Allotropes of Carbon

Allotropes of carbon: Definition

Carbon holds a special place in the periodic table. Carbon exists everywhere - either in elemental form (on its own) or as a compound (in molecules with other elements). It's present in the pencils you write with, the paper you write on, the chalk, sugar, wood - you name it! In fact, about 12% of the atoms in the human body are Carbon atoms. In this article, you will learn about the molecule which Carbon forms in its elemental form.

For example, H₂ is the elemental form of Hydrogen, because Hydrogen exists in the environment as H₂. When an element can exist in multiple elemental forms, those elemental forms are called allotropes.

Similarly, Carbon has numerous allotropes. The electronic configuration of Carbon is 1s2 2s2 2p2. It has a valency of 4 since there are 4 electrons in its valence shell. To complete its Octet, it needs 4 more electrons. Hence, Carbon makes 4 covalent bonds with other atoms.

Carbon has the ability to form stable long chain molecules with atoms bonded by covalent bonds. This ability allows Carbon to have so many allotropes - some even containing 70 Carbon atoms! There are two types of allotropes of Carbon:

• Crystalline Carbon.
• Amorphous Carbon.

Let us look at the examples of some of each of these two.

Crystalline Carbons

Examples of crystalline allotropes of Carbon are Diamond, Graphite and Fullerene. You will read about some properties of each of these and their underlying structure that makes them crystalline.

Types of allotropes of carbon

We will study the types of allotropes of carbon.

Diamond

Yes, the same diamond that is used in jewellery. Diamond is completely made up of Carbon atoms. Each Carbon atom forms 4 single covalent bonds with 4 other Carbon atoms in Diamond. The molecular structure in which the atoms are arranged in Diamond, and the presence of strong covalent bonds, make Diamond the hardest material in the world.

The atoms in Diamond are arranged in a tetrahedral structure. The figure will help you visualize the structure of Diamond.

Since Diamond is made of repetitions of tetrahedrons of Carbon atoms, it forms a crystalline structure.8

Other important properties of Diamond include:

• Excellent electrical insulator.
• Good conductor of heat.
• High melting point.
• Very low chemical reactivity.
• Broad optical transparency.

Diamond being the hardest material in the world renders itself useful for many applications such as use in industrial cutting tools, milling tools, grinding tools, saws, etc. Its rarity, low chemical reactivity (hence, high biological compatibility), and of-course beauty makes it a valuable material in the jewellery industry. Diamond only forms naturally under the high temperature and pressure conditions found under the Earth's crust and cannot be created in a lab. Diamond has many other interesting uses and properties, which you can read about in an article dedicated to Diamond.

Graphite

Graphite is another crystalline allotrope of Carbon. Like Diamond, it's formed naturally under high temperature and pressure conditions found under the Earth's crust. In Graphite, each Carbon atom makes 3 single covalent bonds with 3 other Carbon atoms. The atoms form a flat layer of joined hexagonal rings. Each ring has 6 carbon atoms, and numerous such layers stacked on top of each other form the structure of Graphite. Look at the image for better visualisation.

Since each Carbon atom only makes 3 covalent bonds, it has a free unpaired electron. This unpaired electron is delocalised in the structure of Graphite i.e., it is not associated to any particular atom and can move around in the entire structure. This allows Graphite to conduct electricity. Also, it's because of these electrons that there are weak intermolecular forces between the layers (shown by dotted lines) This allows the layers to to slide over one another, which is why Graphite is soft and smooth. This makes Graphite the only solid-state lubricator which is found naturally. Other properties of Graphite include -

• Good conductor of heat.
• High melting and boiling point.
• High thermal stability.

Did you know that the pencil "leads" are actually made of Graphite? And because of that, pencil leads can actually conduct electricity? If you want to learn more interesting properties of Graphite, head over to this article.

Fullerene

Fullerenes are molecules of Carbon atoms with hollow structures. Fullerenes generally have 6-Carbon hexagonal rings but sometimes also have 5-Carbon pentagonal rings. The first fullerene molecule that was discovered is Buckminsterfullerene (C60).

Buckminsterfullerene is a molecule of 60 Carbon atoms arranged in a hollow spherical shape as you can see in the figure. There are 20 hexagonal rings and 12 pentagonal rings in its structure. Fullerenes can be used as lubricants, catalysts, or even in pharmaceuticals to target specific bacteria in the body.

Another interesting and a relatively newer group of fullerene molecules is called Carbon nanotube, and they are exactly what their name suggests.

Imagine slicing off a single layer of Graphite. A single layer of Graphite will be 1-atom thick, and will have joined 6-Carbon rings. Now fold this layer to form a tube, and what you have is a Carbon nanotube. Carbon nanotubes have high tensile strength, which means they can be stretched without breaking. They are also good conductors of electricity and heat because of the delocalized electrons, same as Graphite. If you find these extremely interesting and want to know more about fullerenes, click here.

Applications of allotropes of carbon: Amorphous Carbons

In examples of Crystalline Carbon allotropes, we saw that all molecules have a repetitive underlying structure. When the underlying structure of a material is random and not orderly or repetitive, they are called amorphous. At the molecular level, some short-range order may be observed in the structure, but never long-range orderly arrangement. All amorphous allotropes of Carbon are opaque i.e., they do not let light pass through them. Some examples of amorphous Carbon are charcoal, coal, lamp black, gas carbon, soot, and coke. It should be noted that these amorphous carbons are generally formed with a lot more impurities (presence of elements other than Carbon) than crystalline carbons.

Amorphous carbons have found applications in:

• Textile.
• Plastic.
• Food packaging.
• Electrical applications.
• Gas and water filtering.
• Healthcare industry.