Why does silicon dioxide is not a molecule but a lattice crystal?

The electronegativity ?

SiO2 tends to form a hexagonal lattice structure. here is a picture:

http://www.aml.arizona.edu/classes/mse222/1998/quartz/Quartzbs.gif

and another:

http://www.aml.arizona.edu/classes/mse222/1998/quartz/Image6.gif

SiO2 is also known as quartz (a type of rock). it is a member of the trigonal trapezohedral class of the rhombohedral subsystem.

 

in a large groups of SiO2, contrary to what the formula states as the bonding, each Si is bonded to 4 O atoms, and each O is bonded to 2 Si atoms. it forms this lattice structure because of intermolecular forces acting upon it. just as H2O "bonds" to other H2O in a sample of ice, SiO2 "bonds" to other SiO2 in a sample of quartz.

 

Note: i use the term "bond" because it's really just van der waal's forces acting upon molecules. i dont consider these bonds, although many chemists do.

*jumping on my bonding soapbox again*

 

Actually, SiO2 crystallizes using bonds with COVALENT character. H2O crystallizes through HYDROGEN BONDS. van der Waals are the absolute weakest type of lattice forces, and, as you say, do not form what we would consider REAL bonds (although they are sometimes termed such by molecular modelling programs, particularly in molecular mechanics forcefields such as MM2, for instance `1,4-van der waals interactions'). But you are not correct in saying that there are `just' van der waals forces acting to form the lattices in either water or SiO2. There are many types of bonds and bonding forces that co-operate to form crystals of these substances. Wherever there is an overlap of electronic orbitals, then there is SOME type of bonding going on. Simply compare the interatomic distances and the van der waals radii of the participating atoms.

 

Hydrogen bonds are REAL. Ionic bonds are REAL. Co-ordination bonds are REAL. Resonance and non-resonance pi-bonds (double and triple), also REAL. You should really think of them as proper bonds, as they are not just van der Waals interactions. Even other supramolecular interactions such as pi-stacking and hydrophobic interactions are all significant packing forces that are not van der Waals interactions, but more electromagnetic and solvent/polarity microenvironment effects.