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My question is why size is not taken into consideration when deciding major contributors of resonance structures, particularly: why is the formal charge always assigned to the most electronegative atom, but not necessarily to the one that can "better hold a negative charge"?

Example: Thiocyanate. Everywhere I've found says the major resonance contributor is that where the negative charge lies on the nitrogen atom due to the higher electronegativity of $\ce{N}$ vs $\ce{S}$, but when we talk about acids/bases, clearly $\ce{H2S}$ is a stronger acid than $\ce{NH3}$ due to the "better stabilization of the negative charge by the sulfur atom due to its larger size/polarizability" (or so we teach in every OChem course). So why do we disregard this fact when assigning formal charges?

PS. I'm aware formal charges are an artificial construct and do not necessarily represent real-life electron density, I am just curious about these rules from an educator's perspective.

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  • $\begingroup$ There is a difference between the two cases. In the first, you create (partial) charges in a neutral molecule, while in the second, you have a formal charge on an atom. $\endgroup$ – Raphaël Aug 6 at 23:49
  • $\begingroup$ @Raphaël I think I see what you're saying, but I fail to see the difference in the end. In the first one you have a formal charge on an atom too. $\endgroup$ – ralk912 Aug 7 at 0:13

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