Stability gets a bit fiddly when you start considering polarisation as well.
The reason why Group 1 metal carbonates have a higher temperature for thermal decomposition is, as sam1234 pointed out, they have a lower charge.
The carbonate ion, CO
32- has three oxygen atoms; two of them have the negative charge. In this case, it's best to think of all three oxygen atoms being negative and the cloud of negative charge being spread out over the whole of these three oxygen atoms.
If you get a Mg
2+ ion, it's going to strongly polarise the carbonate ion and strongly attract the negative charge at one oxygen atom. This is going to leave the carbon and the other two oxygen atoms less negative and more ready to leave as CO
2But if you get a Na
+ ion, it's not going to polarise the carbonate ion as much. This means the carbon atom and two oxygen atoms are still quite negative, and it's not going to be released as CO
2 so easily. Thus, in order to liberate it, we need to add a higher temperature. That makes Na
2CO
3 more stable.
Normally, between two ions there is an electrostatic force of attraction that binds them together. And you're right, if you increase the charge of the ions, the electrostatic force will increase which increases strength of bonding, which makes the ionic lattice much more stable. That's ionic bonding.
But there is a point where the electrostatic force of attraction becomes so great that it actually starts bringing back the electrons on the negative atom. It distorts the electron cloud of the anion and forms, what seems to be, a covalent bond.
What we actually say is that the bond shows
covalent character. (you might come across this term if you're doing A2, or AS even, if not, don't use it)
So as you can see there is a point in which the charge of the ion actually decreases the stability of a compound (there's a lot of factors in play which is why it muddles people up).