Trend 1.5 of the Lithium Group (Group IA) Trends is that the thermal stabilities of the carbonates, nitrates, sulfates, peroxides and superoxides increase down the column.
When you create a lattice of one of the large anions (Carbonate, Nitrate, Sulfate, Peroxide, or Superperoxide) and fill the interstitial holes with one of the alkali cations (Li+ Na+ K+ Rb+ Cs+ Fr+), the species become more stable if you fill the interstitial holes with the cations further down the column.
This is because Carbonate, Nitrate, Sulfate, Peroxide, and Superoxide are all large anions; this means that when they form a lattice the interstitial holes are large. As you go down the column the atoms become larger because there is a larger electron cloud. Ideally you want the cation to fit well into the interstitial holes of the anion lattice. When you have large holes and the smaller cations do not fit well, in fact they move around a lot. This creates less stability. Larger cations, however, do not move around as much in the interstitial holes so they are more stable.
Another way to look at this trend is to look at the stabilities of the products of all of these thermal decompositions, which in this case is the metal oxide (M2O). Since the metal oxides for Li and Na (Li2O and Na2O) are relatively stable, the decomposition occurs with much less heat, making the carbonates, nitrates and so on less thermally stable. As you go down the group to Rb and Cs, the metal oxides (Rb2O and Cs2O) are very unstable because of the bigger ionic radii blowing apart the lattice. Because of this, the carbonates, nitrates, etc... will not want to decompose to the oxide becasue it is far too unstable, so it takes a large amount of heat to push the reaction that way.
How can you apply this information?
Remember when we looked at the reactivities of metals towards O2 and H2O? We learned that the products that form in an O2 reaction are: Li2O, Na2O2, KO2, RbO2, and CsO2. The reason why the oxide changes as you go down the column can be explained by this trend: Li2O is an oxide, Na2O2 is a peroxide, KO2 and below are superoxides. In the table you can see which anions are the largest based on the number of electrons that it contains. The larger cations tend to fit in the larger anion holes. In other words the larger cations are more stable in the large anion lattices.
|Number of Electrons||2(8) = 16||2(8) +1 = 17||2(9) = 18||9 + 10 = 19||10+10=20|