The solubility of carbonates, bicarbonates and hydroxides of s-block elements increases down the group.

But, the solubility of their sulphates and perchlorates decreases down the group.

The reason for this is: down the groups, ions get larger. Thus, hydration enthalpy decreases, and solubility should take a hit. However, for hydroxides and (bi)carbonates, solubility increases. Lattice enthalpy is related to the Coulombic force between the charge centres, which decreases as the square of internuclear distance. Thus, lattice enthalpy goes down much faster than hydration enthalpy does, so on the whole, solubility does increase.

However, according to this answer, sulphates and perchlorates have a shape that ensures that lattice enthalpy decreases much slower than hydration enthalpy. Of course, this is only pushing the question one rung deeper. What is it about the shape of sulphates and perchlorates that takes care of the square factor?

  • $\begingroup$ They are tetrahedral and rather bulky, maybe? $\endgroup$ Mar 15 '21 at 15:42
  • $\begingroup$ chemistry.stackexchange.com/questions/96822/… $\endgroup$
    – Mithoron
    Mar 15 '21 at 15:43
  • $\begingroup$ @OscarLanzi I thought their tetrahedral shape might have something to do with it, but what? $\endgroup$ Mar 15 '21 at 15:44
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    $\begingroup$ Maybe at least delete this "special shape" from title, my nonsense detector is screaming ;) I don't think this old post is nearly as useful as its score would suggest. $\endgroup$
    – Mithoron
    Mar 15 '21 at 15:53
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    $\begingroup$ The one you linked. With "special shape". $\endgroup$
    – Mithoron
    Mar 15 '21 at 17:47