Explain why carbon dioxide is a gas at room temperature but silicon dioxide is a solid substance.

Explanation

As the atomic sizes of carbon and oxygen are almost equal, carbon and oxygen atoms can easily form double bond between them by effective `p pi - p pi` overlapping (O=C=O) and because of this, carbon dioxide exists as discrete molecules. The linear `CO_2` molecules are non-polar and so the intermolecular forces in carbon dioxide are too weak to allow the formation of molecular aggregates. That is why carbon dioxide is a gas at room temperature.
On the other hand, Si-atom is much larger than O atom and 3p -orbital of silicon and 2p -orbital of oxygen differ quite appreciably in their sizes and energies. So formation of double bond by effective `p pi-p pi` overlapping does not take place. Therefore, silicon dioxide cannot have molecular structure of the type O =Si =O, similar to carbon dioxide. In other words, silicon dioxide does not exist as discrete `SiO_2` molecules. Instead, silica possesses a giant three-dimensional structure `[(SiO_2)_n]` in which each silicon atom is linked to four 0-atoms tetrahedrally and each O-atom is linked to two silicon atoms. Because of its giant three dimensional polymeric structure, silicon dioxide is a solid at room temperature.