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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 Χ10-6
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 Χ 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 |
| |
| SI units & STP
are used except where noted. |
Carbon (Wiktionary: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.
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
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
(See also organic chemistry.)
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 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.
Reference
External links
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