The BasicsEdit

Because Silicon is directly below Carbon in the Periodic Table it shares some properties.

The fact that the formula of the Oxide of Silicon is often written as SiO2 it is tempting to think that it will be similar to CO2.

But nothing could be less true.

CO2 is a simple covalent molecule. Each molecule is only weakly attracted to each other by Intermolecular forces and so it is a gas at Room Temperature.

SiO2 is a Giant Covalent structure. It is held together by strong Covalent bonds. Consequently, it's a solid with a very high melting point.

Its structure is very similar to that of Diamond.


Exam HintsEdit

It's difficult to see how an examiner could ask you much about Silica except to ask for a comparison with CO2.

In which case, you must be able to explain the differences in boiling/melting points (see above).

If they ask for some other physical property you could point out that neither conduct electricity since neither have delocalised electrons.

Or, you could explain why Silica is brittle - but then you would have to explain how CO2 forms a solid (so-called dry ice). Remember, C=O bonds may be polar but the molecule is symmetrical so it is non-polar overall, leading to weaker intermolecular forces.

In which case, you should probably point out that the forces are so weak that when there's enough energy to overcome a these the whole structure sublimes rather than melts.