It is all about the wave-functions mixing i.e. hybridization and the electronic configuration of the carbon atoms. The planar structure of the graphene is a result of mixing between the 2s and two 2p orbitals giving rise to the sp2 hybridization.
The third p orbital is left unchanged (its energy is also not changed). In silicon the situation is a little bit different as the hybridization in silicene is rather a sp2-like and the sp3 hybridization is more favoured for silicon.
Similar situation may be found at the surface of bulk silicon crystal for which unpaired broken bonds are responsible for formation of surface charge which is a driving force for the surface reconstruction. The surface located Si atoms are also characterized by the sp2-like hybridization.
It is also possible to obtain such buckled structure in graphene. You can induce a sp3-like character of the bond placing for example a hydrogen atom at a particular carbon atom in graphene layer. A good example of such structure is graphane.
I hope I could help.
Best regards
What I meant is that silicene doesn’t have purely sp2 hybridized orbitals. That is why I have written sp2-like.
According to your question, the 2 electrons at s and 2 electrons at p orbitals gives you four electrons at hybridized sp3 orbitals. It is quite similar like in carbon.
There is a lot of information about the hybridization mechanisms on the web.
Best regards.
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