Charge density, the ratio of the charge to the size of the ion, is highest for the small highly charged ions by elements at the top of groups and further to the right in the periodic table.
For Group 1 and 2, Be²⁺ has the highest charge density and Cs⁺ the lowest.

Ions of high charge density commonly exhibit covalent bonding in their compounds have an acidic hydrated ion have an amphoteric oxide (hydroxide)
In addition, and pertinent to the rest of this file, their compounds containing oxyanions are of lower thermal stability. Decomposition to the oxide is more favorable if the cation in the compound is small as lattices having a cation and anion of similar size are more stable.

Hence MCO₃ ˜ MO + CO₂ is more favourable for M at the top of Group 2.