The phenomenon by which an element can exist in more than one physical state is called allotropy. The allotropes of carbon can be categorized into two:
- Amorphous Carbon Allotropes
- Crystalline Carbon Allotropes
What are Allotropes of Carbon?
Carbon with atomic number 6 and represented by the symbol ‘C’ in the periodic table is one of the most influential elements we see around us. Carbon is one of the elements which shows allotropy. The allotropes of carbon can be either amorphous or crystalline (Diamond, Graphite).
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Carbon due to its capability of having variable oxidation states or coordination number makes carbon one of the few elements to have multiple numbers of allotropic forms. Carbon’s ability to catenate is another contributing factor. Thus, it leads to the formation of various allotropes of carbon.
How many Carbon Allotropes are there?
- Diamond: It is extremely hard, transparent crystal, with the carbon atoms arranged in a tetrahedral lattice. This allotrope of carbon is a poor electrical conductor and an excellent thermal conductor.
- Lonsdaleite: These are also called hexagonal diamond.
- Graphene: It is the basic structural element of other allotropes, nanotubes, charcoal, and fullerenes.
- Q-carbon: These carbon allotropes are ferromagnetic, tough, and brilliant crystal structure that is harder and brighter than diamonds.
- Graphite: It is a soft, black, flaky solid, a moderate electrical conductor. The C atoms are bonded in flat hexagonal lattices (graphene), which are then layered in sheets.
- Linear acetylenic carbon (Carbyne)
- Amorphous carbon
- Fullerenes, including Buckminsterfullerene, also known as “buckyballs”, such as C60.
- Carbon nanotubes: Allotropes of carbon with a cylindrical nanostructure.
Let us now take a look into the more widely known allotropes of carbon:
It is also a pure form of carbon. This allotrope of carbon is composed of flat two dimensional layers of carbon atoms which are arranged hexagonally. It is a soft, black and slippery solid. This property of graphite persists because it cleaves easily between the layers.
In each layer, each C atom is linked to three C atoms via a C-C covalent bond. Each carbon here is sp2 hybridized. The fourth bond is formed as a pi bond. Since the π-electrons are delocalized, they are mobile and can conduct electricity.
Graphite is of two forms: α and ß.
In α form, the layers are arranged in the sequence of ABAB with the third layer exactly above the first layer.
In the ß form, the layers are arranged as ABCABC.
Properties of Graphite:
- Since the layers are stacked over each other, this carbon allotrope can act as a lubricant.
- It also has metallic lustre which helps in the conduction of electricity. It is a very good conductor of both heat and electricity
- One of the most important properties of graphite is that it is used as a dry lubricant for machines at high temperature where we cannot use oil.
- Graphite is used to make crucibles which have the property that they are inert to dilute acids as well as to alkalis.
Note: In comparison to diamond, Graphite is thermodynamically more stable.
Structure of Carbon Allotrope (Graphite):
Graphite has a unique honeycomb layered structure. Each layer is composed of planar hexagonal rings of carbon atoms in which carbon-carbon bond length within the layer is 141.5 picometers.
Out of four carbon atoms three forms sigma bonds whereas the fourth carbon forms pi-bond. The layers in graphite are held together by Vander Waal forces.
It is the purest crystalline allotrope of carbon. It has a number of carbons, linked together tetrahedrally. Each tetrahedral unit consists of carbon bonded to four carbon atoms which are in turn bonded to other carbons. This gives rise to an allotrope of carbon having a three-dimensional arrangement of C-atoms.
⇒ Also Read: Chemical Bonding
Each carbon is sp3 hybridized and forms covalent bonds with four other carbon atoms at the corners of the tetrahedral structure.
Do you know why a Diamond is Hard?
It is hard because breaking a diamond crystal involves rupturing a large number of strong covalent bonds. Breaking covalent bonds is no easy task. This property makes this carbon allotrope the hardest element on earth.
Physical Properties of Diamond
- It is extremely hard
- It has a very high melting point
- It has a high relative density
- It is transparent to X-rays
- It has a high value of the refractive index
- It is a bad conductor of electricity
- It is a good conductor of heat
- It is insoluble in all solvents
Other Carbon Allotropes
Buckminsterfullerene (C60) is also one of the allotropes of carbon. The structure of fullerene is like in a cage shape due to which it looks like a football.
They are spheroidal molecules having the composition, C2n, where n ≥ 30. These carbon allotropes can be prepared by evaporating graphite with a laser.
Unlike diamond, fullerenes dissolve in organic solvents. The fullerene C60 is called ‘Buckminster Fullerene’. The carbon atoms are sp2 hybridized.
Fusing alkali oxides with SiO2 gives silicates. They contain discrete tetrahedral units. Silicon is sp3 hybridized. These allotropes of carbon are classified based on their structures.
1. Orthosilicates: They contain discrete SiO4 units. For example, Willemite ( ZrSiO4).
2. Pyrosilicate: Two units are linked together via an oxygen atom. The simplest ion of this type is Si2O76-. For example, Thortveite ( Sc2[Si2O7]).
3. Cyclic Silicates: The units share two oxygen atoms. Only two ions are known as of now, Si3O96- and Si6O1812-. For example, Beryl – Be3Al2Si6O18.
4. Chain Silicates: The linking of the units linearly results in the formation of chain silicates. They are of two types:
- Metasilicates: Each tetrahedral unit shares two oxygen atoms to form a single chain carbon allotrope. For example, Spodumene NaAl(SiO3)2.
- Amphiboles: When two linear chains are linked together, it results in amphiboles carbon allotrope. The parallel chains are held by sharing the oxygen atoms. For example, Asbestos: CaMg3O(Si4O11).
5. Two-dimensional silicates: Sharing of three oxygen atoms results in the formation of a two-dimensional silicate. For example, mica.
6. Three-dimensional silicate: When all the oxygen atoms are shared, it results in a three-dimensional network. For example, Zeolites.