Using wet-chemical self-assembly, we demonstrate that standard surface reactions can be markedly altered. Although HF etching of Si surfaces is known to produce H-terminated surfaces, we show that up to similar to 30% of a monolayer of stable Si-F bonds can be formed on atomically smooth Si(111) surfaces on HF reaction, when chemically isolated Si atoms are the target of the reaction. Similarly, similar to 30% Si-OH termination can be achieved by immersion of the partially covered F-Si(111) surface in water without oxidation of the underlying Si substrate. Such reactions are possible when H-terminated (111)-oriented Si surfaces are initially uniformly patterned with methoxy groups. These findings are contrary to the knowledge built over the past twenty years and highlight the importance of steric interactions at surfaces and the possibility to stabilize products at surfaces that cannot be obtained on chemically homogeneous surfaces.