Because of its small size and good p-overlap with other small atoms, carbon forms strong double bonds with two oxygen atoms to give discrete CO 2 molecules. Silicon atom, on account of large size, does not have good p-overlap with other atoms. It uses its four valence electrons to form four single bonds directed towards the four apices of a tetrahedron (sp 3 -hybridisation). Each oxygen atom is linked with two silicon atoms, i.e. a giant three-dimensional structure comes into existence which is very stable. Thus, CO 2 is a gas and SiO 2 is a solid.