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The Element
Carbon
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| General |
| Name, Symbol, Number |
Carbon, C, 6 |
| Chemical series |
Nonmetals |
| Group, Period, Block |
14 (IVA), 2, p |
| Density, Hardness |
2267 kg/m3,
0.5 (graphite)
10.0 (diamond) |
| Appearance |
black (graphite)
colourless (diamond)
|
| Atomic
properties |
| Atomic weight |
12.0107 amu |
| Atomic radius (calc.) |
70 (67)pm |
| Covalent radius |
77 pm |
| van der Waals radius |
170 pm |
| Electron configuration |
[He]2s22p2 |
| e- 's per energy level |
2, 4 |
| Oxidation states (Oxide) |
4, 2 (mildly acidic) |
| Crystal structure |
Hexagonal |
| Physical
properties |
| State
of matter |
solid (nonmagnetic) |
| Melting point |
3773 K (6332 °F) |
| Boiling point |
5100 K (8721 °F) |
| Molar volume |
5.29 ×10-6 m3/mol |
| Heat of vaporization |
355.8 kJ/mol (sublimes) |
| Heat of fusion |
N/A (sublimes) |
| Vapor pressure |
0 Pa |
| Speed of sound |
18350 m/s |
| Miscellaneous |
| Electronegativity |
2.55 (Pauling scale) |
| Specific heat capacity |
710 J/(kg*K) |
| Electrical conductivity |
0.061 × 106/m ohm |
| Thermal conductivity |
129 W/(m*K) |
| 1st ionization potential |
1086.5 kJ/mol |
| 2nd ionization potential |
2352.6 kJ/mol |
| 3rd ionization potential |
4620.5 kJ/mol |
| 4th ionization potential |
6222.7 kJ/mol |
| 5th ionization potential |
37831 kJ/mol |
| 6th ionization potential |
47277.0 kJ/mol |
| Most stable
isotopes |
| iso |
NA |
half-life |
DM |
DE MeV |
DP |
| 12C |
98.9% |
C is stable with 6 neutrons |
| 13C |
1.1% |
C is stable with 7 neutrons |
| 14C |
trace |
5730 y |
beta- |
0.156 |
14N |
|
| SI
units & STP are used except where noted. |
Carbon is the chemical element in the periodic
table that has the symbol C and atomic number
6. An abundant nonmetallic, tetravalent element, carbon has
several allotropic forms:
- diamonds
(hardest known mineral). Binding structure: 4 electrons in
3-dimensional so-called sp3-orbitals
- graphite (one
of the softest substances). Binding structure: 3 electrons
in 2-dimensional sp2-orbitals and 1 electron in s-orbitals.
- Covalent bound sp1 orbitals are of chemical interest only.
Fullerite (fullerenes)
are nanometer-scale molecules. In the simple form 60 carbon
atoms form a graphitic layer which is bent to a 3-dimensional
structure, similar to a soccer ball.
Lamp black consists of small graphitic areas. These areas are
randomly distributed, so the whole structure is isotropic.
So-called 'glassy carbon' is isotropic and as strong as glass.
Unlike normal graphite, the graphitic layers are not arranged
like pages in a book, but are crumpled like crumpled paper.
Carbon fibers are similar to glassy carbon. Under special treatment
(stretching of organic fibers and carbonization) it is possible
to arrange the carbon planes in direction of the fiber. Perpendicular
to the fiber axis there is no orientation of the carbon planes.
The result are fibers with a higher specific strength than steel.
The element carbon occurs in all organic life and is the basis of organic
chemistry. This nonmetal also has the interesting chemical property
of being able to bond with itself and a wide variety of other
elements, forming nearly 10 million known compounds. When united
with
oxygen it forms carbon dioxide which is absolutely vital
to plant growth. When united with hydrogen,
it forms various compounds called hydrocarbons which are essential
to industry in the form of fossil fuels. When combined with
both oxygen and hydrogen it can form many groups of compounds
including fatty
acids, which are essential to life, and esters, which give
flavor to many fruits. The isotope carbon-14 is commonly used
in radioactive dating.
