While studying resonance from this Chemistry LibreTexts article, I found the rules to follow in order to decide which structure is the most stable. Among them:
- The structure with the least number of formal charges is more stable
- The structure with the least separation of formal charges is more stable.
Firstly I'm not sure how to interpret the "number of formal charges" in rule 1: does it refer to the number of atoms with non-zero formal charge or to the magnitude of each formal charge? It confuses me a bit also because the total number of formal charge (the sum of them) is clearly constant (and equal to the total charge or molecule).
Secondly I don't understand which of these two rules (1 or 2) is more important and can dominate the other one.
For example two possible structures of $\ce{CNS^{-}}$ are $$\ce{C=N=S}\qquad\ce{C#N-S}$$
The formal charge for the first one are (in order) -2, 1, 0 while for the second one are -1, 1, -1.
The first is preferable according to rule 1 (interpreting it as the number of atoms with non-zero formal charge) while the second is preferable according to rule 2 since the separation of charge is not as strong as in the first structure (I hope I'm understanding correctly rule 2).
Am I reading incorrectly the two rules or is there one that is actually "stronger" than the other?