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?
