State one major difference between having a nitrogen atom in a ring structure and having an oxygen atom in that structure

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One major difference between having a Nitrogen atom in a ring structure and having an oxygen atom in that structure would be that Oxygen atoms are electron attractors by resonance and Nitrogen atoms are electron donors by resonance. 

In a ring structure, as electrons relocate in resonance structures Oxygen can attract an electron from the ring to become negatively charged, and Nitrogen can donate an electron and become positively charged. This can produce changes in terms of stability and reactivity of the ring.

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