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The following line was given in a chemistry textbook

"In case of polyatomic ions, the net charge is possessed by an ion as a whole and not by a particular atom."

I understand this in cases when that polyatomic ion has conjugated double bonds(nitrate, sulphate etc) allowing the charge to be spread throughout the molecule by means of resonance, but I fail to understand how charge could possibly be distributed in cases of ions like Ammonium(NH4+). Shouldn't the positive charge be on the Nitrogen atom alone?

This line was given under a topic discussing why formal charge isn't true charge.

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  • $\begingroup$ N in NH3 significantly pulls electron density to itself, making H atoms partially positively charge. It happens even more in NH4+. N would not keep positive charge, if electrons can be pulled from N-H bonds. $\endgroup$
    – Poutnik
    Commented Feb 9 at 13:21
  • $\begingroup$ Okay, but what about organic anions like acetate? The negative charge is shared only among the two oxygens, but not throughout the whole molecule. $\endgroup$
    – Nothing
    Commented Feb 9 at 13:26
  • $\begingroup$ Ion structures are different. The textbook notes were meant for more or less symmetric ions like ClO4-, NO3-, SO4^2- or NH4-. It obviously does not apply on big molecules with scattered charge groups like -OOC-(CH2)n-COO-, proteins or ion exchanging resins. $\endgroup$
    – Poutnik
    Commented Feb 9 at 13:55
  • $\begingroup$ Understood, Thanks! $\endgroup$
    – Nothing
    Commented Feb 9 at 14:18
  • $\begingroup$ You're not considering "inductive effects". They spread charge between neighbouring atoms. Mesomeric have longer range. $\endgroup$
    – Mithoron
    Commented Feb 9 at 18:31

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