The dianion of squaric acid (squarate ion, C₄O₄²-) is a resonance hybrid. How many equivalent resonance structures, each with two C=O groups, one ring C=C and two O⁻, can be written for it ?

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Each resonance structure has two O⁻ groups on neighbouring carbons, one ring C=C between them, and two C=O groups. The two O⁻ can sit on any of the four edges of the square, giving 4 structures. A structure with O⁻ on diagonal carbons is impossible because the other two carbons would have no partner for a C=C bond.

Number of structures = 4. The charge is spread over all four oxygens, which is why the ion is more stable than acetate.

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