Allotropes are the distinguishable forms of an element when it exists in two or more forms. The different forms have similar chemicals but different physical properties. The element carbon has many allotropes. This is because it has an electronic configuration of 2,4 (Atomic number 6). Since carbon has 4 electrons in its outermost shell, it shares those electrons to form covalent bonds with other carbon atoms and thus reach an inert state. There are two types of allotropic forms of carbon:

• Crystalline allotropic form
• Amorphous allotropic form

Crystalline allotropic forms have a crystalline structure, while amorphous allotropic forms do not.

Crystalline allotropes

Carbon has the following crystalline allotropes:

Diamond

• Each carbon atom is in the state of sp3 hybridisation and is tetrahedrally connected to four neighbouring carbon atoms. 
• This is a rigid and three-dimensional structure.
• The bond length between each carbon atom is 154 pm.
• Diamond is the hardest material on Earth due to directional covalent bonds throughout the lattice, which is difficult to break.
• It has a high melting point and high density.
• Since each valence electron is bonded to the valence electron of the neighbouring atom, there are no free electrons, and hence, it is a bad conductor of electricity.
• It is used for sharpening tools, drilling, cutting, making dyes, etc.

Graphite

• This has a layered structure consisting of hexagonal rings where each carbon atom forms three bonds and is sp2 hybridised.
• The weak Van der Waals force of attraction holds the layers together.
• A distance of 3.40  Å separates the layers of graphite.
• It has a bond length of 1.41  Å.
• It is a good conductor of heat and electricity since each carbon atom forms three bonds which allow the fourth electron to move freely.
• The force which holds the layers together is weak, and therefore, the layers can slip over one another. Hence, graphite is used as a lubricant in machines.
• The cores of lead pencils are made from graphite. Hence it is also called the black lead.

Fullerenes

• When graphite is heated in an electric arc in the presence of inert gas, fullerenes are formed.
• When heated, a sooty material is formed, which consists of C60 (Buckminsterfullerene), C70 fullerene and other traces of other fullerenes.
• Buckminsterfullerene is a ball-shaped structure with 60 carbon atoms, each of which is sp2 hybridised.  
• The Buckminsterfullerene is the most stable allotrope since it does not have an edge or surface and does not attract any carbon atom.
• Buckminsterfullerene consists of both single and double bonds.
• Buckminsterfullerene is also known as a buckyball.
• Carbon nanotubes
  • It consists of single-layer carbon atoms in rolled-up sheets. 
  • Its molecules have a cylindrical structure.
  • They can be single-walled or multi-walled. The single-walled structure has a diameter of less than 1 nm, while the multi-walled structure has a diameter of more than 100 nm.
  • They contain sp2 hybridised bonds.
  • They can be used to replace transistors made of silicon, in solar cells, in electric wires, etc.

Amorphous allotropes

Carbon has the following amorphous allotropes:

• Coke

• This is a hard solid which has a greyish black colour.
• It is fetched by destructive distillation.

• Charcoal

• Wood charcoal is yielded when the wood is heated in a limited air supply.
• Sugar charcoal (the purest form of amorphous carbon) is yielded when concentrated H2SO4 reacts with cane sugar or sucrose.
• Animal sucrose is yielded by destructive distillation of bones. Another name for animal sucrose is bone black.
• Activated charcoal is obtained by heating charcoal at 1273 K.

• Carbon black

• This is obtained by heating hydrocarbons rich in carbon in a finite supply of air.
• The soot yielded when the hydrocarbon is ignited made to cling on a wet blanket. The soot is removed when the blanket dries. This soot is known as carbon black or lamp black.
• It is composed of 98-99 percent carbon.

Uses of carbon allotropes

• Graphite reduces the speed of electrons in nuclear reactors and is hence used as a moderator.
• Graphite embedded in plastic is used in fishing rods, tennis rackets, air crafts, etc. since it forms a strong, lightweight material.
• Graphite is used as a lubricant for machines.
• Graphite is used in industrial electrolysis.
• Activated charcoal is highly porous. Therefore, it is used to absorb organic contaminators in purifiers, harmful gases in masks and odour in air conditions.
• Carbon black and coke are used as reducing agents.
• Diamond is used in making jewellery.

Conclusion 

Allotropes of carbon are the different forms of carbon that have similar chemical but different physical properties. Carbon has many allotropes because it has four valence electrons in its outermost shell which bonds with other carbon atoms to reach an inert state. There are two types of allotropes of carbon – Crystalline allotropes and amorphous allotropes.

Allotropes that have a crystalline structure are termed crystalline allotropes. Diamond, graphite, fullerenes and carbon nanotubes are some crystalline allotropes. Allotropes that do not have a crystalline structure are termed amorphous allotropes. Coke, charcoal and carbon black are some examples of amorphous allotropes.