What are allotropes of carbon?

The word allotrope refers to the different molecular forms in which a chemical element can occur. Therefore, the allotropes of carbon are all forms, natural or not, as the element pure carbon can be presented.

Despite having exactly the same composition, allotropes can have completely different properties.

This is especially true in the case of carbon allotropes. As we will see later, one of the allotropes, diamond, forms perfectly transparent and incredibly hard crystals, while others are soft and brittle substances.

All the carbon allotropes known to date are described below, along with their most outstanding properties and their applications:


Diamond is the most stable form of carbon and is one of the two best-known allotropes of the element. It is composed of sp 3 hybridized tetrahedral carbon atoms linked by CC single covalent bonds in a three-dimensional covalent network.


  • It consists of a colorless, transparent crystalline solid that can be polished to a high gloss.
  • It is the hardest mineral known to man. This means that a diamond is capable of scratching the surface of any other material and can only be scratched with another diamond.
  • It is one of the most expensive natural elements in the world.
  • It is the best known thermal conductor, with a thermal conductivity of between 1300 and 2300 W / mK
  • It does not conduct electricity.
  • It has a very low coefficient of thermal expansion.
  • It has a very low coefficient of friction.


  • Its best known daily use is in jewelry.
  • Due to its hardness, it is an excellent abrasive material, which is why it is added to the surface of instruments for cutting and polishing metals, rocks, etc.
  • Due to its thermal properties, it is used in the manufacture of heat sinks.
  • It is used in the manufacture of low friction micro bearings.


It is another of the best known allotropes of carbon. It is a very common material that has a structure made up of sheets of carbon atoms stacked one on top of the other.

All carbon atoms in graphite have sp 2 hybridization , so they have a trigonal planar structure. Each carbon is bonded to three neighboring carbons forming benzene-like hexagons, with a delocalized system of pi electrons on the surface of each sheet, which makes the graphite able to conduct electricity.


  • It is a solid black material at room temperature.
  • It is combustible, so it can burn in the presence of oxygen.
  • Graphite is a good electrical conductor.
  • It has a high melting point.
  • It is a soft material that gives the impression of being slippery.
  • It has a low coefficient of friction.
  • It is a good thermal conductor.


  • It is frequently used in the manufacture of pencils for writing or drawing.
  • It is frequently used as a dry lubricant.
  • Due to its electrical conductivity and low chemical reactivity, it is frequently used in electrical contacts in different electronic devices.
  • It is used in electrodes for chemical analysis.


Graphene is nothing more than an isolated graphite sheet. It is defined as a two-dimensional crystal, and is obtained by separating the graphite layers with adhesive tape. Graphene possesses unique properties that distinguish it from graphite, despite being very similar in structure.


  • High electrical and thermal conductivity.
  • It is an elastic and flexible material, but at the same time with high hardness and resistance.
  • Being a single carbon atom thick, it is a completely transparent material.
  • It can generate electricity by exposure to sunlight.


  • Flexible screens.
  • Flexible electronic circuits.
  • Anticorrosive coatings.
  • Very efficient solar panels.
  • DNA sequencing systems.
  • High precision biosensors.

Fullerenes or nanocarbon

The fullerenes are closed molecular structures formed only by carbon atoms in which they are arranged in the form of rings of 4 to 7 members.

Buckminsterfulerene (C 60 )

This is shaped like a sphere very similar to a soccer ball. It is also called a bucky ball.


  • High electrical and thermal conductivity
  • High tensile strength.
  • They form a very ductile material.
  • They are relatively inert to chemical reactions.


  • Antivirals
  • Antioxidants
  • Drug delivery systems

Carbon nanotubes

These form the basis and the beginning of nanotechnology. They are long fullerenes with a tubular shape and are one of the most studied materials today for the countless applications of their unique properties.


They have the same basic properties as bucky balls, but being elongated, they are better conductors of electricity in the axial direction.


  • In the manufacture of nanocomposite materials with unique mechanical, thermal and electrical properties.
  • Molecular electronics systems.
  • High sensitivity biosensors.
  • Energy storage systems.


This allotrope has a two-dimensional crystal structure very similar to graphene, but made up of biphenylene units with four-membered rings sandwiched between rings of six, rather than containing only hexagonal arrangements.

Graphite AA ‘

This is a form of carbon discovered in 2008. It is a special type of graphite that differs only in the way graphite sheets are stacked on top of each other. It is a metastable form of graphite that can explain the formation and properties of some multilayered carbon nanotubes.


Graphenylene is a recently discovered new allotrope of carbon. They consist of a two-dimensional diamond shape that is obtained by compressing graphite at very high pressures. This causes the sheets of graphite to bond together, forming a diamond only a few carbon atoms thick.

Amorphous carbon

It generally consists of a mixture of different forms of carbon in different proportions. It can contain nanotubes or other types of fullerenes, graphite, etc. It is found in natural samples such as mineral coal or charcoal produced by pyrolysis of wood.

It is mainly used as a fuel for heating, power generation or cooking food.

Vitreous carbon

It is a form of carbon obtained by heating some organic precursors to high temperatures. It is a very resistant material both to high temperatures and to chemical attack by acids and oxygen. It is also impermeable to gases.

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