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.

(N/A) In $CO_2$, the carbon atom undergoes $sp$ hybridization. Two $sp$ hybridized orbitals of the carbon atom overlap with two $p$-orbitals of oxygen atoms to form two sigma bonds, while the other two electrons of the carbon atom are involved in $p\pi-p\pi$ bonding with oxygen atoms. This results in a linear shape with both $C-O$ bonds of equal length $(115 \ pm)$ and no dipole moment. $O=C=O \leftrightarrow -O-C \equiv O^{+} \leftrightarrow O^{+} \equiv C-O^{-}$
Silicon dioxide is a covalent, three-dimensional network solid in which each silicon atom is covalently bonded in a tetrahedral manner to four oxygen atoms. Each oxygen atom is in turn covalently bonded to another silicon atom. Each corner is shared with another tetrahedron. The entire crystal may be considered as a giant molecule in which eight-membered rings are formed with alternate silicon and oxygen atoms.
$(b)$ Silicon has vacant $d$-orbitals in its valence shell, due to which it can accommodate $6$ electrons from $6$ fluorine atoms, whereas carbon does not have $d$-orbitals and cannot expand its covalence beyond four.