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Your formal charges should "make sense" when drawing resonance structures. Take your first structure and see if you can move any double bonds. Then, take the formal charge of the atoms with your resonance structure and evaluate them. Usually, they should be 0 or negative 1 and positive 1.
In a resonance structure, the formal charges for same atoms should all be equal in reality. This is because in reality, the structure is a hybrid of all the resonance structures. For example, in the SO4 2- resonance structures with 2 Os connected to S with double bonds and 2 Os connected to S with single bonds, the calculated formal charges for the Os with double bonds are 0 and the Os with single bonds are +1. However, in reality, all the of the Os should have a +1/2 formal charge because they all average out to be the same in the hybrid.
Formal charge is used to determine which resonance structure is "best" or the most stable. By calculating the formal charge, you can determine which resonance structure is most likely to occur. Ideally, you want the higher electronegative atoms to have the negative charges and you want to disperse the charge on as many atoms as possible.
Formal charge for the same atom in resonance structures should be the same. For example in SO4 2-, when you draw out the lewis structure you get S as the central atom with two Os attached to it by double bonds and two Os attached to it by single bonds. No matter how you orientate the double bonds in different resonance structures, in each structure the formal charges for the central atom S and O with double bond and O will dingle bond should not change.
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