Question

How do you calculate the formal charge of `NH_3`?

Answer 1

Ammonia is a neutral gas, and thus has a formal charge of zero. Moreover its constituent atoms are each formally neutral.

Explanation:

A useful comparison is to consider the formal neutral charge of ammonia versus ammonium ion, `NH_4^+`, in which nitrogen bears a formal positive charge.

Around nitrogen in ammonia there 2 electrons from the lone pair, 3 electrons from the `N-H` bonds (the other 3 electrons devolve to hydrogen), and 2 inner core electrons. There are thus 7 electrons to balance the 7 protons (i.e. the positively charged, massive particles in the nitrogen nucleus). Nitrogen is thus neutral; the hydrogens are neutral, and ammonia is a neutral molecule.

When ammonium forms from ammonia, we write:

`NH_3 + H^(+) rarr NH_4^+`

Of course, here, mass and charge are conserved, as they must be. But in ammonium ion, nitrogen has a half share only of the 8 electrons that comprise the `N-H` bonds, so four from the `N-H` bonds, and 2 inner core electrons. Around nitrogen in ammonium there are thus 6 electrons only, and here nitrogen has a formal positive charge necessarily.

Are you happy with this treatment?