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Graphite is an allotrope of carbon. The molecular geometry of graphite is interesting to study as it attributes various unique properties to graphite. It is a crystalline form of carbon and its molecules are arranged in layers. These layers are made up of hexagonal rings of carbon atoms. Graphite is the most stable form of carbon and occurs naturally in this form. Because of the graphite molecular geometry, it is a very good conductor of electricity and is used in electronic products like electrodes, batteries, and even solar panels.
Types of graphite
Several graphite ores are found in nature. The following are the types of graphite ores:
- Amorphous graphite: This kind of graphite is flake-like and seems like a fine powder.
- Highly ordered pyrolytic graphite: In this kind of graphite, the angular spread between sheets is less than one degree.
- Graphite fibre: Sometimes carbon fibres or carbon-reinforced polymers are known as graphite fibres.
- Lump graphite: This kind of graphite usually occurs in veins and is also known as fissure graphite. It is found as enormous ingrowths of plates of acicular crystalline aggregates. In all probability, this kind of graphite is hydrothermal in origin.
- Crystalline graphite: This kind of graphite occurs as small flakes. If it is unbroken, its edges are hexagonal and become irregular when broken.
Graphite molecular geometry
The graphite structure is composed of layers of carbon atoms that are arranged in hexagonal rings. Each hexagonal ring is made up of six carbon atoms. The layers are formed by the rings joining together at the edges. These can be visualised as arrays of benzene rings without the hydrogen atoms. These carbon atoms in the layers of graphite are in the sp2-hybridised state. According to the sp2 molecular orbital model, one carbon atom is attached to three more carbon atoms. These carbon atoms are attached at an angle of 120°. These rings are arranged in large sheets, and each sheet is known as a graphene layer. The bond length of a carbon-carbon bond in a layer is 1.418. Graphite crystallises in a4 axis system, and the layers are arranged in a “C” crystallographic axis arrangement. The importance of graphite molecular geometry and bond angles becomes evident in the crystalline structure of graphite. Graphite has a crystalline structure because of the development and repetitions of a three-dimensional order of atoms and their layers throughout its structure. This is possible because each layer is “indexed”. This is done in the following manner:
Each layer aligns itself to the layers above and below it in a particular manner.
This alignment aligns in a manner that each second layer repeats this system of arrangement.
This means that the carbon atoms of the first layer are arranged in alignment with those in the third layer, and the carbon atoms of the second layer are in alignment with those in the fourth layer.
The sp2-hybridised state
The sp2-hybridised state that the carbon atoms exist in makes two kinds of bonding components possible – the sigma and the pi component. The sigma bonding component is one of the strongest bonding components possible in a 2-dimensional structure. It accounts for the strength of carbon fibres and carbon nanotubes. The sigma component of bonding is extremely strong even within the graphene layers. It is a hard bond and is similar to the sp3-hybridised state of diamonds.
The hard bond of the sigma bonding component does not offer any strength to the attachment between the graphene layers. Thus, the sigma bond does not bind the layers together in the upward direction or the ‘c’ direction. These layers do require any support to maintain the alignment and a stabilising distance. This is provided by the pi bonding component. The pi orbitals of the sp2 network of the carbon atoms overlap with each other inside the graphene sheets. This overlapping causes weak electrical bonds to form between the carbon atoms of adjacent graphene sheets.
Conclusion
Graphite molecular geometry accounts for the various properties of graphite. It is also responsible for its uniqueness. The importance of graphite molecular geometry and bond angles is evident in the number of ways it influences the nature of the substance. Therefore, its study becomes essential.
