What is the trend of thermal stability of group 1 oxides?

Question

The thermal stability of most compounds of Group 1 elememts (hydroxides, carbonates, nitrates) increases down the group due to decrement in charge density of the cation. Now, according to one of my study sources, thermal stability of oxides is as follows:

normal oxide(that of Lithium)>peroxide(that of Sodium)>superoxide(that of Potassium, Rubidium, Cesium).

However another source exactly says: "The stability of peroxides and superoxides increases as the size of metal ion increases."

I can not understand whether these two statements contradict each other or not. If yes, then which statement is true and what is the actual trend of thermal stability of oxides down a group?

Answer 1

The stability of "normal" oxides decreases, while that of superoxides increases going down the group. This can be reasoned as such - The charge density of metals at the top is higher than those at the bottom, so they attract the "normal" oxide ion with a much higher force which doesn't give it enough time to combine with another oxygen atom to form a peroxide, so they prefer to form normal oxides, but when we go down the group, like in sodium the charge density becomes lower so it doesn't attract the oxide ion with as high force and thus allows it to combine with another oxygen atom. This is why sodim prefers to form a peroxide over a normal oxide. When you look at the bottom metals, you will observe a similar pattern, seeing Rubidium or cesium forming superoxides.

You can also see the stability of oxides by decomposition of the corresponding carbonate of the metal ion i.e more stable the carbonate, more will be the heat required by it to be decomposed and hence lesser stable will it's normal oxide be.