Introduction

Carbon is the main component of the animal and plant world. It is the fourth most abundant element. Carbon is a nonmetal that is located in group 14 of the 2^nd period of the periodic table. The word carbon comes from the Latin word ‘carbo’, which means coal. Scientists Lavoisier was the first to show that carbon is an element. Carbon dioxide, a component of air, contains carbon. Petroleum and natural gas contain carbon in the form of hydrocarbons combined with hydrogen. Carbon as a mineral is in the form of carbonate in various compounds. Various plant and animal substances also contain carbon. Carbon resides in nature in various forms. So carbon is a polymorphic element (1). In the following article, we are going to discuss the allotropes of carbon.

What is an allotrope of carbon?

Allotropy is a property for which a basic element may exist in two or more forms leaving its original chemical properties unchanged. Some elements may have two or more different forms without changing their basic chemical properties. Those different forms of the element are called allotropes. Carbon is found in abundance in nature as an element in the free state and as a compound when combined with other elements. This element has the ability to form many different compounds because each carbon element can have four chemical bonds with other elements. That is, carbon is an element that has two or more different forms or shows allotropy  (1).

1. Diamond

Diamonds are a special form of an allotrope of carbon. It is a gemstone that is widely used in jewelry making. This gem is made from a single pure material carbon. The diamond is transparent. Scientists thought that diamonds take about 1 to 3 billion years to form due to intense heat and pressure between the earth’s center and earth’s crust, about 140 to 190 km below the earth’s interior. These allotropes of carbon are usually colorless (1) & (2).

2. Graphite

Graphite is a form or allotrope of carbon. Its crystals are hexagonal in shape. Graphite is found in granite, gneiss, mica, and crystalline limestone cracks in the form of large lumps or scattered in fibrous layers. This allotropes of carbon is a dark gray crystalline substance. It is shiny like metal. Graphite can be non-metallic and is a good conductor of electricity. In the formation of graphite, each carbon atom is bounded in a covalent bond to the three surrounding carbon atoms (1) & (2).

3. Lonsdaleite

Lonsdaleite is an allotrope of carbon. It is also called a hexagonal diamond. Lonsdaleite is transparent. It is formed due to asteroidal impacts when meteorites with graphite hit the Earth. This allotrope of carbon played a central role during the transformation of diamonds from graphite. Lonsdaleite is 58% stronger than diamond. It was first identified in 1967. The specific gravity of Lonsdaleite is 3.2. It is brownish-yellow and grayish in color (2).

4. C₆₀ (Buckminsterfullerene)

Buckminsterfullerene is a crystalline allotrope of carbon. Its molecule is made up of 60 carbon atoms therefore has a formula is C₆₀. The structure of the C₆₀ molecule is like a football. Its molecular structure consists of a ring made up of five and six carbon atoms, forming a multi-layered molecule. Its molecular structure is similar to that of a geodesic dome. Famous American architect Buckminster Fuller discovered the Geodesic Dome. Therefore, according to the name of Buckminster Fuller, C₆₀ is called Buckminsterfullerene (2).

5. C₅₄₀ (Fullerite)

This allotrope of carbon is a molecular crystal structure with fullerene molecules. It was first discovered over 30 years ago. Fullerite is a hollow spherical shape in structure. It is used in nanotechnology. The solid form of fullerenes is called fullerite (C₅₄₀) (2).

6. C₇₀

There is another type of fullerenes made up of 70 carbon atoms. Its formula is C₇₀. C₇₀ fullerene structure is a lot like a rugby ball. Fullerenes were first discovered in 1985 by the American scientist Robert F. Curl, Richard E. Smalley, and the British scientist Harold W. Kroto. It is made of 25 hexagons and 12 pentagons. The melting point of C₇₀ is low. These allotropes of carbon are insoluble in water. The density of C₇₀ fullerene is 1.7 gm/cmᶾ. Its structure is a lot like C₆₀ fullerene (2).

7. Amorphous carbon

Amorphous carbon is an allotrope of carbon. It is free and reactive carbon that does not have any crystalline structure. It is a solid allotropic form of carbon. Coke, coal, charcoal, lamp black, gas carbon, carbon black, etc. are amorphous forms of carbon.

Coal is created in nature by the carbonization of wood. Coal consists of about 94-95% of carbon. Charcoal is a dark, solid form of carbon. It is used as fuel. Carbon black is a form of carbon that is produced as a result of incomplete combustion of petroleum products. Gas carbon is produced when petroleum products are heated at high temperatures. It is also an allotrope of carbon (2).

