Not really much of a problem - just a general question and some speculation on its answer.
Why do some ionic compounds such as aluminum fluoride form hydrates while other ionic compounds such as sodium fluoride don't form hydrates upon being dissolved in solution?
Water is a polar molecule.
Coulombic attraction. Attraction (q) is inversely proportional to the distance between the radii of the two charged particles.
The Attempt at a Solution
I'm guessing this is due to the fact that sodium is a much bigger molecule than aluminium and sodium has only a +1 charge while aluminium has a +3 charge. These two factors cause aluminum ions in solution to form aluminium hydrate (aluminium with 6 water molecules attached) while sodium doesn't do anything in solution; it stays as an ion.