Reason why Silicondioxide(SiO2)and Carbondioxide(CO2) are in different state

   
      Silicon(Si) and Carbon(C) are elements of the same group but Silicon dioxide(SiO2) and Carbon dioxide(CO2) are in different state of solid and gas.
        This difference is due to the linear structure with two double bond between carbon and oxygen. It is a small molecule and non-polar with only weak bonds between the molecules. Two of carbon's valence electrons hybridize into two sp hybrid orbital. As a result, the molecule is one dimension with an angle of 180 degrees between bonds and completely non-polar, hence, it is a gas.
        Silicon(Si) on the other hand does not form such bonds as Carbon(C) and the angle is far from 180 degrees, which in conjunction with oxygen's high electronegativity makes it quite polar. Silicon dioxide (SiO2) is not formed of small molecules , it has crystal structure that is made up of infinite array of silicon where each silicon is bonded to four separate oxygen (and each oxygen is shared between two silicon dioxide)  and subsequently form a dimensional network which is large enough to make the substance a solid.
         So, the same apparent overall formula doesn't describe the actual structure of the compounds at all. But the structures explain the difference in behaviour.