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Why do some covalent compounds form lattice while others do not? I'm not only talking about amorphous solids but covalent compounds that form molecular lattice in general. For Example: Compounds like SiO2, SiC and AlN form covalent lattices while some covalent compounds like methane and SiF4 form molecular lattices. The question is - Why do only some covalent compounds do this and also, is there a way to identify which compound forms covalent lattice and which forms a molecular lattice? If so, then how?

NOTE: I've looked this up over Concise Inorganic Chemistry by JD Lee and also on the internet but have found nothing.

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  • $\begingroup$ Off the top of my head, Fluorine has negligible tendency for catenation and multiple bond formation. While on the other hand, C has a high catenation tendency, O can involve itself in an ether kind of linkage, and similarly N can involve itself in multiple bonds on either side. Thus SiF4 would probably interact majorly via Van der Waals forces due to lack of options for polymerization. But of course there may be a better reason here $\endgroup$ – Yusuf Hasan Nov 12 '18 at 14:28
  • $\begingroup$ @YUSUFHASAN Yes. I think so too. But can we treat polymerization the same as lattice formation because if this is true then I've found my answer $\endgroup$ – Lucifer - Nov 12 '18 at 14:37
  • $\begingroup$ No I wouldn't say that polymerization is the same as lattice formation, as lattices tend to have a more ordered, 3-D structure. But still, the idea is to form bonds in both cases, and that will probably depend on the factors I mentioned above $\endgroup$ – Yusuf Hasan Nov 12 '18 at 14:40
  • $\begingroup$ @YUSUFHASAN It actually sounds very convincing. I've trying using the logic on a few compounds and seems like this works. Can you please also tell how to compare the catenation properties of elements and how this sits in the bond formation of AlN and SiO2. $\endgroup$ – Lucifer - Nov 12 '18 at 14:46
  • $\begingroup$ This is not a duplicate of the covalent network question. IT seems to be asking why anything forms a crystalline lattice not just network solids and also why some things don't seem to. That is a much broader question than the dupe. $\endgroup$ – matt_black Nov 12 '18 at 16:04
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See, as far as I know, their is no fixed way to compare catenation properties of elements randomly without any context. Some general pointers however, to keep in mind are-: Carbon has one of the highest catenation powers, as soap molecules like sodium stearate can contain up to 17 carbon atoms. After carbon, some other elements having significant catenation tendencies are silicon, sulphur, boron etc. (roughly in that order). I will not be elaborating further here as I'm not very sure myself about this except perhaps a few rough pointers such as medium-ish size of atom and the presence of group 13 and 14(which have the maximum number of covalent sigma bonds formed without expansion of octet) The thing you really need to understand is that the ability to form multiple bonds is more important here to predict the bonding nature of the crystal lattice. Oxygen and nitrogen can form multiple bonds, which would allow them to kind form cross links while satisfying the valency of the central atom. While on the other hand, fluorine will finish it's covalency in satisfying the needs of the central atom (Si) only, so the tetrahedral unit formed will majorly interact via Van der Waals forces (checking it with the facts, this is a pretty good prediction). The exact crystal structure of SiO2 and ALN are quartz and wurtzite respectively, which would be difficult to predict simply by making the covalent structures. However, a fair idea can be taken about the kind of interactions which will prevail in the crystal lattice, provided it is also covalent.

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