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They just have to sum up to the same overall charge, as far as I know. Not sure if this is what you mean, but some resonant structures are more stable than others due to the formal charges of all the atoms being lower (i.e, all of them being 0 or +1 versus one atom being +2 and others being 0)
The most stable and ideal resonance structure would have a formal charge of 0 on each individual atom. However, that may not happen if the molecule is charged. If we have something such as SO4(2-), then the individual formal charges on each individual atom should add up to a total of -2. If a resonance structure's charge adds up correctly, it is valid, but the most ideal structure would have formal charges of 0 on each atom or on at least the central atom.
I believe that the textbook said a resonance structure is any structure in which the atoms are in the same place, but the bonds or location of electrons can move around. In this sense, resonance structures do not all have to be the lowest charge, but I think if we were asked about resonance structures on a test, we are only expected to put the most stable ones. Hope this helps!
no the resonance structures do not need to have the same formal charge for all the atoms. The only thing that must stay the same is the overall charge. the difference in formal charge is what causes one molecule structure to be more stable or favorable than others.
They should just sum up to the overall charge! For example if your structure is an ion that has a charge of -2, the formal charges should add up to that. But the formal charges on the atom for each resonance structure will vary because the bonds vary.
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