The C-O bond order in the carbonate ion, CO3^2-, taking the average over its resonance structures, is

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The C-O bond order in the carbonate ion, $CO X_{3} X^{2 -}$ , taking the average over its resonance structures, is

Four bonds' worth of bonding (one double + two single) is spread over three equivalent C-O links. Average bond order =

(2 + 1 + 1) /3 = 4/3