Why is Silicon dioxide macromolecular?
Both compounds involve a sigma bond between the central atom (C or Si) and oxygen. As it happens, the Si-O sigma bond is actually somewhat stronger (452 kJ/mole) than a C-O sigma bond (358 kJ/mole). In the CO2 molecule, however, there is excellent overlap between the carbon 2px and 2py orbitals and the oxygen 2px and 2py orbitals, with the result being the formation of very strong pi bonds between the carbon and oxygen atoms. These pi bonds provide a home for the electrons on the carbon. By sharing the electrons with the oxygens to form pi bonds, no other bonding interactions are possible for the carbon. Thus CO2 exists as discrete molecules, which just happen to be nonpolar owing to the geometry of the molecule. Consequently the only intermolecular forces in pure carbon dioxide are London dispersion forces. The electron structure of SiO2 is very different. Unlike CO2, where there is excellent overlap between carbon and oxygen 2px orbitals and between 2py orbitals, there is very poor o
