Question

How many covalent bonds are there in one molecule of silica, SiO2?

Answer 1

The number of covalent bonds that are present in one molecule of silica, SiO2 is two. The fact that the number of valence electrons actually determine the number of covalent bonds that can be made. In this case there are two oxygen atoms and two bonds are required to form the bond with Si. this is why this situation needs two covalent bonds.

Answer 2

The number of covalent bonds is determined by the number of valence electrons of an element. The number of electrons tells the maximum number of bonds an element allows. In this case, an oxygen atom requires two bonds. So for Sio2, each oxygen  atoms are bonded in double bonds to Si.  Hence there are two covalent bonds.