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

SiO4?

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

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.

The difference in melting points between BeCl2 and SiO4 can be explained by the difference in the types of covalent bonds present in the two compounds.

BeCl2 is a compound composed of beryllium (Be) and chlorine (Cl) atoms. Beryllium belongs to Group 2 of the periodic table, and chlorine belongs to Group 17. In BeCl2, beryllium forms two covalent bonds with two chlorine atoms. Since both beryllium and chlorine have relatively small atomic sizes, the bond between them is highly polar covalent.

On the other hand, SiO4 is a compound composed of silicon (Si) and oxygen (O) atoms. Silicon belongs to Group 14, and oxygen belongs to Group 16. In SiO4, silicon forms four covalent bonds with four oxygen atoms. The Si-O bonds in SiO4 are purely covalent, with relatively large atomic sizes of silicon and oxygen.

The difference in melting points between BeCl2 and SiO4 is primarily due to the difference in bond polarity and bond strength.

In BeCl2, the polar covalent bonds result in a smaller bond strength, as the electron distribution between the beryllium and chlorine atoms is not equal. Consequently, the intermolecular forces in BeCl2 (van der Waals forces) are relatively weak. These weak forces do not require much energy to break, resulting in a low melting point for BeCl2.

In SiO4, the purely covalent bonds have a relatively larger bond strength, as the electron distribution between the silicon and oxygen atoms is more evenly shared. As a result, the intermolecular forces in SiO4 (van der Waals forces) are relatively strong. These strong forces require more energy to overcome, resulting in a higher melting point for SiO4.

In summary, even though both BeCl2 and SiO4 have covalent bonds, the difference in bond polarity and bond strength leads to the difference in their melting points.