Allotropes of Carbon – Properties, Structure of Carbon Allotropes, Graphite, Diamond, Fullerenes

The phenomenon by which an element can exist in more than one physical state is called allotropy. The allotropes of carbon can be categorised into two:

  • Amorphous Carbon Allotropes
  • Crystalline Carbon Allotropes

What Are the 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).

Carbon, due to its capability of having variable oxidation states or coordination numbers, 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 an extremely hard, transparent crystal with carbon atoms arranged in a tetrahedral lattice. This allotrope of carbon is a poor electrical conductor and an excellent thermal conductor.
  • Lonsdaleite: It is also called a 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:

Graphite

It is 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 a 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 temperatures 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 the carbon-carbon bond length within the layer is 141.5 picometers.

Out of four carbon atoms, three form sigma bonds, whereas the fourth carbon forms pi-bonds. The layers in graphite are held together by Vander Waal forces.

The phenomenon by which an element can exist in more than one physical state is called allotropy. The allotropes of carbon can be categorised into two:

Graphite Structure – Allotropes of Carbon

Diamond

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.

Each carbon is sp3 hybridized and forms covalent bonds with four other carbon atoms at the corners of the tetrahedral structure.

The phenomenon by which an element can exist in more than one physical state is called allotropy. The allotropes of carbon can be categorised into two:

Structure of Diamond

Do you know why a diamond is hard?

It is hard because breaking a diamond crystal involves rupturing many strong covalent bonds. Breaking covalent bonds is not an 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

Buckminsterfullerene (C60) is also one of the allotropes of carbon. The structure of fullerene is like a cage shape, due to which it looks like a football.

Fullerenes

They are spheroidal molecules having the composition C2n, where n ≥ 30. These carbon allotropes can be prepared by evaporating graphite with a laser.

Unlike diamonds, fullerenes dissolve in organic solvents. The fullerene C60 is called ‘Buckminster Fullerene’. The carbon atoms are sp2 hybridized.

Note: There are 12 five-membered rings and 20 six-membered rings in C60.

Silicates

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 the amphibole’s 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.

Frequently Asked Questions (FAQs)

Q1

What is meant by an allotrope of an element?

The existence of an element in more than one physical form, differing in the arrangement of atoms, is called an allotrope.

Q2

Name a few allotropes of carbon.

Diamond, graphite, fullerene, and carbon nanotubes are prominent allotropes of carbon.

Q3

Which allotropes of carbon occur in the solid state?

Graphite and diamond occur in the solid state.

Q4

Which allotrope of carbon is the hardest?

Diamond is the hardest allotrope of carbon.

Q5

Is diamond a good conductor of electricity?

No, diamond is a bad conductor of electricity.

Er. Neeraj K.Anand is a freelance mentor and writer who specializes in Engineering & Science subjects. Neeraj Anand received a B.Tech degree in Electronics and Communication Engineering from N.I.T Warangal & M.Tech Post Graduation from IETE, New Delhi. He has over 30 years of teaching experience and serves as the Head of Department of ANAND CLASSES. He concentrated all his energy and experiences in academics and subsequently grew up as one of the best mentors in the country for students aspiring for success in competitive examinations. In parallel, he started a Technical Publication "ANAND TECHNICAL PUBLISHERS" in 2002 and Educational Newspaper "NATIONAL EDUCATION NEWS" in 2014 at Jalandhar. Now he is a Director of leading publication "ANAND TECHNICAL PUBLISHERS", "ANAND CLASSES" and "NATIONAL EDUCATION NEWS". He has published more than hundred books in the field of Physics, Mathematics, Computers and Information Technology. Besides this he has written many books to help students prepare for IIT-JEE and AIPMT entrance exams. He is an executive member of the IEEE (Institute of Electrical & Electronics Engineers. USA) and honorary member of many Indian scientific societies such as Institution of Electronics & Telecommunication Engineers, Aeronautical Society of India, Bioinformatics Institute of India, Institution of Engineers. He has got award from American Biographical Institute Board of International Research in the year 2005.