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How is positive charge delocalized? I understand how negative charge can be delocalized. Electrons are mobile and they can move around. But what about positive charge? For example, consider this resonance structure and its composite:

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How exactly is that positive charge delocalized? Protons aren't mobile. I can't see any protons moving through the remaining pi-system.

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There are no protons moving - protons cannot move in resonance structures. In resonance structures all nuclei must remain fixed, only electrons can move. As you progress from the top-left resonance structure to the top-middle resonance structure you have taken the two pi electrons in what I'll call the 3-4 bond (position 1 being where the bromine is attached) and moved them between carbons 2 and 3, creating a pi bond between carbons 2 and 3 and simultaneously moving the electron vacancy (the positive charge, what a physicist would call a "hole") to carbon 4.

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