What are Allotropes of Carbon?
The phenomenon of the existence of allotropic forms of an element is called allotropy. Allotropes are the different forms of the same element having different physical properties but almost similar chemical properties due to different arrangements of atoms.
The allotropes of carbon can be categorized into two:
- Amorphous Carbon Allotropes
- Crystalline Carbon Allotropes
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:
Diamond is a crystalline allotrope of carbon. Its atomic symbol & empirical formula is 'C'
In a diamond, each carbon atom is covalently bonded to four other carbon atoms in a tetrahedral arrangement. This tetrahedral arrangement of carbon atoms gives a rigid, three-dimensional structure to diamond.
It is due to this rigid structure that diamond:
- It is a very hard crystalline structure.
- It has a high melting point.
- It is a non-conductor of heat and electricity.
Structure of Diamond
- Pure diamond is a transparent and colourless solid.
- Polished diamond sparkles brightly because it reflects most of the light (refractive index of diamond is 2.4175)
- Diamond is not attacked by acids, alkalis and solvents like water, ether, benzene or carbon tetrachloride but diamond is attacked by fluorine at 750°C.
- C (Diamond) + 2 F2 (Flourine) → CF4 (Carbontetrafluoride) (at 750°C).
- The density of diamond is 3.51 g per cm3 at 20°C.
- A saw fitted with diamond is used for sawing marbles.
- A chip of diamond is used for glass cutting.
- Black diamonds are used in making drills.
- Diamonds are used for making dice for drawing very thin wires of harder metals.
- Diamonds are also used for making high precision tools for use in surgery such as for the removal of the cataract.
- Diamonds are used for making precision thermometers and protective windows for space crafts.
Graphite is also known as black lead, it marks paper black. The name graphite has been taken from the Greek word ''graphein" (which means to write) in reference to its uses as 'lead' in lead pencils.
- Graphite is an opaque and dark grey solid.
- In a crystal of graphite, the carbon atoms are arranged in hexagonal patterns in parallel planes.
- In a layer of graphite, each carbon atom is strongly bonded to three carbon atoms by covalent bonds. Thus, one valence electron of each carbon atom is free in every layer of graphite crystal.
- The free-electron makes graphite a good conductor of electricity.
- Each layer is bonded to the adjacent layers by weak Vander Waal forces. As a result, each layer can easily slide over the other.
- Graphite is greyish-black, opaque material having metallic (shiny) lustre.
- It is soft and slippery.
- Graphite is lighter than diamond. The density of graphite is 2.26 g per cm3 at 20°C.
- Graphite is a good conductor of heat and electricity.
- Graphite has a very high melting point.
- Graphite is insoluble in all common solvents.
- For making electrodes in dry cells and electric arc furnaces.
- Graphite is a good dry lubricant for those parts of machines where grease and oil cannot be used.
- For making crucibles for melting metals.
- For manufacturing lead pencils.
- Graphite is used as a neutron moderator in nuclear reactors.
- For the manufacture of gramophone records and in electrotyping.
- For the manufacture of an artificial diamond.
- For the manufacture of black dye and paints.
Try yourself:Graphite is used as a lubricant in machines because:
Graphite has a layered structure of hexagonal rings. It has one carbon atom covalently bonded to three other carbon atoms, forming a giant hexagonal molecular structure with layers held by weak Van der Waals' forces. As the Van der Waals' forces between the layers are weak, the layers can easily slide over each other when pressured and is soft. Thus, it can be used as a lubricant.
- Fullerene was discovered in 1985 by Robert F. Curl Jr, Harold Kroto and Richard E. Smally.
- This molecule containing sixty atoms of carbon has been named Buckminster Fullerene. Fullerenes has been named after American architect and engineer R. Buckminster-fuller whose geodesic domes follow similar building principles.
- Types of fullerene: C60, C70, C74 and C78
But C60 is the most stable and most studied form of fullerenes.
- Buckminsterfullerene molecule (C60) is nearly spherical.
- It consists of 12 pentagonal faces and 20 hexagonal faces giving it 60 corners. Thus, Buckminsterfullerene has a hollow, cage-like structure.
Structure of Fullerene
- In the figure, it has a ball-like structure whose molecules contain C atoms.
- By electrically heating a graphite rod in the atmosphere of helium.
- By vaporising graphite by using laser.
- Fullerene is soluble in benzene and forms a deep violet colour solution.
- Crystalline fullerene has semiconductor properties.
- Compounds of fullerene with alkali metals are called fullerides and they are superconductors.
- As a superconductor.
- As a semiconductor.
- As a lubricant and catalyst.
- As fibres to reinforce plastics.
Try yourself:Which of the following is not considered as a crystalline allotrope of carbon?
Carbon exists in 2 allotropic forms: Crystalline and Amorphous.
Crystalline Allotropes: Diamond, Graphite and Fullerene.
Amorphous Allotropes: Coal, Charcoal, Lampblack and Coke.
What are Hydrocarbons?
Compounds formed by the combination of carbon and hydrogen atoms only are known as hydrocarbons.
Classification of Hydrocarbons
- The hydrocarbons which contain only single carbon-carbon covalent bonds are called saturated hydrocarbons.
- They are also called alkanes.
- The general formula for alkanes is CnH2n+2 where 'n' is the number of carbon atoms.
Table: Some examples for saturated hydrocarbon
- The hydrocarbon in which two carbon atoms are bonded to each other by a double (=) or a triple (≡) bond is called an unsaturated hydrocarbon.
- Unsaturated hydrocarbons are of two types:
(a) Alkenes (>C=C<): The hydrocarbons in which the two carbon atoms are bonded by a double bond are called alkenes.
Their general formula is CnH2n where "n" is the number of carbon atoms.
Table: Some examples for alkenes(b) Alkyne (-C≡C-): The hydrocarbons in which two carbon atoms are bonded by a triple bond are called alkynes.
Their general formula is CnH2n-2 where 'n' is the number of carbon atoms.
Table: Some examples for alkynes
Try yourself:Which among the following are unsaturated hydrocarbons?
Unsaturated hydrocarbons are hydrocarbons that have double or triple covalent bonds between adjacent carbon atoms. The term "unsaturated" means more hydrogen atoms may be added to the hydrocarbon to make it saturated (i.e. consisting all single bonds).
So, option C is correct as (ii) and (iv) have double or triple covalent bonds.