alkali and alkaline earth metals

[cracked_doc]

Now I don't understand this : in the alkali and alkaline earth metals first they say that the solubility of their oxides increases down the group...then they say that the solubility of their hydroxides also increases down the group. this is because lower members have lower lattice energies so they dissociate easily and dissolve in water...

But they say that the solubility of their carbonates and sulphates decreases down the group....WHY??...it should also increase following the above reason they have given......Please give a comprehensive answer

[NanoMachine]

I've jotted down some thoughts. I'll need to think some more to get the full explanation and then come back to this.

Originally Posted by cracked_doc

Now I don't understand this : in the alkali and alkaline earth metals first they say that the solubility of their oxides increases down the group...then they say that the solubility of their hydroxides also increases down the group. this is because lower members have lower lattice energies so they dissociate easily and dissolve in water...

Correct. As you go down the group Be, Mg, Ca, Sr, Ba, the metal ions get bigger. The smallest ions make the strongest ionic bonds with oxide and hydroxide ions. The strongest bonds are hardest to pull apart and so BeO is the worst at dissolving. The salts get easier to dissolve as you go down the group. Or to put it another way, the elements nearest to the bottom of the group have the lowest lattice energies and dissolve most easily.

Originally Posted by cracked_doc

But they say that the solubility of their carbonates and sulphates decreases down the group....WHY??...it should also increase following the above reason they have given......Please give a comprehensive answer

Carbonate and sulfate are much bigger ions than oxide or hydroxide. This means they are more polarizable - their electron clouds can be pulled more out of shape by small positive ions like Be²⁺ than bigger ions like like Ba²⁺.