated beverages often contain fruit acids and phosphoric acid.
Carbon dioxide is a normal constituent of air. However, the burning of fossil fuels is releasing additional carbon dioxide into the atmosphere, which is feared to result in global warming.
CARBONATES:
Carbon dioxide dissolves in water to form an aqueous solution of carbonic acid.
The acid is diprotic (has two acidic H atoms per molecule) and dissociates to form hydrogen carbonate ion and carbonate ion.
Carbonic acid has never been isolated from solution, but its salts, hydrogen carbonates and carbonates are well known. When carbon dioxide is dissolved and shaken in an aqueous solution of calcium hydroxide, a milky white precipitate of calcium carbonate is formed.
This is a standard test for carbon dioxide, and the same reaction is used in the manufacture of pure calcium carbonate, which has medical applications as antacid. Carbonate minerals, such as limestone, are very common and many are of commercial importance.