Notable characteristics
Carbon is a remarkable element for many reasons. Its different
forms include one of the softest (graphite) and one of the hardest
(diamond) substances known to man. Moreover, it has a great
affinity for bonding with other small atoms, including other
carbon atoms, and its small size makes it capable of forming
multiple bonds. Because of these properties, carbon is known
to form nearly ten million different compounds. Carbon compounds
form the basis of all life on Earth and the carbon-nitrogen
cycle provides some of the energy produced by the sun and other
stars.
Carbon was not created in the big bang due to the fact that
it needs a triple collision of alpha particles (helium nuclei)
to be produced. The universe initially expanded and cooled too
fast for that to be possible. It is produced, however, in the
interior of stars in the horizontal branch, where stars transform
a helium core into carbon by means of the triple-alpha process.
Applications
The element carbon is a vital component of all known living
systems, and without it life as we know it could not exist (see
carbon chauvinism). The major economic use of carbon is in the
form of hydrocarbons, most notably the fossil fuels methane
gas and crude oil. Crude oil is used by the petrochemical industry
to produce, amongst others, petroleum, gasoline and kerosene,
through a distillation process, in so-called refineries. Crude
oil forms the raw material for many synthetic substances, many
of which are collectively called plastics.
Other uses:
- The isotope 14C, discovered February 27th, 1940,
is used in radiocarbon dating.
- Some smoke detectors use tiny amounts of a radioactive isotope
of carbon as source of ionizing radiation (Most smoke detectors
of this type use an isotope of Americium)
- Graphite is combined with clays to form the 'lead' used
in pencils.
- Diamond is used for decorative purposes, and also as drill
bits and other applications making use of its hardness.
- Carbon is added to iron to make steel.
- Carbon is used for control rods in nuclear reactors.
- Graphite carbon in a powdered, caked form is used as charcoal
for cooking, artwork and other uses.
- Charcoal pills are used in medicine in pill or powder form
to adsorb toxins or poisons from the digestive system.
The chemical and structural properties of fullerenes, in the
form of carbon nanotubes, has
promising potential uses in the nascent field of nanotechnology.
History
Carbon (Latin carbo meaning "charcoal") was discovered
in prehistory and was known to the ancients, who manufactured
it by burning organic material in insufficient oxygen (making
charcoal). Diamonds have long been considered rare and beautiful.
The last-known allotrope of carbon, fullerenes, were discovered
as byproducts of molecular beam experiments in the 1980's.
Allotropes
Four allotropes of carbon are known to exist: amorphous, graphite,
diamond and fullerenes. The discovery of a fifth form was announced
on March 22, 2004 [1] (http://www.nature.com/nsu/040322/040322-5.html).
In its amorphous form, carbon is essentially graphite but not
held in a crystalline macrostructure. It is, rather, present
as a powder which is the main constituent of substances such
as charcoal and lamp black (soot).
At normal pressures carbon takes the form of graphite, in which
each atom is bonded to three others in a plane composed of fused
hexagonal rings, just like those in aromatic hydrocarbons. The
two known forms of graphite, alpha (hexagonal) and beta (rhombohedral),
both have identical physical properties, except for their crystal
structure. Graphites that naturally occur have been found to
contain up to 30% of the beta form, when synthetically-produced
graphite only contains the alpha form. The alpha form can be
converted to the beta form through mechanical treatment and
the beta form reverts back to the alpha form when it is heated
above 1000 °C.
Because of the delocalization of the pi-cloud, graphite conducts
electricity. The material is soft and the sheets, frequently
separated by other atoms, are held together only by van der
Waals forces, so easily slip past one another.
At very high pressures carbon has an allotrope called diamond,
in which each atom is bonded to four others. Diamond has the
same cubic structure as silicon and germanium and, thanks to
the strength of the carbon-carbon bonds, is together with the
isoelectronic boron nitride (BN) the hardest substance in terms
of resistance to scratching. The transition to graphite at room
temperature is so slow as to be unnoticeable. Under some conditions,
carbon crystallizes as Lonsdaleite, a form similar to diamond
but hexagonal.
Fullerenes have a graphite-like structure, but instead of purely
hexagonal packing, also contain pentagons (or possibly heptagons)
of carbon atoms, which bend the sheet into spheres, ellipses
or cylinders. The properties of fullerenes (also called "buckyballs"
and "buckytubes") have not yet been fully analyzed. All the
names of fullerenes are after Buckminster Fuller, developer
of the geodesic dome, which mimics the structure of "buckyballs".
