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enter image description hereFeSO4.7H2OCuSO4.5H2O, FeSO4.7H2O, ZnSO4.7H2O are some well known compounds. Known as blue, green and white vitriol respectively. As we can see the first compound has 5 molecules of water while the latter two have 7 each. What seems to decide the number?

It is stranger if we look into CaCl2.6H2O and MgCl2.7H2O. These compounds are chlorides of Ca and Mg, present in the same group. However they don't have the same number of water of crystallization molecules. But why? What dictates the number of water of crystallization molecules in a compound?

Ps: 1. I know that the hydrated form of FeSO4 is FeSO4.nH2O where n= 1,2,3,.. However, n=7 seems to be the most common. This is true for other examples in this question.

  1. I thought about it for quite a while. I couldn't find a clear answer. However I have a guess.

FeSO4.7H2O has it's central atom Fe2+ actually is bonded with 6 water molecules via coordinate bonding.

Fe2+ = ... 3px2 3py2 3pz2 3dxy2 3dyz1 3dxz1 3dx2-z21 3dz21 4s0

Fe2+ * = ... 3px1 3py1 3pz1 3dxy1 3dyz1 3dxz1 3dx2-z21 3dz21 4s1 4px1 4py1 4pz1

and then 6 water molecules create coordination bonding with Fe2+* by donating electrons to these twelve orbitals and filling thsese orbitals. However, this is simply an educated guess and can't explain the examples.

I would much appreciate any help. I am open for a discussion. Thanks for reading!

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