Consider the sulfate ion, to be made up of sulfur(VI) or S^6+ and O^2 ions even

Question

Consider the sulfate ion, to be made up of sulfur(VI) or S^6+ and O^2 ions even though it behaves more covalently. (Why covalent? Think of top ten stability concepts.) State what the electrostatic bond character of the S-O might be. If we consider calcium sulfate, anhydrite, CaSo_4, used in plaster for walls and casts, how many oxygen oxide atoms must surround each calcium ion? Why? Having determined the number of bonds, what would be the overall crystal structure of calcium sulfate? The radius of the calcium ion is 114 pm whereas that of the sulfate ion is 244 pm. (Does the radius ratio help you decide positions...)

Explanation / Answer

For a compound to be ionic, there should be a big difference in their electronegativities (say one partener as metal and other as non-metal). This will facilitate the electron transfer from metal forming +ve charge which inturn this electron would be accepted by nonmetal (say element with high electronegativity). Because these charges are localizes this leads to the formation of strong ionic bond.

This is not the case with SO42- Here, S-O bond is polarized towards O, rather than localized.

c) The crystal structure of calcium sulfate is Orthorhombic.

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