8. Single-walled carbon nanotube

A single-walled carbon nanotube is hollow and cylindrical in shape. These are one of the allotropes of carbon that intersect between fullerene and graphene. The single-walled carbon nanotube is called the Bucky tube. They have very high electrical conductivity. The single-walled carbon nanotubes have a low density. It is an allotrope of carbon. The diameter of Single-walled carbon nanotubes is 1 nm (2).

Three main allotropes of carbon

There are three allotropes of carbon. These are

1. Diamond
2. Graphite
3. Fullerenes

Diamond

Source

About 95% of the world’s diamonds are found in South Africa. Besides this, they are also found in Australia, Brazil, America, and India.

Physical properties

• Pure diamond is a transparent, colorless, bright crystalline substance.
• This is the purest allotropes of carbon.
• Among the allotropes of carbon, diamond is the heaviest.
• Diamonds do not transport heat and electricity.
• The refractive index of a diamond is 2.42. So it looks very transparent and bright.
• X-rays can go through pure diamonds. But X-rays cannot go through glass or artificial diamonds.
• Diamonds do not melt even at very high temperatures.
• They are insoluble in any type of solvent (1) & (2).

Chemical properties (1) & (2).

• Diamonds are chemically highly inert at normal temperatures.
• They are not attacked by acid, alkali, salt, etc. however at high temperatures, they react with some substances. Such as-
• Diamonds oxidize with oxygen at 800°c to 900°c to produce pure carbon dioxide.

C         +       O₂         =        CO₂

(Diamonds)      (Oxygen)         (Carbon dioxide)

• It produces sodium monoxide and carbon monoxide when it is attacked by molten sodium carbonate.

C       +        Na₂CO₃   =       Na₂O + 2CO ↑

• At a temperature of 700°c, the diamonds are oxidized by fluorine to form carbon tetrafluoride.

C     +         2F₂      =    CF₄

(Diamond)    (Fluorine)    (Carbon tetrafluoride)

• Diamonds produce carbon disulfide by reacting with vapor Sulphur at a temperature of 1000°

C       +          2S       =       CS₂

(Diamond)    (Sulphur)      (Carbon disulfide)

Uses

• Diamonds are used as precious gems for bright luster.
• For extreme hardness, diamonds are used for cutting glass, cutting stones, perforating stones, and polishing. The opaque diamond used to cut glass and other solid objects is called bort. Black diamonds used for cutting and polishing stones are called carbonado. (1) & (2)

Graphite

Source

Graphite is found as a mineral in Sri Lanka, Siberia, Mexico, Italy, Canada, California in the USA, and Sikkim in India. Artificial graphite is obtained by heating a mixture of sand, coke powder, and ferric oxide at a temperature of 3000°c for 24 to 30 hours.

Physical properties

• Graphite is a dark gray crystalline substance. It is shiny like metal.
• It is soft and slippery when touched.
• Graphite is lighter than a diamond. Its density is 2.2 gm/cmᶾ.
• Although graphite is a nonmetal, it transfers heat and electricity.
• The melting point of graphite is very high, about 3750°c.
• X-ray and ultraviolet rays cannot go through the graphite.
• Rubbing graphite on paper causes black spots, hence graphite is called a black lead (1) & (2).

Chemical properties (1) & (2).

• Graphite has slightly higher activity than diamond.
• Graphite oxidizes with oxygen at 700°c to produce carbon dioxide. This proves that graphite is an allotrope of carbon.

C         +       O₂         =        CO₂

(Graphite)        (Oxygen)       (Carbon dioxide)

• Graphite reacts with fluorine at a temperature of 500°c to produce carbon tetrafluoride.
• Graphite is not attacked by acids or alkalis.
• When graphite is heated with nitric and sulfuric acid mixed with a small amount of potassium chlorate (KClO₃), it turns a greenish-yellow solid graphitic acid.

Uses

• Graphite is used to make pencil lead.
• It is also used to polish iron products.
• Graphite rods are used in nuclear reactors.
• It is used for the preparation of batteries.
• Graphite powder is mixed with oil or water and used as a lubricant.
• It also is used as the electrode of electric reactors (1) & (2).

Fullerenes

Fullerene is a crystalline allotrope of carbon. The fullerenes molecule is made up of 60 carbon atoms its formula is, therefore, C₆₀. There is another type of fullerenes made up of 70 carbon atoms whose formula is C₇₀. To date 30 to 96 carbon atomic fullerenes have been found.

