Explain the following
(a) `CO_(2)` is a gas whereas `SiO_(2)` is a solid
(b) Silicon forms `SiF_(6)^(2-)` ion whereas corresponding fluoro compound of carbon is not known.

Explanation

(a) `CO_(2)` has a linear structure. Its dipole moment is zero. It is believed that `CO_(2)` molecule is a resonance hybrid has the following structures
`O=C=O harr^(-)O-C-=O^(+)harrO^(+)-=C-O^(-)`
The `CO_(2)` molecules are held together by weak van der Waals' forces and thus, it exists as gas. In `SiO_(2)`, due to large electronegative difference between Si and O, the Si-O bonds have considerable ionic nature
Therefore, silica has three dimensional network like structure in which Si-atom is tetrahedrally bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms by covalent bonds

There is no discrete `SiO_(2)` molecule. It is a network solid with octahedral coordinaton.
(b) In silicon, vacant 3d orbitals are available due to which it can accomodate electrons from 6 fluorine atoms, thereby forming `SiF_(6)^(2-)` ion. However, in case of C only `2p^(2)` filled orbitals are available thus, it cannot expand their covalency expand their covalency more then 4. Thus, `C Cl_(6)^(2)` is not known.