-4
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I was doing some practice problems, and apparently 6 is the main contributor, due to having all full octets, but I'm not seeing how the oxygen with the + formal charge has a full octet in this case. I thought neither of the molecules had full octets for everything, so 6 would be the major contributor because it has more bonds.

What is the reason for 6 being the major contributor?

Thanks!

Major Resonance contributor

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3
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Don't forget about lone pairs that are not drawn. In (5), O has 2 lone pairs and a full octet, but C doesn't. Compare that to (6), where O contributes one of its lone pairs to (form an extra)* bond with C. Now C has a full octet and O still has a full octet and a formal charge of +1. The formal charge +1 means there are 5 electrons around O, which includes a lone pair that is not shown.

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