[avantgarde_]

Either Pt or Mg(NO3)2 fails to conduct electricity - I'm not sure which one it is...seems to me that both of them are competent conductivity-wise...Pt is a metal and I'm guessing Mg(NO3)2 is an ionic solid. I'd be grateful for someone to explain the answer for me. Thanks.

[jdurg]

Well, think about the properties of the two items and the properties of electricity.  In order for electricity to flow, electrons have to be able to move.  So which one of those items will allow electrons to move around without hinderance based upon it's structure?  Would it be the metal which has a 'sea' of electrons surrounding the individual platinum protons?  Would it be the ionic solid in which the nuclei and electrons are bound into a crystalline structure?

[avantgarde_]

Oh, right. Mg(NO3)2 would have a rigid ionic structure, and therefore its electrons would have less mobility. Thanks a lot, not too solid on these things!

[xiankai]

yeah... but when magnesium nitrate is dissolved in water, it has ions floating around... and can conduct electricity too.

magnesium nitrate in molten state conduct electricity too...

[jdurg]

yeah... but when magnesium nitrate is dissolved in water, it has ions floating around... and can conduct electricity too.

magnesium nitrate in molten state conduct electricity too...

Correct.  Because when dissolved into solution, the charges are separated which allows current to flow from one side to the other.  When molten, those charges are also able to move around so the substance is able to conduct electricity.  But at STP and standard states (I.E. not dissolved or transformed in any manner), magnesium nitrate is unable to conduct electricity while platinum can.