Remember that the “true” picture of a molecule is a hybrid of resonance forms. The more stable the resonance form, the greater the contribution it will make to the resonance hybrid. Two main things to think about when evaluating resonance forms: legality and significance
Don’t ever draw more than 8 electrons for carbon, nitrogen, or oxygen. That would be illegal. It’s amazingly easy to make this mistake. Try to do it while doing problems, not during an exam.
Second, significance. Here are a few main points.
The fewer charges the resonance form has, the more stable it will be
Resonance forms where every atom has a full octets are more stable than resonance forms where an atom has less than a full octet.
Don’t ever give nitrogen or oxygen less than a full octet ; these are highly unstable species. (If you get far enough in organic chemistry you will know the rare occasions when to break this rule: it’s not now)
Negative charges are stabilized by highly electronegative atoms (e.g. O better than C) and adjacent electron-withdrawing groups (such as halogens or carboxylic acids)
Positive charges are stabilized by more electropositive atoms (e.g. C better than O) and adjacent electron donating groups (such as alkyl groups)
Stabilization of negative and positive charges is a BIG deal in organic chemistry. It will come up again and again in different contexts. More on that in a few days.
Tomorrow: the second KEY accounting convention to learn in organic chemistry.Thanks for reading! James