why does BeCl2 have low melting point but SiO4 has high melting point even though they are both covalent bonds between the atoms

Answer 1

SiO4?

Answer 2

I mean why are their bonds different? why does Becl2 form covalent molecule and SiO4 a covalent network solid?

Answer 3

BeCl2 has a linear crystalline molecular geometry with bond angle of 180-deg. X-A-X geometry by the VSEPR Theory and SiO4 is a tetrahedral molecular geometry with 104.5-degree bond angles with an AX4 geometry. SiO4 can not form closely packed molecular solids b/c of its geometry as can BeCl2.
Search molecular geometry of BeCl2 and SiO4 on internet. I'm sure you'll find some images to compare.