I'm not clear what you mean by "stability"
If you were looking at say oxides or chlorides or carbonates of group 2 metals then "stability" would probably mean whether the molecules or ionic lattices would hold together under different levels of temperature and pressure. For example: group 2 carbonates are more thermally stable as one progresses "down" the group
http://www.s-cool.co.uk/alevel/chemistry/group-ii-and-group-iv/group-ii.htmlIs that the question you are being asked for the "compounds" given?
A detail: CO
3-, is not a compound, it is an ion. I'm not sure that "thermal stability of carbonate ion" actually has any meaning because it can't be isolated.
So if the question is about thermal stability, how did you arrive at the order you suggested in your first post? I'm not saying you are wrong, I'm just not clear what the question you have been asked means.
A suggestion: experiment with the "sup" and "sub" buttons on the forum editor it makes formulae easier to read: e.g. H2CO4 becomes H
2CO
4A question for you. Do you think H
2CO
4 is percarbonic acid and H
2CO is hypocarbonous acid? See
http://wci.wrdsb.on.ca/www/Science/PreAP%20Enrichment/10-1%20Nomenclature%204.pdf . I have never come across such oxyacids of carbon - but that's just my lack of a broad chemical knowledge.
Another thing I'm not sure about is I feel these compounds, although formed from hydrogen, carbon and oxygen are not really organic compounds. My definition of organic compounds is that they are molecules with a variety of "functional groups" on a "backbone" of carbon atoms in chains or rings - although that's a bit of a wooly definition and maybe completely wrong, I guess !
Clive