Occurrence
There are nearly ten million carbon compounds that are known
to science and many thousands of these are vital to life processes
and very economically important organic-based reactions. This
element 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. In combination with other elements, carbon is found the
earth's atmosphere and dissolved in all bodies of water. With
smaller amounts of calcium, magnesium, and iron, it is a major
component of very large masses carbonate rock (limestone, dolomite,
marble etc.). When combined with hydrogen, carbon form coal,
petroleum, and natural gas which are called hydrocarbons.
Graphite is found in large quantities in New York and Texas,
the United States; Russia; Mexico; Greenland and India.
Natural diamonds occur in the mineral 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 Canada,
the Russian Arctic, Brazil and in Northern and Western Australia.
Inorganic compounds
The most prominent oxide of carbon is carbon dioxide, CO2.
This is a minor component of the Earth's atmosphere, produced
and used by living things, and a common volatile elsewhere.
In water it forms trace amounts of carbonic acid, H2CO3,
but as most compounds with multiple single-bonded oxygens on
a single carbon it is unstable. Through this intermediate, though,
resonance-stabilized carbonate ions are produced. Some important
minerals are carbonates, notably calcite. Carbon disulfide,
CS2, is similar.
The other oxides are carbon monoxide, CO, and the uncommon
carbon suboxide, C3O2. Carbon monoxide
is formed by incomplete combustion, and is a colorless, odorless
gas. The molecules each contain a triple bond and are fairly
polar, resulting in a tendency to bind permanently to hemoglobin
molecules, so that the gas is highly poisonous. Cyanide, CN-,
has a similar structure and behaves a lot like a halide ion;
the nitride cyanogen, (CN)2, is related.
With strong metals carbon forms either carbides, C-,
or acetylides, C22-; these are associated
with methane and acetylene, both incredibly pathetic acids.
All in all, with an electronegativity of 2.5, carbon prefers
to form covalent bonds. A few carbides are covalent lattices,
like carborundum, SiC, which resembles diamond.
Carbon chains
It´s the atomic structure of hydrocarbons in which a series
of carbon atoms, saturated by hydrogen atoms, form a chain.
Volatile oils have shorter chains. Fats have longer chain lengths,
and waxes have extremely long chains.
Carbon cycle
The continuous process of combining and releasing carbon and
oxygen thereby storing and emitting heat and energy. Catabolism
+ anabolism = metabolism. See carbon cycle.
Isotopes
In 1961 the International Union of Pure and Applied Chemistry
adopted the isotope carbon-12 for basis for atomic weights.
- Carbon-14 is a radioisotope with a half-life of 5715 years
and has been used extensively for radiocarbon dating wood,
archaeological sites and specimens.
Carbon has two stable, naturally-occurring isotopes: C-12 (98.89%)
and C-13 (1.11%). Ratios of these isotopes are reported in ?
relative to the standard VPDB (Vienna Pee Dee Belemnite from
the Peedee Formation of South Carolina). The dC-13 of the atmosphere
is -7?. During photosynthesis, the carbon that becomes fixed
in plant tissue is significantly depleted in C-13 relative to
the atmosphere.
There is two mode distribution in the dC-13 values of terrestrial
plants resulting from differences in the photosynthetic reaction
used by the plant. Most terrestrial plants are C3 pathway plants
and have dC-13 values range from -24 to -34?. A second category
of plants (C4 pathway plants), composed of aquatic plants, desert
plants, salt marsh plants, and tropical grasses, have dC-13
values that range from -6 to -19. An intermediate group (CAM
plants) composed of algae and lichens has dC-13 values range
from -12 to -23?. The dC-13 of plants and organisms can provide
useful information about sources of nutrients and food web relations.
Precautions
Compounds of carbon have a wide range of toxic action. Carbon
monoxide (CO), which is present in the exhaust of combustion
engines, and cyanide (CN-), which is sometimes in
mining pollution, are extremely toxic
to mammals. Many other carbon compounds are not toxic and are
in fact absolutely essential for life. Organic gases such as
ethene (CH2=CH2), ethyne (HCCH), and methane
(CH4) are dangerously explosive and
flammable when mixed with air.
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