Allotropes of Carbon
Allotropes of Carbon
Allotropes are the existence of an element in more than one form. Carbon is one of the elements that contain many allotropes. Each allotrope of carbon has its own different properties. The allotropes of carbon are diamond, graphite, fullerenes, carbon nanotubes, carbon nanobuds, amorphous carbon, glassy carbon, lonsdaleite, and carbon nanofoam.
Diamond is the best know allotrope of carbon. It is the hardest known natural mineral which makes it very rough. In the structure of the diamond, each carbon atom is covalently bonded to four other carbon atoms and has a tetrahedral shape, meaning it is a nonpolar molecule. Diamond has a very melting point and has a density of 3.51 grams per cubic centimeter.
Graphite is the second allotrope of carbon. Its atoms of carbon are arranged in layers. Within each layer of carbon atoms, the atoms join to form six-membered rings and each carbon atom is connected to three others. Two of the three bonds is a single covalent bond and the other bond is a double covalent bond. Graphite is very soft and slippery because the layers are not chemically bonded and can slide over the other. Graphite also has electrical conductivity and is less dense than Carbon since its density is 2.22 grams per cubic centimeter.
A third allotrope of carbon is the fullerenes. Each atom is bonded to three other atoms of carbon just like in the graphite structure, however, the atoms join to form both five-membered rings and six-membered rings. The chemical properties of fullerenes are still being researched by scientists and they are also studied for potential medical use in the belief that they can be put into specific antibiotics to target bacteria and cancer cells.
Another allotrope of carbon is carbon nanotubes. They have a cylindrical or circular cylinder shape. The name nanotube comes from their size because they only have a diameter of a few nanometers. They are made up of layers of graphite in the form of a cylinder and they are often closed at both ends(wikipedia). They exist as single or multi walled nanotubes. Carbon nanotubes have many properties that make them useful in many things. They are extremely strong, and they have special electrical properties. Their unique electrical properties make them a very good conductor of heat. Nanotubes can be made in two ways. One is by mixing the soot of nanotubes of carbon in water and surfactant. The solution is rotated into a substance that is able to collect the nanotubes. Then a liquid is injected to help the formation of the rectangular shape, which later rolls itself into a cylinder shape. The other way is to decompose methane with a catalyst.
Carbon nanobuds are a newly discovered allotrope of carbon. They have fullerene like buds that are covalently attached to the outer walls of nanotubes. It is basically a combination of carbon nanotubes and fullerenes. This is because nanobuds have a combination of the useful properties of both nanotubes and fullerenes. Carbon nanobuds are exceptionally good field emitters.
Amorphous carbon is the name used for carbon when it does not have any crystalline structure. Some short range order can be seen but there is no long range pattern of atomic position. Amorphous carbon is used to describe the carbon materials found in soot and coal that is neither graphite nor diamond.
Another allotrope of carbon is glassy carbon. Glassy carbon is a class of non-graphitizing carbon that is mostly used as an electrode material used in electrochemistry. It was first made by using cellulose but later it started being produced with a similar material phenolic resin. The making of glassy carbon is by taking organic precursors and exposing it to a series of heat treatments. Glassy carbon is impermeable to gases. The oxidation of certain glassy carbons in oxygen, carbon dioxide and water vapor are much lower then of any other carbon. They are also highly resistant to acids.
Lonsdaleite is a hexagonal allotrope of the carbon allotrope diamond. This allotrope is also known as “hexagonal diamond”. It is believed to be formed when meteoric graphite falls to the Earth. The great heat and stress of the impact causes it to turn into diamond however it retains the graphite hexagonal crystal lattice. Lonsdaleite has a transparent brownish yellow color. It has a hardness of 7-8 on Mohs hardness scale. The reason why it has a lower hardness then diamond is because of the imperfections in the naturally occurring materials. Lonsdaleite can also be made by the thermal decomposition of a polymer at the atmospheric pressure under argon starting at 110 degrees Celsius.
A final allotrope of carbon is carbon nanofoam. It consists of a low density cluster assembly of carbon atoms strung together to form a three dimensional web. Each cluster of carbons are about 6 nanometers wide and have about 4000 carbon atoms that are linked together in graphite like sheets that are given negative curvatures. This is because there are heptagons within the regular hexagonal pattern(saskschools). Carbon nanofoam is a poor conductor of electricity and they have many unpaired electrons. It is believed that the unpaired electrons are due to bonding defects.
http://www.chemsoc.org/ExemplarChem/entries/2003/bristol_shanley/carbonallotropes/ http://www.saskschools.ca/curr_content/chem20/networks/allotrop.html http://en.wikipedia.org/wiki/Allotropes_of_carbon