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

$N {H}_{3}$ 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 ${\left[H : N {H}_{3}\right]}^{+}$ is not really correct, as all $H$-atoms are completely equal, and the electron deficit (the overall +) is shared around the whole ${\left[N {H}_{4}\right]}^{+}$ 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.