........whereas silicon tetrachloride is molecular. And this criterion of molecularity strongly differentiates boiling point and melting point.
$SiCl_4$ consists of discrete molecules, which are bound together in the condensed phases by fairly weak dispersion forces. The result? $SiCl_4$ is a volatile liquid with a $"normal boiling point = "58$ $""^@C$.
On the other hand, silicon dioxide is NON-MOLECULAR with an infinite array of $Si-O-Si-O$ bonds that persists thruout the entire crystalline lattice in three dimensions. Because these bonds are fairly strong, and are not terminated by molecular boundaries, the boiling points and melting points of this substance are substantially elevated. If they can be measured, $SiO_2$ melting points and boiling in the $1000's$ of degree Celsius. And this illustrates the strength of the $Si-O$ bond, which must be disrupted.
