Carbon
There are several allotropes of carbon of which the best known are graphite, diamond, and amorphous carbon. The physical properties of carbon vary widely with the allotropic form. For example, diamond is highly transparent, while graphite is opaque and black. Diamond is the hardest naturally-occurring material known, while graphite is soft enough to form a streak on paper (hence its name, from the Greek word "to write"). Diamond has a very low electrical conductivity, while graphite is a very good conductor. Under normal conditions, diamond, carbon nanotube and graphene have the highest thermal conductivities of all known materials.
All carbon allotropes are solids under normal conditions with graphite being the most thermodynamically stable form. They are chemically resistant and require high temperature to react even with oxygen. The most common oxidation state of carbon in inorganic compounds is +4, while +2 is found in carbon monoxide and other transition metal carbonyl complexes. The largest sources of inorganic carbon are limestones, dolomites and carbon dioxide, but significant quantities occur in organic deposits of coal, peat, oil and methane clathrates. Carbon forms more compounds than any other element, with almost ten million pure organic compounds described to date, which in turn are a tiny fraction of such compounds that are theoretically possible under standard conditions.
Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is present in all known life forms, and in the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen. This abundance, together with the unique diversity of organic compounds and their unusual polymer-forming ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life.
Occurrence
Carbon is the fourth most abundant chemical element in the universe by mass after hydrogen, helium, and oxygen. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets. Some meteorites contain microscopic diamonds that were formed when the solar system was still a protoplanetary disk. Microscopic diamonds may also be formed by the intense pressure and high temperature at the sites of meteorite impacts.
In combination with oxygen in carbon dioxide, carbon is found in the Earth's atmosphere (approximately 810 gigatonnes of carbon) and dissolved in all water bodies (approximately 36,000 gigatonnes of carbon). Around 1,900 gigatonnes of carbon are present in the biosphere. Hydrocarbons (such as coal, petroleum, and natural gas) contain carbon as well—coal "reserves" (not "resources") amount to around 900 gigatonnes, and oil reserves around 150 gigatonnes. Proven sources of natural gas are about 175 1012 cubic metres (representing about 105 gigatonnes carbon), but it is estimated that there are also about 900 1012 cubic metres of "unconventional" gas such as shale gas, representing about 540 gigatonnes of carbon. Carbon is also locked up as methane hydrates in polar regions and under the seas. Various estimates of the amount of carbon this represents have been made: 500 to 2500 Gt, or 3000 Gt. (In the past, quantities of hydrocarbons were greater. In the period from 1751 to 2008 about 347 gigatonnes of carbon were released as carbon dioxide to the atmosphere from burning of fossil fuels.)
Carbon is a major component in very large masses of carbonate rock (limestone, dolomite, marble and so on). Coal is the largest commercial source of mineral carbon, accounting for 4,000 gigatonnes or 80% of fossil carbon fuel. It is also rich in carbon – for example, anthracite contains 92–98%.
As for individual carbon allotropes, graphite is found in large quantities in the United States (mostly in New York and Texas), Russia, Mexico, Greenland, and India. Natural diamonds occur in the rock kimberlite, found in ancient volcanic "necks", or "pipes". Most diamond deposits are in Africa, notably in South Africa, Namibia, Botswana, the Republic of the Congo, and Sierra Leone. There are also deposits in Arkansas, Canada, the Russian Arctic, Brazil and in Northern and Western Australia. Diamonds are now also being recovered from the ocean floor off the Cape of Good Hope. However, though diamonds are found naturally, about 30% of all industrial diamonds used in the U.S. are now made synthetically.
Carbon-14 is formed in upper layers of the troposphere and the stratosphere, at altitudes of 9–15 km, by a reaction that is precipitated by cosmic rays. Thermal neutrons are produced that collide with the nuclei of nitrogen-14, forming carbon-14 and a proton.
| Symbol | C | |
| Atomic Number | 6 | |
| Atomic Weight | 12.011 | |
| Oxidation States | +2, +4, -4 | |
| Electronegativity, Pauling | 2.55 | |
| State at RT | Solid, Nonmetal | |
| Melting Point, K | 3820 | |
| Boiling Point, K | 5100 |
Interesting Facts about Carbon
- About 20% of the weight of living organisms is carbon.
- More compounds are known which contain carbon than don’t.
- Diamond is an excellent abrasive because it is the hardest common material and it also has the highest thermal conductivity. It can grind down any substance, while the heat generated by friction is swiftly conducted away.
- The carbon atoms in your body were all once part of the carbon dioxide fraction of the atmosphere.
- Graphene is the thinnest, strongest material ever known.
- Graphene is made of 2-dimensional atomic crystals, the first time such structures have ever been seen.
- The graphite in a typical mechanical pencil has a diameter of 0.7 mm. This is equal to 2 million layers of graphene.
- Car tires are black because they are about 30% carbon black, which is added to rubber to strengthen it. The carbon black also helps protect against UV damage to the tires.(8)
- Carbon is made within stars when they burn helium in nuclear fusion reactions. Carbon is part of the ‘ash’ formed by helium burning.
- Carbon undergoes nuclear fusion reactions in heavy stars to make neon, magnesium and oxygen.
- Carbon is the fourth most abundant element in the universe.
Appearance and Characteristics
Harmful effects:
Pure carbon has very low toxicity. Inhalation of large quantities of carbon black dust (soot/coal dust) can cause irritation and damage to the lungs.
Characteristics:
- Carbon can exist with several different 3 dimensional structures in which its atoms are arranged differently (allotropes).
- Three common crystalline allotropes are graphite, diamond, and (usually) fullerenes. Graphene has a 2D crystal structure.(Fullerenes can sometimes exist in amorphous form.)
- Carbon can also exist in an amorphous state. Many allotropes commonly described as amorphous, however, such as glassy carbon, soot, or carbon black usually have enough structure to not be truly amorphous. Although crystalline nanotubes have been observed, they are generally amorphous.
- The structures of eight allotropes are shown at the bottom of this page.
- Interestingly, graphite is one of the softest substances and diamond was thought, until recently, to be the hardest naturally occurring substance.
- An extremely rare allotrope of carbon, lonsdaleite, has been calculated, in pure form, to be 58% stronger than diamond. Lonsdaleite is a diamond-like carbon network which has graphite’s hexagonal structure. It is made when meteorites containing graphite hit another body, such as Earth. The high temperatures and pressures of the impact transform the graphite into lonsdaleite.
- Carbon has the highest melting/sublimation point of all the elements and, in the form of diamond, has the highest thermal conductivity of any element.
- Diamond’s high thermal conductivity is the origin of the slang term ‘ice.’ At typical room temperatures your body temperature is higher than the room’s – including any large diamonds you may just happen to have lying around. If you touch any of these diamonds, their high thermal conductivity carries heat away from your skin faster than any other material. Your brain interprets this rapid transfer of heat energy away from your skin as meaning you are touching something very cold – so diamonds at room temperature can feel like ice.
Uses of Carbon
- Carbon (in the form of coal, which is mainly carbon) is used as a fuel.
- Graphite is used for pencil tips, high temperature crucibles, dry cells, electrodes and as a lubricant.
- Diamonds are used in jewelry and – because they are so hard – in industry for cutting, drilling, grinding, and polishing.
- Carbon black is used as the black pigment in printing ink.
- Carbon can form alloys with iron, of which the most common is carbon steel.
- The 14C radioactive isotope is used in archaeological dating.
- Carbon compounds are important in many areas of the chemical industry – carbon forms a vast number of compounds with hydrogen, oxygen, nitrogen and other elements.
