Why does silicon dioxide form a covalent structure and carbon dioxide a simple molecular?
In polycarbonate, material with the summary formula CO2 can be convinced to form linear covalent structures. Double bonds with elements of the third main group are problematic, partly because of bond length, partly because the p3-orbitals aren’t the only ones available for hybridization. The empty d3-orbitals can influence the geometry of the hybrid orbitals as well. On the other hand, oxygen-bridged tetrahedral lattices are very stable for Al, Si and P, and will tolerate substitutions with each other over a non-stochiometric range. I have no idea whether gaseous SIO2 molecules or (SiO2)n molecules exist – SiO2 will melt at about 1700°C after having undergone at least one structural change.
