Well this is because because silicon dioxide  has a giant molecular structure. the molecules are bonded strongly to each other and hence have very strong intermolecular forces. (:
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Si O2 is a solid at room temperature because the chemical bonds among the molecules of Si O2 are very strong at that temperature.  The molecules are very close to each other and some times next to each other without a gap. The molecules are closely packed so that they cannot move any where. They can only vibrate with small deviations.

CO2 is a gas because the attractive force between the molecules is very less and they are separated by  large distances much higher than their molecular size. The molecules are very mobile in all directions to move about or vibrate. They have a lot of energy compared to molecules in a solid. This is at room temperature.
    At a very low temperatures like - 80 degrees centigrade, CO2 can become a solid and it is called  DRY ICE.  Then at that temperature, molecules of CO2 are tightly bonded together to be in solid form.

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