[xshadow]

Hi

I don't understand why SO₄^2- is  considered a hard base while So₃^2- is a soft (borderline) one


For monoatomic ionic species like Mg²⁺ vs K⁺ (or Ba2+)  is say that Mg²⁺ is hard because has a much higher  charge/ionic radius of the metal ion than Ba²⁺ or K⁺


But for polyatomic ions like SO₄^2- or SO₃^2- how can i say this?
what  parameter I have to consider to say that SO42- is hard while SO32- is a borderline base?

The negative charge is in both case 2-
THe ionic radius ...of the entire ion I think is not so much different...

thanks