Why is ammonia a base and ammonium an acid?

Question

Why can't ammonia NH3 donate a proton (when in water) but ammonium NH4 can? and why can't ammonium accept a proton and ammonia can?

btw all I learned so far are 2 theories of acids/bases: Arrhenius and Bronsted-Lowry.

Answer 1

NH3 CAN donate a proton, but it much more often accepts a proton. Basically, as you move from right to left accross the periodic table, the tendency of the atoms to give up a proton decreases, so an uncharged molecule consisting of F and H is acidic (hydrofluoric acid), An uncharged molecule consisting of O and H is neutral (water has a pH of 7) and so the next element over, Nitrogen, is basic when consisting of only Nitrogen and Hydrogen and uncharged. Adding a proton (hydrogen cation) gives the molecule a positive charge, which makes the molecule more likely to donate a proton, and thus act as an acid.

Ammonium CAN'T accept a proton because it already has 4 bonds, the maximum possible since it is in the second period. This is why CH4 doesn't accept a proton; it should be even more basic than ammonia, but it has a full 4 bonds already, so it can't accept any more.

Answer 2

First of all, ammonia is NH3. Secondly, when it dissolves in water, it forms NH4+ and OH-, which shows that it is a base. Actually, ammonia is the only base with no metal ions. But ammonia is also a weak base because it dissociates very less, i.e. most of the dissolved NH3 in H2O exists as NH3 and not NH4+. Therefore it is better to call ammonium hydroxide as aqueous ammonia.

Answer 3

NH3 donates a proton when reacting with sodium.

2Na + 2NH3 ===> 2Na+ + 2NH2- + H2

But NH2- is so strong a base that it cannot exist in water.

NH2- + H2O ===> NH3 + OH-

NH4+ cannot accept a proton, because then it would be NH5++, and there are not enough valence electrons or accessible orbitals to allow this. NH3 accepts a proton, because H+ sits down on the nonbonding electron pair of N, and there is still an octet.

Answer 4

Ammonia will accept proton which is a property of bases
ammonium will donate proton which is a property of acids