We have investigated atomic arrangements and electronic properties of two-dimensional Si compounds having the Si(111) skeletons with H and/or OH-radicals terminators, using first-principles calculations within the density functional theory. A mechanism of the onset of a direct gap for the planar siloxene has been clarified from the chemical bonding points of view. The direct energy gap does not change with increasing thickness of the Si-skeleton of the planar siloxene, but the indirect one decreases: the direct and indirect gaps of the planar siloxene are associated with the surface state and the Si-skeleton states, respectively. If both surfaces of the planar siloxene are terminated with the OH-radicals, the stacking of the Si-skeleton changes from the diamond structure to the Wurtzite one as the Si-skeleton thickness increases. In such artificial compounds, a direct gap appears only at the thinnest Si thickness as well as the case of the planar siloxene. [DOI: 10.1380/ejssnt.2006.528]