Carbon – facts and properties
- Symbol
- C
- Atomic number
- 6
- Atomic mass
- 12.011 u
- Period
- 2
- Group
- 14
- Category
- polyatomic nonmetal
- Electron configuration
- [He] 2s² 2p²
- Electronegativity
- 2.55
- Atomic radius
- 67 pm
- Valence electrons
- 4
- State at standard conditions
- solid
- Melting point
- 3550 °C
- Boiling point
- 3825 °C
- Density
- 2.267 g/cm³
- Discovered
- known since antiquity
Carbon is the element on which all known life is built. With atomic number 6 and four valence electrons, it links to itself and to other elements in chains, rings and networks more readily than any other element. The same atoms can form soft, slippery graphite or diamond, the hardest natural material. Carbon in coal, oil and gas powers modern society, and carbon in the form of carbon dioxide is changing the climate.
Position in the periodic table
Occurrence and the carbon cycle
Carbon occurs both free and combined. The free element is found as graphite, as diamond and, in tiny amounts, as fullerenes. Far more carbon is locked in carbonate rocks such as limestone, chalk, marble and dolomite, which store it as calcium and magnesium carbonate. The fossil fuels coal, oil and natural gas formed over millions of years from buried plants and marine organisms. The atmosphere holds carbon as carbon dioxide, currently about four hundredths of one per cent of the air by volume and rising because of fossil fuel combustion. Every living organism is largely built from carbon compounds, and the oceans hold a vast store of dissolved carbon dioxide and carbonate.
These reservoirs exchange carbon continuously in the carbon cycle. Plants and algae take up carbon dioxide by photosynthesis and build it into sugars; animals, fungi and bacteria return it to the air by respiration and decay; the oceans absorb it and deposit it as carbonate shells and sediments. Carbon is obtained in several forms. Graphite is mined or made synthetically from petroleum coke. Coke is produced by heating coal without air. Carbon black comes from the incomplete combustion of oil or gas. Activated carbon is made by charring wood or coconut shells and then roughening the material with steam to create an enormous internal surface. Synthetic diamonds are grown at high pressure and temperature or deposited from a carbon-containing gas. Carbon has been known since antiquity: charcoal and soot were used by the earliest human cultures.
Properties and reactions
Carbon has the electron configuration [He] 2s² 2p² and belongs to period 2 and group 14. Its four valence electrons form four covalent bonds, to other carbon atoms as readily as to hydrogen, oxygen or nitrogen. With an electronegativity of 2.55, carbon sits in the middle of the scale and forms neither cations nor anions in ordinary chemistry; instead it builds molecules and extended networks. This is the foundation of organic chemistry, with many millions of known compounds.
The element exists in several allotropes. In diamond every atom is bonded tetrahedrally to four neighbours, giving the hardest natural material, transparent, electrically insulating and an outstanding conductor of heat. In graphite the atoms form flat sheets of hexagons in which each atom has only three bonds; the fourth electron is delocalised across the sheet, so graphite conducts electricity, appears black and feels greasy because the sheets slide over one another. A single sheet is graphene; rolled up it becomes a nanotube, and closed into a cage it becomes a fullerene. Carbon does not melt at atmospheric pressure but passes straight into the gas phase, so the melting point of 3550 °C and the boiling point of 3825 °C are reference values for special conditions. Graphite has a density of 2.267 g/cm³. At room temperature carbon is unreactive, but when heated it burns to carbon dioxide or, with too little oxygen, to poisonous carbon monoxide. Hot carbon strips oxygen from metal oxides, which is the basis of iron smelting. The radioactive isotope carbon-14 makes it possible to date organic remains.
Uses in everyday life and technology
Carbon is all around you. Pencil leads are graphite mixed with clay, barbecue charcoal is charred wood, car tyres are filled with carbon black, and activated carbon filters drinking water, air and, in emergencies, poisons in the stomach. Most diamonds are not jewels but industrial grit in drill bits, saw blades and polishing pastes.
In industry, coke is indispensable. In the blast furnace it reduces iron ore to pig iron and supplies the heat at the same time. Steel is iron with a small proportion of carbon, which makes it hard. Graphite serves as electrodes in arc furnaces and in lithium-ion batteries, as a dry lubricant and as a neutron moderator in some reactors. Carbon fibres embedded in resin give bicycle frames, aircraft parts and racing cars their combination of low weight and stiffness. The entire petrochemical industry rests on carbon: plastics, medicines, dyes, fuels and solvents are made from the carbon compounds in oil and gas. At the same time, carbon is at the heart of a global problem: burning fossil fuels releases carbon dioxide, a greenhouse gas that warms the planet. Renewable energy, carbon capture and storage, and more efficient use of materials are the main strategies for bringing the carbon cycle back into balance.
Molecules containing this element
- Methane CH4
- Ethane C2H6
- Ethene C2H4
Quick question
Which of these elements has the highest electronegativity?
FAQ
Why is diamond hard and graphite soft when both are pure carbon?
In diamond every carbon atom holds four equally strong bonds to its neighbours in a three-dimensional network, so deforming the crystal means breaking those bonds. In graphite the atoms are strongly bonded only within flat sheets; between the sheets act only weak forces, so the sheets slide past one another easily, which is exactly what makes a pencil write.
How does radiocarbon dating work?
Cosmic rays constantly produce radioactive carbon-14 in the upper atmosphere, and plants take it up along with ordinary carbon dioxide. While an organism lives, the proportion of carbon-14 in its tissues stays constant. After death no new carbon-14 is taken in and the existing atoms decay with a half-life of about five thousand seven hundred years. Measuring how much remains gives the age of wood, bone or cloth up to roughly fifty thousand years old.
Why are there so many carbon compounds?
Carbon forms four strong, stable bonds and links to itself almost indefinitely in chains, rings and branched frameworks. It can form single, double and triple bonds and combines with hydrogen, oxygen, nitrogen, sulfur, the halogens and many metals. No other element offers this range of stable combinations, which is why organic chemistry is a subject in its own right.