Different compounds have different properties based on the strengths of the bonds between atoms (and other factors), this means that all different compounds have different boiling and freezing points - determining whether they are solid at room temperature or not. Hope that helps :)

Short answer: CO2 is O=C=O, with double bonds, fully saturated, chemically inert. SiO2 is O-Si-O, single bond, with free bonds to coordinate into a crystal. ________________________________________________ Long answer: What makes an atom happy? If all of its electrons are paired and the shells are full. Case of Carbon: two electrons in the inner shell: full! Four electrons in the second shell, four missing (8 would be full). Oxygen: two electrons in the inner shell: full! Six electrons in the second shell, two missing. Carbon dioxide makes everybody happy: O=C=O. The "=" means double bond, two electrons shared between O and C. So all vacancies filled, and Carbon dioxide is all happy, a very inert gas (not even polar, since linearly stretched). Now Silicon: two electrons in the inner shell: full! Eight electrons in the second shell: full! Third shell: four electrons, four missing for at least full 3p-orbital. 3d electrons would give double bond in the third period. So only single bonds for the 3p-electrons, coordinates like: | O | -O-Si-O- | O | (fixed width font not really fixed width??)