Similarities and trends in the properties of the Group 2 metals, magnesium to barium, and their compounds — FAQ

It's a balance between lattice enthalpy (energy needed to break the ionic lattice) and the sum of hydration enthalpies of the ions (energy released when ions are hydrated). Both decrease down the group. For large anions like sulfate, the lattice enthalpy changes very little, so the rapid decrease in hydration enthalpy dominates, making dissolution less favourable down the group. For the smaller hydroxide ion, the lattice enthalpy change is more significant, and the overall balance results in increased solubility down the group.

Polarising power is the ability of a cation to distort the electron cloud of a nearby anion. High charge density (a large charge packed into a small volume) leads to high polarising power. For example, the small $Mg^{2+}$ ion is more polarising than the larger $Ba^{2+}$ ion.

Beryllium is the first member of Group 2 and shows anomalous properties due to its very small size and high charge density. Its bonding has a significant covalent character, unlike the other members which are almost purely ionic. A-Level Chemistry focuses on the trends from magnesium downwards, where the properties are more regularly metallic and ionic.

Yes, Group 2 carbonates are all largely insoluble in water. However, their solubility does technically increase down the group, but they are all so insoluble that for most practical purposes in A-Level Chemistry, they are considered insoluble.