# Why is Graphene So Strong?

There has been a lot of news about Graphene since its discovery in 2004. And as we are all told it is a revolutionary material which is very strong, conductive and transparent.

But what is it about the structure of Graphene which makes it so strong?

In a perfect graphene sheet, all carbon atoms are sp$^2$-hybridized, with three in-plane $\sigma$-orbitals and two out-of-plan $\pi$-orbitals. This means that each carbon atom can form equivalent s-bonds to each of its three neighboring atoms. The bonding energy of one C-C bond in graphene amounts to 4.93 eV [doi:10.1088/0953-8984/14/4/312]. These strong (covalent) $\sigma$-bonds are responsible for the extraordinary mechanical properties of graphene.
The out-of-plane orbitals are responsible for the $\pi$ bands (Dirac cones) and lead to the nice electric properties of graphene, but they do not significantly contribute to the mechanical "strongness" of graphene.