A detailed schematic of step (4) is shown in the bottom row of Figure . In step 4a the missing hydrogen sites are identified by checking the type of hybridization of each carbon atom against the number of valence electrons participating in covalent bonds. In this example, all C atoms in the aromatic rings have sp 2 hybridization. In step 4b H atoms are attached to atoms with unpaired valence electrons. The bond length and angle are determined based on the bonded neighbors and hybridization type.