Kekulé structures of different carbon species have been determined. On the basis of Kekulé structure and C-C bond counts as well as the surface curvature, stability of diverse carbon species, driving force for curling of graphite fragments and formation of fullerenes and nanotubes, have been discussed. Curling of graphite flat fragments, end-capping of nanotubes, and closure of curved structures are driven by a tremendous increase in Kekulé structures as terminal carbon atoms couple their dangling bonds into C-C σ bonds. The increasing tendency becomes particularly striking for large cages and nanotube. Resonance among numerous Kekulé structures will stabilize the curved structure and dominate formation of closed carbon species. For similar carbon cages with comparable Kekulé structure counts in magnitude, the surface curvature of carbon cages, as a measure for the strain energy, also plays an important role in determining their most stable forms.
