• # question_answer 73)   Explain the following : (a) $C{{O}_{2}}$ is a gas whereas $Si{{O}_{2}}$ is a solid. (b) Silicon forms $SiF_{6}^{2-}$ ion whereas corresponding fluoro compound of carbon is not known.

(a) Because of its small size and good $\pi$-overlap with other small atoms, carbon forms strong double bonds with two oxygen atoms to give discrete $C{{O}_{2}}$ molecules. Silicon atom, on account of large size, does not have good$\pi$-overlap with other atoms. It uses its four valence electrons to form four single bonds directed towards the four apices of a tetrahedron ($s{{p}^{3}}$-hybridization). Each oxygen is linked with two silicon atoms, i.e., a giant three dimensional structure comes into existence which is very stable. Thus, $C{{O}_{2}}$ is a gas and $Si{{O}_{2}}$ is a solid. (b) Silicon has 3d-orbitals in the valence shell and thus expands its octet giving $s{{p}^{3}}{{d}^{2}}$-hybridization while d-orbitals are not present in the valence shell of carbon. It can undergo sp3 -hybridization only. Thus, carbon is unable to form $CF_{6}^{2-}$ anion.