Consider the ammonia molecule, #H_3N:#, which is formally neutral Why so? Because it has 2 inner shell electrons, it has a half share of the 6 electrons that comprise the #3xxN-H# bonds, and it gets the full contribution from the lone pair of electrons: #2+3+2=7e^-#. This electronic charge balances the 7 positively charged protons present in the nitrogen nucleus. In ammonium, it still gets the 2 inner core electrons, but only half of the #4xxN-H# electrons, i.e. 6 electrons in total, and thus a formal positive charge.
This can also be seen in the acid-base reaction:
#NH_3(aq) + H_2O rightleftharpoonsNH_4^+ + HO^-#.
Charge is conserved as always.
In liquid ammonia, the amide ion, #NH_2^-#, is known, as well as the imide ion, #NH^(2-)#. By means of the formalisms above, can you tell me where the formal charge lies?
