The Chemistry of Exotic Materials, Part 3 - Carbon Nanotubes and The Hybrid Atomic Orbital Model of Covalent Bonding

Properties of Carbon Nanotubes and the Graphene Fused Ring Structure

The nanotube allotrope of the carbon element exhibits interesting properties of strength and electrical conduction.  Those properties can be directly connected to the atomic-scale structure of the carbon nanotube.  In a perfect display of the bonding versatility of carbon, atoms become arranged in a hexagonal fused ring structure with a continuous conjugated bond system.  The fused ring structure is very stable; tremendous energy is required to break it, which contributes to a carbon nanotube's physical strength.  In order for carbon nanotubes to have any conductance, electrons must be free to roam throughout the length of the structure.  This conductance, which is a "hallmark" property of a conjugated system, can be explained by the resonance which occurs across the entire nanotube.  Resonance results in a theoretical "middle" state, in essence a hybrid electron arrangement between the two "static" distributions.

The Hybrid Atomic Orbital Covalent Bonding Model

An interesting conceptual model for visualizing the formation of a double bond is the hybridized atomic orbital.  The theory is that when two separate atoms (which will become bonded) approach each other there exists an "energy of perturbation".  This perturbation promotes ground state electrons to a higher potential energy state whence new hybrid "bonding" atomic orbitals are formed.  Generally, the different geometry of the hybrid orbitals provide a more spatially stable arrangement of electrons.

 

In the present case, for each carbon atom, one ground-state s-orbital mixes with two ground-state p-orbitals to form three hybrid sp2 orbitals.  Note that one p-orbital remains unhybridized.  A double bond is formed from the combined overlaps of an adjacent sp2 orbital pair and a pair of unhybridized p-orbitals.  This is depicted in the drawing below.

 

The drawing also shows the difference between sigma and pi bonding in the hybrid orbital model.  Note that the resulting geometry is trigonal planar, which is a much more stable electron arrangement than the original unhybridized orbitals with their orthogonal geometry.

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