Why do Group 2 hydroxides become more soluble down the group?

This is a question that arose from learning about the solubility of Group 2 hydroxides in class... 


Structure of Metal Hydroxides

Metal hydroxides consist of metal cations and negative hydroxide ions. Therefore when bonded they form giant ionic lattices.


What determines solubility?

Solubility can be determined by how hydrophilic (water loving) a compound is. Most ionic compounds are soluble in water due to the polarity of water. 

Water has a positive an negative partial pole. One end of the water molecule is negative due to the lone pair of electrons located on the oxygen atom. The other end of the water molecule is positive due to the slightly positive hydrogen.

The partial poles attract the oppositely charged ions in the metal hydroxides causing the molecules to split.


Why do Group 2 hydroxides become more soluble down the group?

The size of the metal cations increases down the group as electrons fill an additional energy level. This decreases the lattice energy (the energy required to separate ions in a lattice) as attraction between the oppositely charged ions decrease due to the positive nucleus of the metal ions are further from the negative OH- ions. 

This makes the metal cations and negative hydroxide ions easier to separate and dissolve.  

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