[alfagamizado]

I've been asking myself this question since yesterday evening. The proton "gets close" to the last avaiable electron, since the other three electrons are already taken, and the same proton "steals" that electron, making Nitrogen+. Is that right? Why then don't the other three electrons Nitrogen receives from the three "neutral hydrogens" makes it N3- ? Is the only reason these three bonds are shared and the last one, not?

Thanks in advance,
Alfa.

[Schrödinger]

About the bond between H⁺ and NH₃, it is a co-ordinate bond (google it). The lone pair of electrons of electrons are 'donated' by the N to the H⁺.

And about the N becoming N^3- by accepting electrons from 3 neutral H's, it's not possible because the electronegativity difference between N and H is not huge enough for an ionic bond to form (See Pauling's electronegativity scale). Hence the 3 bonds in NH₃ are polar covalent and the the NH₃-H⁺ bond is co-ordinate.