Why does ammonia, nh3, behave as a base when it reacts with an acid?

Ammonia NH3 behaves as a base when it reacts with an acid because it accepts a proton and becomes NH4+. hope this helps! :) have a good day

The answer is "it accepts a proton and becomes NH4" It's the answer because I just got it correct

They are all full and don't need any more electrons in there outer shell.

Less stable  hope this