Allotropes of carbon: Diamond, Graphite, and Fullerenes

Allotropes are elements that occur in more than one form. The most common examples of allotropes are Carbon and sulfur. The forms of an allotrope have different physical properties but identical chemical properties. This is caused by the difference in bonding between two atoms of the element. Although the physical properties of allotropes of any element are different, the physical state or state of matter is the same. In the crystalline form, diamond and graphite are the two allotropes of carbon while the amorphous forms of carbon are coal, coke, petroleum coke, gas carbon, etc. In this article, we will learn more about the allotropes of carbon: diamond, graphite, and fullerenes.

Key takeaways: Allotropes of carbon, graphite, diamond, fullerenes, properties of diamond, properties of graphite

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What are allotropes?

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Allotropes or allotropy may be defined as the existence of a chemical element in one or more physical forms occurring in the same physical state. Allotropes may show different physical or chemical states depending upon the atom arrangement or the number of existent atoms. While carbon and sulfur is the common element displaying allotropy, Carbon can make more allotropes due to their five-membered valency.

What are allotropes of carbon?

Carbon is a p-block element. The atomic number of carbon is 6 and it belongs to group 14. It makes up about 18.5% of our body.

Now, when it comes to carbon, it has only two types of allotropes.

1. Crystalline
2. Amorphous

The crystalline forms of carbon. The crystalline form of carbon consists of allotropes such as:

1. Diamond
2. Graphite
3. Fullerenes

While the amorphous allotropes of carbon are:

1. Coal
2. Coke
3. Wood charcoal
4. Animal charcoal
5. Lampblack
6. Gas carbon

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Diamond

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Diamond is the crystalline form of carbon. It is one of the purest forms of crystalline carbon present on earth.

• Each atom of carbon is tetrahedrally linked to other atoms of carbon in the diamond.
• So one carbon atom in diamond is linked to four other carbon atoms.
• This creates the 3-dimensional structure of the diamond.

Structure of diamond

Carbon atoms are sp3 hybridized in diamond. Each carbon in a diamond is linked to four other carbon atoms via four σ (sigma) bonds (strongest covalent bonds).

Structure of Diamond

Read More: Hybridization

Properties of Diamond

Some of the properties of diamond are:

• Diamond is the purest and densest form of carbon.
• It is extremely hard. Diamond is the hardest naturally occurring substance.
• The melting point of the diamond is 3843K (approx 3570 celsius).
• In diamonds, all the valence electrons are involved in bond formation. There is no lone pair of electrons, due to this diamond having a very low electrical conductivity.
• Diamond is insoluble in all solvents.
• When heated at 475K in presence of sulphuric acid and potassium dichromate, diamond is oxidized directly into carbon dioxide, leaving no residue.
• Diamond has a very high refractive index.
• It is a good conductor of heat.

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Graphite

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Graphite is one of the most commonly found naturally occurring allotropes of carbon.

• Graphite consists of hexagonally arranged two-dimensional flat layers of carbon atoms.
• It is soft, black, and slippery.

Structure of graphite

In graphite, carbons are in sp2 hybridization. Each carbon is linked to three other carbon atoms via σ bonds in a single layer. The fourth bond of carbon is via π bond. Two subsequent layers are linked by weak Van Der Waal’s forces.

Structure of graphite

Properties of graphite

Some of the properties of graphite are:

• Graphite is slippery because of its characteristic layered structure. It is often used as a dry lubricant.
• Graphites are good conductors of electricity because the fourth electron of each carbon atom is free.
• Graphite is a good conductor of heat. But due to its layered structure, it has lower heat conductivity compared to diamond.
• Graphite is black in color and has a metallic luster.
• Graphite isn’t soluble in polar and non-polar solvents. But it does get oxidized into carbon dioxide upon treating it with potassium dichromate and sulphuric acid.
• Graphite is inert to both alkalis and acids. And hence are often used to make crucibles.

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Fullerenes

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Unlike graphite and diamond, fullerene doesn’t have any dazzling edges or surface bonds to attract atoms hence it is considered the only purest form of carbon.

• It is a spherical molecule of composition C_2n, where n ≥ 30.
• The preparation of fullerenes occurs when graphite is heated in an electric arc in an inert gas like helium or argon forming a sooty material by condensation of Cn molecules.

Structure of fullerenes

The Fullerene, also known as buckminsterfullerene, consists of 60 vertices with a carbon atom at the end of each vertex. It consists of 20 six-membered rings and 12 five-membered rings. This gives it a spherical, ball-like structure.

The six-membered rings are fused to the other six and five-membered rings, while the five-membered rings are only fused with the six-membered rings. Fullerene contains both single and double Carbon-Carbon bonds of distance 142pm and 138.3 pm.

Structure of fullerenes

Properties of Fullerene

Fullerene has the following properties:

• Fullerene is soluble in organic solvents.
• C₆₀ fullerene when added to toluene turns purple while a C₇₀ fullerene is orange-red.
• Fullerene can be reduced electrochemically. It can then react with group 1 alkali metals. It will then act as a superconductor under 18K carrying electricity with zero resistance.
• It can also form a platinum complex.

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Why is diamond hard?

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Each carbon has four electrons on its outermost shell. In the case of diamonds, these electrons are bonded with other carbon atoms resulting in a strong chemical bond. The presence of this bond creates an extremely rigid tetrahedral crystal. In simple words, the tightly arranged structure makes diamond the hardest substance on the planet.

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Uses of allotropes of carbon

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Each allotrope of carbon has its unique physical and chemical properties. Based on their properties each allotrope of carbon is used commercially for different purposes.

Uses of diamond: Diamond is used for:

• Being the hardest substance on earth, diamond is used to cut glasses, rock drilling, as well as cutting marbles.
• Diamond is the most precious stone and has a huge demand for jewelry.
• Diamond is used in surgical tools, for example, to remove cataracts from the eyes.
• It is used to make dies to draw thin wires of metals like tungsten.

Uses of graphite: The uses of graphite include:

• Graphite is used as a dry lubricant in machines where wet lubricants cannot be used.
• The most common use of graphite is making pencil leads.
• It is inert in acids and alkalis and hence is used to make crucible.
• Graphite is used as a reducing agent in the steel industry.
• Graphite is used in making carbon arcs, batteries, and electrodes.

Uses of fullerenes: The uses of fullerenes are:

• Fullerene is used as a lubricant.
• It is used to absorb gasses.
• It has various uses in the biomedical industry like drug delivery systems, light-activated antimicrobial agents.
• Fullerene is a good conductor of electricity and is used in batteries and electronic devices.
• Fullerene is used in nanotechnology.

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Things to remember

• Allotrope refers to one or more forms of an element occurring in the same state of matter. Each allotrope of an element has distinct physical and chemical properties.
• Crystalline allotropes of carbon are diamond, graphite, and fullerene. The amorphous allotropes of carbon are coal. Coke, charcoal, etc.
• Diamond is a pure form of carbon. It has a tetrahedral structure with sp³ hybridization.
• Graphite consists of multiple flat layers of bonded carbon. Each carbon is attached to 3 other carbons.
• Fullerene is a spherical molecule of composition C_2n, where n ≥ 30.
• Carbon in diamond is involved in sigma bond formation with other carbons. This makes diamond the hardest substance on earth.

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