What is the most likely Lewis structure for SO42-? I know it has 6 resonance structure but I do not know which one is the best of them.

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The one that is most stable. To solve for this, you have to find the formal charge for each one. Here's how to do this:   (Total # of Valence Electrons) - (# Unbonded Electrons) - (# of Bonds)   Whichever structure has a charge that is higher than 1, than it is the least stable, I think.