The electronic properties of two-dimensional hexagonal silicon (silicene) are investigated using first-principles simulations. Though silicene is predicted to be a gapless semiconductor, due to the sp(2)-hybridization of the Si atoms, the weak overlapping between their 3 pz orbitals leads to a very reactive surface, resulting in a more energetically stable semiconducting surface upon the adsorption of foreign chemical species. It is predicted that silicene inserted into a graphite-like lattice, like ultrathin AlN stacks, preserves its sp(2)-hybridization, and hence its graphene-like electronic properties.