Graphene structure - the honeycomb structure comprises two atoms per unit mesh with bond length $l = 142 \mathrm{pm} = 1.42 \mathrm{\AA}$, and lattice parameter $a = l \sqrt{3}$. The yellow and blue atoms are inequivalent (look at the bond directions) with each occupying a primitive hexagonal sublattice.
The unit mesh area is 0.0524 $\mathrm{nm}^{2}$ = or 5.24 $\mathrm{\AA}^{2}$, so the atomic density of graphene is 38.17 atoms per $\mathrm{nm}^{2}$.