[bmu123]

The most common oxides of C, Si and Pb are CO, CO2, SiO2, PbO, Pb3O4 and PbO2. For all of these, state if they exist at standard temperature and pressure as discrete molecules, polymeric solids or extended (ionic) solids and discuss why these compositions and states form. Factors to consider are bond strengths, ionic radii, common oxidation states and any other(s) that you think are important.

OK so CO and CO2 exist as gases at STP - as carbon is a small size, this allows considerable p-orbital overlap and allows pi bonds to form.
Down the group the ionic radii get larger and therefore the p orbital overlap is less efficient
SiO2 dimerises and is a solid
PbO, Pb3O4 and PbO2 exist as extended ionic solids
Pb's most common oxidation states are +2 and +4 (found in PbO and PbO2)

Really struggling with the full reasoning here? Am I on the right tracks? Many thanks

[Corribus]

Definitely on the right track with silicon and carbon compounds. I think you have a little more work to do with the lead-based species. Why, for example, do you think the lead solids are ionic vs. covalent?  There are some hints given in the last sentence of the problem write-up. You may also want to look around for what's often called the inert pair effect.

[Enthalpy]

Why do you say that silicon dioxide dimerizes? Different oxygen atoms bond different pairs of silicon atoms, so I don't grasp where the limits of a dimer should be. The crystal seems to be one single molecule.
http://en.wikipedia.org/wiki/Silica#Crystalline_forms_of_silicon_dioxide
Or did I miss something? A matter of wording?