In situ scanning tunneling microscopy and density functional theory ͑DFT͒ calculations are employed to investigate single NH 3 -molecule-reacted Si͑111͒-7 ϫ 7 half cells at room temperature. An NH 3 molecule is dissociatively adsorbed on the surface, and each of its fragments, H or NH 2 , may be adsorbed at an adatom or at a rest atom site. Intermediated by a metastable species, the NH 2 adsorbed atop the adatom is transformed into a stable species when negative sample bias voltages are applied for scanning. By using this property, we determine that 12.1% of the reacted adatom sites are capped by NH 2 and, in turn, 87.9% of NH 2 are bonded with the rest atoms. According to the DFT calculation results, the metastable species and stable species are considered to be the NH 2 -and NH-inserted adatom backbond structures, respectively.