Question

How is `NH_3` acting in the reaction `H^++:NH_3 ->[H:NH_3]^+`according to the Lewis definition?

Answer 1

`NH_3` is acting as an electron donor, so by definition it is a Lewis base.

The lone pair of elctrons from the `N`-atom is (mainly) donated to, are shared with, the proton (`H^+`), after which all four `H`'s fill up a nice tetrahedron around the `N`.

The representation of `[H:NH_3]^+` is not really correct, as all `H`-atoms are completely equal, and the electron deficit (the overall +) is shared around the whole `[NH_4]^+` ammonium -ion.

If the ammonium-ion is dissociated (e.g. in hot alkalic surroundings), there is no telling which of the four `H`'s will leave.