Great question. It delves into molecular orbital theory. There are 4 electrons delocalized in the pi system. Two occupy a molecular orbital containing node in the plane where the oxygen nuclei reside and the rest of the electron density is spread over the entire molecule above and below that plane. The other two occupy and orbital where there is a node in the plane of the nuclei and a node in a plane perpendicular to the plane of the nuclei to it right through central oxygen nucleus is located splitting the molecule in half. This is what spreads that negative charge to both the outer oxygens and why the resonance structure is drawn the way it is.
BTW I was disappointed that you did not select my last answer to one of your questions as the best. In fact, the answer you selected was kind of crappy. Oh well, I’ll give you this one last chance to redeem yourself.
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