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The crystal structure of single-layer graphene in comparison to graphite is discussed with regard to its crystallographic and chemical properties. In both of these polymorphs of carbon, the atomic volume of carbon, reduced to the closest packing of atoms, is practically the same and considerably smaller than in diamond. This indicates pentavalent carbon in graphene as well as in graphite. The observed elastic corrugations of the graphene layers which probably cause their amazing rigidity seem to be due to numerous weak chemical bonds within the layers.