Using high resolution transmission electron microscopy, we identify the specific atomic scale features in chemically derived graphene monolayers that originate from the oxidation-reduction treatment of graphene. The layers are found to comprise defect-free graphene areas with sizes of a few nanometers interspersed with defect areas dominated by clustered pentagons and heptagons. Interestingly, all carbon atoms in these defective areas are bonded to three neighbors maintaining a planar sp(2)-configuration, which makes them undetectable by spectroscopic techniques. Furthermore, we observe that they introduce significant in-plane distortions and strain in the surrounding lattice.