So I am asked to find the bond order for both of them. I drew the correct Lewis structure, and there's one C-O double bond (and 2 other single bonds). I determined the bond order in CO3 (2-) to be 1.33, but why is it not the same for H2CO? Any hint would be greatly appreciated guys!!

Thanks in advance

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    $\begingroup$ Because the $\ce{C=O}$ and $\ce{C-H}$ bonds in formaldehyde are not identical. $\endgroup$ – Jan Oct 25 '17 at 7:39

All three oxygens in carbonate are equal and these three share a charge of -2. That means you can draw these three resonance structures, where each bond in 2 out of 3 has an order of 1, and in the third there's a double bond. The average of that gives you exactly 1.33.

But you can't draw any reasonable resonance structure for formaldehyde, which would share the partial charge of oxygen to hydrogens. There's only one other, which has a positive charge on carbon and negative on oxygen, but that won't make the greatest contribution, as carbon would have only 6 electrons

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