Fullerenes were first discovered in 1985 by the American scientist Robert F. Curl, Richard E. Smalley, and the British scientist Harold W. Kroto. The structure of the C₆₀ molecule is like a football. Its molecular structure consists of a ring made up of five and six carbon atoms, forming a multi-layered molecule. Its molecular structure is similar to that of a geodesic dome. The Geodesic Dome is a masterpiece of architecture. Famous American architect Buckminster Fuller discovered the Geodesic Dome.

Therefore, according to the name of Buckminster Fuller, C₆₀ is called BuckminsterFullerene. Another name for C₆₀ is Buckyball. C₇₀ fullerene structure is a lot like a rugby ball (1).

Source

Naphthalene is heated in argon gas at high temperatures (1000°C) and gives fullerenes. Fullerenes have been found in anthracite coal in coal mines in Australia, Russia, the USA, Canada, and New Zealand.

Physical properties

• Fullerenes are a type of yellow powder.
• C₆₀ fullerenes is a dark brown crystalline solid matter.
• The melting point of fullerenes is low.
• C₇₀ fullerenes color is grey-black.
• At high temperatures (more than 1000°C) the molecular structure of fullerenes breaks down.
• Fullerenes are soluble in various organic solvents such as carbon disulfide, benzene, toluene, etc.
• The density of fullerenes is 1.65gm/cmᶾ (1) & (2).

Chemical properties

• The chemical activity of fullerenes is much higher than that of diamond and graphite.
• Fullerenes are converted to carbon dioxide at low temperatures heated with oxygen.
• The compounds produced by the reaction of fullerenes with elements like potassium, and rubidium are superconducting.
• Fullerenes, when dissolved in benzene, reacts with oxygen in the presence of ultraviolet rays to produce fullerenes epoxide. (1)

Uses

• There is a lot of potential for its use in cancer treatment and research.
• Very small nanotubes for research are made of Fullerenes.
• It can be used as a lubricant.
• Fullerenes compounds can be used as superconductors and semiconductors.
• It may be used in solar cells and batteries in the near future. (1)

How is graphite different from other allotropes of carbon?

Graphite is different from other allotropes of carbon (diamond & fullerenes). However, there are some similarities between graphite and other allotropes of carbon. Because these are allotropes of carbon (2).

Diamond, Graphite, and Fullerene similarities 

• All three diamonds, graphite, and fullerenes are solid allotropes of carbon.
• Diamond graphite and fullerenes are both crystalline substances.
• Graphite and diamond react with oxygen at high temperatures and fullerenes react
• With oxygen at low temperatures to produce carbon dioxide.
• graphite and diamond react with molten sodium carbonate to produce carbon monoxide (co) and sodium monoxide (Na₂O).
• Both allotropes of carbon are insoluble in water (2).

But there are some differences between graphite and other allotropes of carbon (1) & (2)

Properties	Diamond	Graphite	Fullerenes
1. Activity	Diamond is the hardest substance and very inert.	Graphite is a soft and slippery substance. Its chemical activity is higher than that of a diamond.	The chemical activity of fullerenes is much higher than that of diamond and graphite.
2. Melting point	Diamond is very hard and has a high melting point.	Graphite is soft and has a very high melting point	Fullerenes have a low melting point.
3. Color	Diamond is a colorless transparent crystalline substance.	Graphite is a gray crystalline substance.	C₆₀ fullerenes is a dark brown crystalline solid matter. C₇₀ fullerenes color is grey-black.
4. Transparency	The diamond is transparent.	Graphite is opaque.	Fullerenes are transparent but not more than diamonds.
5. Boiling point	Diamond has a high boiling point.	Graphite still has a very high boiling point.	It has a much lower boiling point than diamonds and graphite.
6. Electricity conduct	Diamond is an extremely bad conductor of electricity.	Graphite is a good conductor of electricity.	Fullerene is a bad conductor of electricity.
7. Heat conductor	It is a poor conductor of heat.	It is a good conductor of heat.	Fullerenes are a high conductor of heat.
8. Stability	Diamonds are stable substances.	Graphite is the most stable substance in all allotropes of carbon at normal conditions.	Fullerenes are more unstable than diamond and graphite.
9. Chemical bond	Diamond formed only a single bond.	Graphite forms a single bond between carbon atoms. It can form electron bonds between two graphite sheets.	Fullerenes can form single and double bonds.
10. Density	Diamond is highly dense. The density of the diamond is 3.51gm/cmᶾ.	Graphite is medium-dense. Its density is 2.26gm/cmᶾ.	The density of fullerenes is lower than that of graphite and diamond. Its density is 1.65gm/cmᶾ.