Explain silicon dioxide.

(N/A) $95 \%$ of the earth's crust is made up of silica and silicates. Silicon dioxide,commonly known as silica,occurs in several crystallographic forms.
Silica is found in the form of quartz,cristobalite,and tridymite.
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. The entire crystal may be considered as a giant molecule in which eight-membered rings are formed with alternate silicon and oxygen atoms.
Silica in its normal form is almost non-reactive because of the very high $Si-O$ bond enthalpy. It resists the attack by halogens,dihydrogen,and most of the acids and metals even at elevated temperatures.
$SiO_{2}$ reacts with $HF$ and $NaOH$:
$SiO_{2} + 2 NaOH \rightarrow Na_{2}SiO_{3} + H_{2}O$
$SiO_{2} + 4 HF \rightarrow SiF_{4} + 2 H_{2}O$