Question

Explain the following:

CO₂ is a gas whereas SiO₂ is a solid.

Answer 1

Because of its small size and good π-overlap with other small atoms, carbon forms strong double bonds with two oxygen atoms to give discrete CO₂ molecules.

Silicon atom, on account of large size, does not have good π-overlap with other atoms. It uses its four valence electrons to form four single bonds directed towards the four apices of a tetrahedron (sp³-hybridisation). Each oxygen is linked with two silicon atoms, i.e., a giant three-dimensional structure comes into existence which is very stable. Thus, CO₂ is a gas and SiO₂ is a solid.

\[\ce{O = C = O}\]