Carbon has some properties that makes it special: it tends to form covalent bonds, and it prefers to have four of them. Note that the carbon-carbon bonds are record strong (not to be confused with hardness), which certainly paves the way to a lot of weary interesting combinations, as illustrated by many allotropes of carbon (probably more than in any other element) and the immense diversity of organic chemistry.
Graphene has much of its high strength thanks to the 2-D layer structure of carbon atoms. In contrast, Graphite does not have interconnected crystal structure – nor 3-D lattice bonds such as a diamond. Graphite is composed of stacked layers of graphene sheets, which are held together by the weak Van der Waals forces, including attraction and repulsions between atoms, molecules, and surfaces, as well as other intermolecular forces.
But before we go any further, here are some information to remember:
- Amorphous carbon: undefined ratio between sp2 and sp3 bonds arranged in an irregular configuration, making it mechanically weak in every direction.
- Diamond: sp3 bond forming three-dimensionally crystalline structure, not flexible but mechanically strong. Hard and not flexible mean brittle, that is cannot absorb the exterior force energy.
- Graphene: sp2 bond forming a planar 6 rings configuration, making it strong in the direction of the plane of rings, but relatively weak between planes. The pi electrons are free and delocalized across the carbon plane.
The reason the sp orbitals form is to allow the carbon to bond easier with less repulsion between electrons in each of the orbitals. What this means is that carbon’s orbitals change shape to make the carbon “happy”! This is called the hyper conjugation effect. Hyper conjugation can be called the stabilizing interaction that results from the interaction of the electrons in a σ-bond (usually C-H or C-C) with an adjacent empty or partially filled p-orbital or a pi-orbital to give an extended molecular orbital that increases the stability of the system. Hyper conjugation is a factor explaining why increasing the number of alkyl substituents on a carbocation or radical center leads to an increased stability.
When we discuss the “strength of a material”, we refer to the force it takes to break apart the bond between two lattice atoms. While the actual strength of impure graphene is reduced due to voids, dislocations, etc. and can be 2 to 3 times weaker than the theoretical strength, it can be understood that the strength of a nearly flawless graphene can effectively reach its theoretical value.